Cancer in Children: Clinical Management, 5th Edition

Chapter 12. Acute lymphoblastic leukaemia

Kjeld Schmiegelow

Göran Gustafsson

Introduction

Acute lymphoblastic leukaemia (ALL) accounts for 30 per cent of all childhood malignancies. It encompasses a heterogeneous group of biologically and clinically related entities, each with their own characteristic epidemiology, biology, and sensitivity to anticancer agents. Whereas ALL was fatal in the vast majority of patients only 30 years ago, cure is now realistic in 75 per cent of patients.1,2 This impressive success has been obtained by integrating improved understanding of the biology (including cytogenetics and molecular biology) with risk-groupadapted therapy, pharmacology, and supportive care, and not least randomized clinical trials and close international collaboration.3 This approach has served as a general model for the successful treatment of other childhood cancers.

Epidemiology and aetiology

The annual incidence of ALL in Europe and the USA is 3.5–4.0 per 100 000 children <15 years of age with a male-to-female ratio of 1.2.4 The risk of developing ALL before the age of 15 years is one in 2000. There is a striking incidence peak at age 2–7 years, where the incidence is as high as 10 per 100 000 children (Fig. 12.1). This peak has implications for the understanding of epidemiology, biology, and effective therapy. It is conspicuously highest in the most developed countries and is higher among White than among Black children, it consists mainly of B-cell precursor leukaemia with the TEL-AML1 (ETV6-RUNX1) fusion [translocation (t(12;21)(p13;q22)] or a high-hyperdiploid genotype, and the outcome for children aged 2–7 years is superior to that of children in older and younger age groups.

Although a number of risk factors for ALL have been identified, the aetiology and pathogenic mechanisms remain unknown in the vast majority of cases. An increased risk of ALL has been associated with high birth weight (the odds ratio increases by approximately 20 per cent per kilogram), certain congenital syndromes and chromosomal abnormalities (e.g. a 15-fold odds ratio for Down syndrome patients), genetic instability (e.g. ataxia telangiectasia, including ATM gene mutation carriers, Fanconi anaemia, and Bloom syndrome), exposure to ionizing radiation, and certain immune deficiencies (e.g. Wiskott–Aldrich syndrome). In addition, some studies have linked the development of ALL to exposure to electromagnetic fields or vitamin K prophylaxis, but others have failed to confirm these associations. Although specific genotypes of certain polymorphic enzymes that may play a role in detoxification or DNA repair processes have been linked to the risk of ALL, the risk of developing ALL is not significantly increased for siblings of children with ALL (except for monozygous twins) despite their common genetic background and environment.5 Whereas the associations noted above will account for only a small fraction of patients, a growing amount of data indicate that ALL in childhood occurs as a consequence of in utero (first hit) genetic aberrations and postnatal (second or further hits) abnormal immunologic responses to common childhood infections.6,7 The latter hypothesis has been supported by computer modelling, space and time cluster analysis, and the finding of an increased risk of ALL with delayed exposure to common childhood infections, population mixing, certain tissue types, and low levels of mannosebinding lectin.6,7,8 The prenatal occurrence of leukaemia-associated genetic aberrations, such as translocations t(4;11) or t(12;21), hyperdiploidy, and clonal heavy-chain gene rearrangements, have been convincingly demonstrated by their presence in Guthrie cards for a large proportion of patients who later develop ALL, but even more interestingly they also occur in approximately 1 per cent of healthy newborns.6,7,9

Fig. 12.1 Age-specific incidence rates for 3354 cases of childhood ALL diagnosed in Denmark, Finland, Iceland, Norway, and Sweden between 1982 and 2001. The data can roughly be divided into infant ALL (age <1 year) characterized by a high frequency of 11q23 aberrations, an incidence peak group where 70 per cent or more are cases with t(12;21) translocation or high hyperdiploidy, and a ‘base-line’ incidence that includes T-cell leukaemia and B-cell precursor ALL without t(12;21) and hyperdiploids.4

Biologic studies

ALL is driven by mutations in a multistep malignant transformation that leads to a deregulated monoclonal expansion of immature lymphoid progenitor cells. Histology, cytochemistry, immunophenotyping, and cytogenetics are crucial for the correct identification of the involved cell lines as well as their stage of cellular maturation. Based on specific morphologic features, the French–American–British (FAB) morphologic classification of the mid-1970s distinguished between L1, L2, and L3 subtypes. Although the small L1 lymphoblasts have a higher chemosensitivity that the somewhat larger and more pleomorphic L2 subtype, the distinction between L1 and L2 subtypes has been surpassed by more informative clinical and biologic factors. In contrast, the FAB L3 subset of ALL remains important as it identifies mature B cells that express immunoglobulin on the cell surface, and which are indistinguishable from the lymphoblasts seen in Burkitt non-Hodgkin lymphoma. The L3 leukaemic cells are larger than those seen in FAB L1 and L2 ALL, they have moderately abundant and deeply basophilic cytoplasm that contains numerous prominent vacuoles, and the nucleus is regular and contains distinct nucleoli.

Immunophenotype

In acute myeloid leukaemia (AML), ALL, and non-Hodgkin lymphomas (defined with respect to ALL by the presence of <25 per cent lymphoblasts in the bone marrow), the malignant cells resemble normal haemopoietic cells. Immunophenotyping by flow cytometry and/or immunohistochemistry can be applied to cells from blood, bone marrow, ascites, pleural effusions, cerebrospinal fluid, and lymph node biopsies using monoclonal antibodies directed towards differentiation antigens [cluster of differentiation (CD) antigens] in the nucleus or cytoplasm, or on the cell membrane. Flow cytometry allows rapid analysis of thousands of cells, and in the same run it is possible to determine cell size (forward scatter), granularity (side scatter), and (depending on the technical equipment) two, three, or more different CD antigens by applying different fluorochrome-linked antibodies directed towards these antigens.10 Since many of the CD antigens are not lineage specific, the immunophenotypic classification is based on the combination of CD antigen expression, and at least 95 per cent of all leukaemias can be classified as lymphoid or myeloid with or without co-expression of myeloid or lymphoid CD antigens (Table 12.1). Less than 5 per cent of all acute leukaemias lack a uniform lymphoid or myeloid pattern and are classified as biphenotypic leukaemias (carrying a combination of markers from different lineages on each malignant cell) or (more rarely) bilineage leukaemias displaying a mixture of two different lineages of malignant cells. Rarely, morphology and cytochemistry, immunophenotyping, and cytogenetic markers fail to determine whether the leukaemia is of lymphoid or myeloid origin. These undifferentiated leukaemias tend to be very resistant to chemotherapy. Weak aberrant expression of myeloid markers is common in ALL [e.g. frequent in t(12;21)-positive ALL], but this feature does not seem to carry prognostic implications.

Determination of the immunophenotype of the leukaemic cells is primarily crucial for the identification of children with mature B-cell ALL, since they need very intensive therapy (see the section on special subsets), and those with T-lineage ALL which is more commonly associated with other known risk factors than B-cell precursor ALL and also needs special treatment considerations (see section on special subsets). Based on the immunophenotyping, ALL can be classified as T-lineage (10 per cent of ALL), B-cell precursor (85–90 per cent of ALL), or mature B-cell disease expressing clonal surface immunoglobulins (2 per cent of ALL). In addition to the mature B-cell leukaemias, B-lineage ALL can be subdivided into pro-B (CD-10 negative), early pre-B, and pre-B (with intracytoplasmic immunoglobulins), but this subdivision is not generally used in the risk group and treatment stratification. Finally, the pattern of CD antigen expression in most cases of childhood ALL is so unique and specific that it can be applied to flow cytometric determination of residual disease in the bone marrow or peripheral blood during therapy, although the sensitivity rarely falls below one leukaemic cell in 103–104 normal mononuclear cells.11

Table 12.1. Characteristics of subsets of childhood ALL

Subset

Frequency of all leukaemias (per cent)

Frequent positive markers

Typical chromosomal abnormalities

Other characteristics compared with B-cell precursor

5-year pEFS (per cent)

B-cell precursor

75

CD10, 19, 22, cy79, HLA-DR, TdT

High-hyperdiploid, t(9;22)(q34;q11), t(1;19)(q23;p13), t(12;21)(p13;q22), dic(9;12)(p11-p12;p12)

 

75-85

Mature B-cell

2

Surface immunoglobulins, CD19,20

t(8;14)(q24;q32.2), t(8;22)(q24;q11), t(2;8)(p11-12;q24)

Frequently bulky disease, low WBC, male predominance

80-85

Infants

3

CD19, 22, cy79, HLA-DR (NB usually lacks CD10)

t(4;11)(q21;q23), t(11;19)(q23;p13), other 11q23 aberrations

Very high WBC, frequently CNS disease

50

T-lineage

10

CD2, cy3, 4/8, 5, 7, TCRa=b, TdT

t(11;14)(p13;q11), t(7;v)(q34-36;v)

Older age, higher WBC, mediastinal mass, CNS disease

60-65

CD, clusters of antigens; CNS, central nervous system; cy*, cytoplasmatic; pEFS, probability of event-free survival; TdT, terminal deoxynucleotidyl transferase; v, variable, WBC, white blood cell count at diagnosis.

 

Cytogenetics

Clonal numerical and structural chromosomal aberrations can be demonstrated in 90 per cent of children with ALL (see Chapter 2).12,13 As far as we know, many of these carry neither pathogenic nor prognostic significance, and probably merely reflect the genomic instability of the malignant clone. Other non-random chromosomal changes are a hallmark for understanding aetiology, biology, and risk stratification, since they correlate with the drug sensitivity of the malignant clone and thus may guide the choice of treatment. The karyotyping is generally done by Giemsa dye (G-band) staining of metaphase chromosome spreads with a resolution of approximately 5–10 Mbp.12 Although this is a useful method for screening, G-band karyotyping may be hampered by the poor in vitro growth of malignant lymphoblasts, the preferential growth of certain subclones, and a lower quality of the banded chromosomes compared with those in AML samples for instance. Even when the technique is successful, some chromosomal translocations are easily overlooked by G-band karyotyping since they involve regions of almost similar size and band staining [e.g. the t(12;21) translocation]. Hence a number of alternative techniques that do not require metaphase spreads have been applied for genomic screening [e.g. high-resolution comparative genomic hybridization (HR-CGH) with a resolution of ~ 3 Mbp13] or to detect specific gene mutations or translocations of prognostic significance. The latter include Southern blot, interphase fluorescence in situ hybridization (FISH), and reverse transcriptase polymerase chain reaction (RT-PCR) which detects the chimeric mRNA generated by a chromosomal translocation. Other new techniques used to clarify chromosomal aberrations in malignant cells include spectral karyotyping and spectral colour banding, both of which improve the interpretation of the metaphase chromosomes by chromosomeor bandspecific differential colour staining (Chapter 2). Finally, the DNA index or DNA content (ratio of clonal DNA to normal DNA content) quantified by G-band karyotyping or flow cytometry carries prognostic significance. Thus, 3 per cent of children with ALL have a hypodiploid clone (< 45 chromosomes, DNA index <0.90–0.95) and a poor prognosis, while 30 per cent have a high-hyperdiploid clone with a modal number (chromosome number of the dominating clone) of >50 chromosomes (median 55) and a favourable outcome.12 The modal number of 50 represents the antimode of the bimodal distribution of the modal chromosomal number in these leukaemias. The high-hyperdiploid leukaemias, which are most common in the age group 2–7 years (Fig. 12.1), have a very characteristic non-random pattern of trisomies, most commonly involving chromosomes 4, 6, 10, 14, 17, 18, 21, and X. When the modal number exceeds 60–65, a far more random pattern of trisomies emerges. Since the superior prognosis of the high-hyperdiploid cases has been specifically related to the presence of trisomies 4, 10, and 17, interphase FISH techniques to detect these trisomies are applied by the US Children's Oncology Group as part of their risk group classification strategy. An important feature of high-hyperdiploid ALL is the relatively low frequency of chromosomal deletions that involve regions harbouring tumour suppressor genes, and this could be one of the biologic factors determining their prognostic significance.13 In addition, the favourable prognosis of high-hyperdiploid ALL has been related to a high in vitro propensity for apoptosis (programmed cell death), a high in vitro sensitivity to antimetabolites, and an increased in vivo sensitivity to high-dose methotrexate (HD-MTX).14 In G-band karyotyping, the DNA index is usually determined by the best growing clone, whereas the DNA index determined by flow cytometry is that of the quantitatively dominating clone even though smaller subclones with hypoor hyperdiploid DNA indices may be present. The prognostic impact of the presence of such subclones remains to be determined in large clinical studies.

The chromosomal translocations affecting regulatory and coding regions involved in cellular growth, differentiation, and survival (Fig. 12.2) are highly significant for our understanding of the pathogenic processes leading to ALL.

Fig. 12.2 Examples of important and common translocations among children with ALL. Other numerical and structural aberrations are common, but are not shown. The translocations lead to aberrant and dysregulated genes that encode nuclear (most frequently) or cytoplasmic proteins that interfere with the regulatory processes that control cell growth, differentiation, and survival. The translocations are associated with a specific subtype of ALL. The translocations may upregulate proto-oncogenes by placing them under the control of a potent enhancer such as T-cell receptor (T-cell ALL) or immunoglobulin genes (B-cell, commonly Myc oncogene activation (8q24)). Several chimeric fusion genes of prognostic significance have been described. In late (expressing cytoplasmatic immunoglobulins) and early B-lineage precursor (pre-B) leukaemias. The t(9;22) and t(?,11q23) (?, different translocation partners) are found in both myeloid and lymphoid leukaemias, which indicate that malignant events may take place in primitive multilineage stem cells.

t(1;19)

A frequent translocation that identifies a subset of patients with specific treatment requirements is the t(1;19)(q23;p13) translocation, which occurs in 5 per cent of all cases of childhood ALL, especially in the pre-B ALL subset defined by the presence of intracytoplasmatic immunoglobulins. This translocation fuses the E2A gene on chromosome 19 with the PBX1 gene on chromosome 1. Although this translocation was previously found to carry a poor prognosis, intensified induction and consolidation therapy has counteracted this to a large degree.

11q23

The 11q23 aberrations most commonly involve translocations between the very promiscuous MLL (mixed-lineage leukaemia) gene located on chromosome region 11q23 and more than 50 different partner genes of which the t(4;11) translocation accounts for approximately 50 per cent.15 In childhood ALL the 11q23 rearrangements affect 8 per cent of patients. Patients with t(4;11) and t(11;19) translocations are the youngest (most often infants), tend to have very high white blood cell count (WBC) at diagnosis (median > 180 109/liter), rarely have T-cell disease, and have a high frequency of central nervous system (CNS) disease at diagnosis. Among infants, any 11q23 abnormality seems related to a poorer outcome, whereas in older children an inferior outcome has primarily been associated with the t(4;11) and t(9;11) translocations. At least for the t(4;11) translocation, stem cell transplantation (SCT) has not been shown to improve survival compared with chemotherapy only.15

t(9;22)

The translocation t(9;22)(q34;q11), which can be demonstrated in 3 per cent of ALL cases in children and in 25 per cent of adult cases, fuses the c-abl gene on chromosome 9 with the breakpoint cluster region (bcr) on chromosome 22. This yields a minor (185 kDa) or major (210 kDa, more commonly seen in chronic myeloid leukaemia) constitutively activated chimeric gene with high expression of a tyrosine-specific protein kinase. Secondary cytogenetic abnormalities are found in two-thirds of the patients. Because of their lower chance of complete remission after induction therapy (80–85 per cent), their high levels of subclinical disease after induction therapy, and their increased risk of relapse, allogeneic SCT from an HLA-matched related or unrelated donor is indicated in first remission.16 The WBC at diagnosis is negatively correlated with the chances of cure. Patients with a low WBC at diagnosis, aged between 1 and 9 years, and/or with a good response to a 1-week prednisone prephase have a less unfavourable outcome, and a substantial number of such patients can be cured by intensive chemotherapy.16 The oncogenic protein generated by the t(9;22) translocation can effectively be targeted and blocked by the specific tyrosine kinase inhibitor imatinib mesylate (formerly STI-571). Although it has been shown to be effective in both relapsed and refractory Philadelphia chromosome positive (Ph+) ALL, development of resistance occurs rapidly. The usefulness of this targeted therapy in first-line treatment is uncertain, and several study groups are testing its impact on long-term survival.

t(12;21)

The t(12;21)(p13;q22) translocation fuses the TEL gene on chromosome 12 with the AML1 gene on chromosome 21. This common translocation, which occurs in 25 per cent of cases of B-cell precursor ALL, has been crucial in our understanding of the natural history of childhood ALL. It is a main determinant of the characteristic incidence peak in the age group 2–7 years (the other is high-hyperdiploid ALL), it is associated with a favourable risk grouping (a high WBC at diagnosis or CNS disease is rare), and it confirms that the first genetic aberration(s) in a large proportion of ALL occurs prenatally, since it has been demonstrated not only in the Guthrie cards of children who later develop TELAML1-positive ALL, but also in 1 per cent of healthy newborns.6,7,9,17 The vast majority of t(12;21)-positive leukaemias also harbour structural chromosomal changes, e.g. commonly 12p, 6q, and 9p deletions, which include deletions of tumour suppressor genes.13 Leukaemias with this translocation frequently have a low-level expression of (myeloid) CD13 and CD33 antigens, almost never have a modal number >50, are generally aged 1–10 years, and tend to have late relapses (many of which are extramedullary) with a median time to relapse of 4 years. Following a relapse, they seem to have a better outcome than patients without this translocation, but it is as yet unclear to what extent this reflects patient selection and lack of adjustment for the duration of first remission. Patients with the TEL–AML1 translocation are more sensitive to L-asparaginase in vitro than those who lack the translocation, and some protocols including intensive L-asparaginase therapy have yielded high cure rates for this subset of patients.18,19

Microarray studies

Although all the cytogenetic findings discussed above are useful in the risk grouping of children with ALL and offer some hints on the pathogenic mechanisms involved, an even more substantial breakthrough in the mapping of the involved genes and the tumour biology will come from microarray studies in which the expression of thousands of genes can be determined in a single analysis.20 Although the amount of data generated by these studies is overwhelming and requires extensive and complex bioinformatic analyses, in the coming years we are likely to understand the mechanisms behind disease development, aggressiveness, and drug sensitivity. This will probably have significant impact on our risk classification and treatment approach to childhood ALL.

Clonal immune gene rearrangements

The diversity in the normal immune response towards foreign antigens reflects somatic mutations in the B-cell (immunoglobulin) and T-cell receptor genes. Because of the clonal nature of ALL, quantitatively dominating immune gene rearrangements can be demonstrated by PCR at diagnosis in 95 per cent of children with ALL.21 Since the leukaemic cells are degenerate, the clonal immune gene rearrangements are not line specific. Thus T-cell receptor gene rearrangements can be detected in most cases of B-cell precursor ALL. The clonal immune gene rearrangements are useful for diagnosis, since they are rarely found in cases of AML, myelodysplastic syndrome, aplastic anaemia, or non-malignant lymphadenopathies. In addition, they are very specific markers of the malignant lymphoblastic clone, and thus can be used for precise determination of residual bone marrow disease (MRD) during therapy with a detection limit of one malignant cell in 104–106 normal mononuclear cells (see below).21,22

Symptoms and differential diagnosis

At the time of diagnosis, the leukaemic cell burden is of the order of 1011–1012 cells (109cells 1 g tissue). This immense clonal expansion of lymphoblasts gives rise to bone marrow dysfunction generally with pancytopenia and the classical symptoms of fatigue and pallor (80 per cent), fever (65 per cent), bleeding (50 per cent), enlarged liver and spleen (80 per cent), swollen lymph nodes (40 per cent), bone and joint pains (25 per cent), a thymic mass (10 per cent), and a pattern of symptoms that reflects the organs involved (e.g. breathing difficulties, neurologic symptoms, or renal failure). CNS disease with >5 106/liter lympho-blasts in the cerebrospinal fluid (CNS-3) or a lower number of unequivocal blasts (CNS-2) are seen in 3–5 per cent of patients. Only some groups have been able to demonstrate an inferior outcome for patients with CNS-2. Except for the presence of a high number of lymphoblasts in a blood smear, none of the presenting clinical and haematologic findings are pathognomonic for childhood ALL, and the presenting clinical picture may occasionally be atypical. Hence a bone marrow examination is warranted in children with prolonged monocytopenia, and should be considered prior to glucocorticosteroid or methotrexate (MTX) therapy of supposed idiopathic thrombocytopenic purpura or rheumatoid arthritis. Most patients are generally well at the time of diagnosis, and the majority of children with ALL have had symptoms for only a few days or weeks, with a trend for the lower risk groups to have a more protracted course of disease. Half of all children with ALL present with a WBC <10 109/liter, some even without easily detectable lymphoblasts in a blood smear (aleukaemic leukaemia), although PCR amplification of the B- and T-cell receptor genes will demonstrate clonality in the peripheral blood of the vast majority of these ‘aleukaemic’ patients. Fifteen per cent of all patients have a WBC at diagnosis of > 50 109/liter. Eighty per cent will have a thrombocyte count < 100 109/liter, and 85 per cent will have a haemoglobin < 6.0 mmol/l. A thrombocyte count above the upper normal limit is rare in children with ALL and more likely indicates that the patient has an inflammatory disease. Similarly, a high erythrocyte sedimentation rate without a concommitant high C-reactive protein also points towards ALL. Patients who present with high haemoglobin at diagnosis have a somewhat inferior outcome, as this indicates aggressive disease with bone marrow involvement for only a short time period. Other signs of ALL are the indicators of a rapid cell turnover such as high plasma lactate dehydrogenase and high plasma uric acid levels. At diagnosis, some patients are critically ill and may die before or at the initiation of therapy because of septicaemia, severe breathing difficulties (due to a thymic mass, superior vena cava obstruction, or pulmonary leucostasis), CNS disease, or hyperleucocytosis with WBC > 400 109/liter. Patients with a very high WBC have a risk of disseminated intravascular coagulation, tumour lysis syndrome and electrolyte disturbances, renal failure, and leucostasis with life-threatening breathing difficulties, convulsions, and CNS haemorrhage or thrombosis.

The diagnosis of ALL must be established beyond any doubt. A complete workup is generally much more important than rapidly initiated therapy. Since most cases of ALL are very chemosensitive if given the right therapy, failure to perform the necessary investigations may jeopardize the chances of long-term survival. In addition to the bone marrow biopsy and aspirate for histomorphologic examination, immunophenotyping, and karyotyping, the initial programme must include a full history, performance and neurologic status, lymph node palpation, determination of spleen, liver, and testicular size, skin and mucosal membrane pathology, a chest radiograph to detect an anterior mediastinal mass, full blood counts, determination of blasts in blood and spinal fluid, and basic values such as blood group, immunoglobulins, electrolytes and urate, kidney and liver function tests, and serologic tests for viral infections. CT or MRI of the brain and spinal cord is indicated in cases with signs of CNS disease (e.g. headache and nausea, vision disturbances, cranial or peripheral nerve paresis) or the presence of spinal fluid lymphoblasts.

Juvenile rheumatoid arthritis and infectious mononucleosis may occasionally mimic leukaemia. Other diseases that may cause bone marrow failure and be clinically confused with leukaemia include aplastic anaemia, widely disseminated neuroblastoma, and more rarely Ewing sarcoma, rhabdomyosarcoma, and other anaplastic tumours with bone marrow dissemination. In addition, Epstein–Barr virus, cytomegalovirus, and parvovirus B-19 infections, as well as haemophagocytic or virus-associated lymphohistiocytosis with pancytopenia, may be misinterpreted as leukaemia. Pre-ALL with bone marrow failure, but a level of lymphoblasts in the bone marrow <5 per cent, is a rare condition that occasionally can be precipitated by parvovirus B-19 infection. This condition is more common in girls. PCR amplification of the B- and T-cell receptor gene rearrangements will demonstrate clonality in most of these patients.

Risk stratification

Since childhood ALL consists of a heterogeneous group of distinct biologic subtypes, identical therapy cannot be given to all patients. Approximately 40 per cent of all patients have one or more features that predict a significantly poorer outcome. Identification of these features should be attempted in every patient, since this allows correct risk grouping and the subsequent right choice of treatment intensity. Thus the most important prognostic factor is the treatment itself. Not only will all patients die if deprived of antileukaemic therapy, but specific subsets of ALL differ in their sensitivity to combinations of chemotherapy. An international classification that defines standard-risk patients as those with an age of 1.0–9.9 years and WBC < 50 109/liter and high-risk ALL as all other combinations has been useful for the comparison of treatment results from different study groups.23However, each study group needs a risk classification system that reflects its treatment strategy. Although the improvement in riskadapted therapy has eliminated or reduced the importance of some factors (e.g. haemoglobin and FAB L1 versus L2 morphology), others have proved their robustness across study groups. These include gender, age at diagnosis, the tumour burden, immunophenotype and cytogenetics, and the early response to therapy. As an example, the NOPHO ALL-2000 risk grouping is outlined in Table 12.2.

White blood cells

The leukaemic cell mass at diagnosis, reflected by the WBC and extent of organ involvement, has been the most important prognostic factor for several decades, and is included in treatment stratification by nearly all groups (Fig. 12.3 and Table 12.2).1 In addition, the significance of many risk factors in univariate analyses reflects their relation to the WBC. Thus WBC is correlated with immunophenotype (lowest in B-precursor ALL), age (lowest in the age group 2–7 years), and cytogenetics (low in high-hyperdiploid and TEL–AML1 - positive ALL).

Table 12.2. Therapy Groups by NOPHO - ALL 2000 protocol for ALL 1.0-14.9 years

Therapy group

Age (years)

Criteria

Standard

<10

WBC < 10 109/liter
No unfavourable criteria

Intermediate

<10
≥10 y

WBC 10 - < 50 109/liter
WBC < 50 109/liter
No unfavourable criteria

Intensivea

Any age

WBC 50 to < 200 109/liter
Hypodiploidy (34-44 chromosomes)
t(1;19)
T-cell ALL
Overt testicular involvement
CNS involvement
Standard or intermediate at diagnosis with slow response
(day 15 M3 or day 29 M2 bone marrow)

Extra-intensiveb

Any age

WBC ≥200 × 109/liter
11q23/MLL rearrangements
t(9;22)
Hypodiploidy (<34 chromosomes)
Day 29 M3 bone marrow

aOnly patients >5 years at diagnosis with CNS disease, slow response, and/or T-cell disease with mediastinal involvement will receive CNS irradiation.
bThe extra-intensive arm includes stem cell transplantation in first remission. This is optional for 11q23 aberrations in children aged 2.0-9.9 years.

Fig. 12.3 Event-free survival (EFS) of children with (a) B-cell precursor or (b) T-lineage ALL in the NOPHO ALL-92 study. The WBC at diagnosis is one of the strongest prognostic factors, particularly for B-lineage ALL. All T-cell cases received high-risk therapy.

Treatment response

Even when remission is achieved, which is generally the case for 97–98 per cent of all patients, the rapidity of tumour reduction is a very strong predictor of likelihood of cure. Thus the prognosis is inferior for patients with > 1 109/liter lymphoblasts in the peripheral blood after 7 days exposure to prednisone and one intrathecal (i.t.) dose of MTX, >25 per cent lymphoblasts in the bone marrow (M3 bone marrow) after 2 weeks of induction therapy, or >5 per cent lymphoblasts in the bone marrow (M2 bone marrow) at the end of induction therapy. Although these features define a poor risk group of children with ALL, most relapses will eventually occur among patients who seemingly have responded well to the remission induction treatment. Recently, precise monitoring of the subclinical level of MRD by flow-cytometric detection of clone-specific patterns of CD antigens or PCR detection of clonal immune gene rearrangements have emerged as powerful tools to distinguish between the good and poor responders. Immunologic assays reflect the recognition that nearly all T-cell and most B-precursor leukaemias have a phenotype (pattern of CD antigens) that is not normally present in bone marrow. The PCR-based assays identify either the fusion gene transcripts that are generated by the chromosomal translocations [e.g. t(12;21)] present in 35 per cent of ALL cases or the junctional regions of the B-cell (immunoglobulin) or T-cell receptor gene rearrangements. The latter are not leukaemia-associated genetic aberrations, but owing to the vast repertoire of these rearrangements (e.g. 1011 for the immunoglobulin heavy chain), the likelihood of finding non-leukaemic cells with the same rearrangements is extremely low. Together, the immunologic and PCR-based techniques are applicable in 95 per cent of children with ALL, and they will probably change our future definition of remission as well as risk classification. A major pitfall in the quantification of MRD levels based on clonal immune gene rearrangements is the evolution of subclones that differ both in these gene rearrangements and in their chemosensitivity. These subclones may go undetected at diagnosis and be responsible for relapse. However, the variations in the chemosensitivity of subclones seem to be much less than the variations in their immune gene rearrangement. Thus MRD levels early during therapy have already become established as a very strong prognostic feature, surpassing even WBC at diagnosis. Forty per cent of children with ALL will have < 104–105 leukaemic cells in the bone marrow at the end of a 4- to 6-week induction therapy, and these patients will have a relapse risk of 5 per cent irrespective of other known risk factors (Fig. 12.4).21,22 However, it is not known whether these are the patients who can be cured with the less toxic therapy used in the 1970s. In contrast, patients who still have > 103 leukaemic cells in the bone marrow after several months of therapy will have a 70 per cent risk of subsequent and even very late relapses.11,21 So far it is not known whether quantification of low-level residual disease during therapy will allow control of emerging relapse through treatment intensification. In contrast, several groups have demonstrated the persistence of leukaemia-associated immune gene rearrangements at the end of treatment and off-treatment in a substantial number of patients who continue to remain in remission. These may represent normal cells with the same gene rearrangements, non-dividing quiescent leukaemic cells, or preleukaemic cells harbouring the first mutations but lacking the further mutations that lead to overt ALL.6,9 Consequently, it is uncertain whether the demonstration of clonal immune gene rearrangements by PCR at the end of therapy carries prognostic significance.

Gender

Several reports have demonstrated that after the first 1–2 years of therapy boys have a significantly higher risk of relapse than girls, which is only partially explained by their higher frequency of T-cell disease and lower frequency of high hyperdiploidy. However, beyond offering longer maintenance therapy to boys, few collaborative study groups stratify therapy by gender.

Fig. 12.4 Kaplan–Meier plots for 104 patients treated according to the NOPHO ALL-92 protocol. Patients with day 29 MRD levels 0.01 per cent (n=60, lower curve) and <0.01 per cent (n=40, upper curve) (P=0.0007); 0.01 per cent is the median level for patients in remission.22

Age

Although modern therapy has reduced the prognostic impact of age at diagnosis (except for infants), most study groups will offer more intensive treatment to children >10 years of age at diagnosis. The poorer outcome for adolescents compared with children <10 years of age reflects a low frequency of translocation t(12;21) and the high-hyperploid karyotype (Fig. 12.1), a lower chemosensitivity,22 and a lower treatment compliance during oral therapy.

CNS disease

The presence of CNS leukaemia at diagnosis is generally classified as a high risk factor. Although CNS disease requires intensified CNS-directed chemotherapy and/or radiotherapy, it is uncertain whether it is a general feature of more aggressive ALL and thus always justifies more intensive systemic therapy. When high-risk therapy is given, the outcome of these patients is as good as that of other high-risk patients without CNS disease (Fig. 12.5).

Unfortunately, the improvements in outcome during recent decades have been far more pronounced for the lower risk groups, and treatment intensification has had less impact on the cure rates for the higher risk groups (Fig. 12.6).1 Patients with WBC > 200 109/liter at diagnosis, t(4;11) or t(9;22) translocations, a very hypodiploid clone (modal number <34), and those who fail to achieve remission after the first 4 weeks of therapy generally have such a poor outcome that SCT in first remission with a related or matched unrelated donor is a relevant treatment option.16,24,25 However, it is doubtful whether SCTreally improves the longterm survival for some of these subsets of patients at very high risk.15

Therapy

The efficacy of antileukaemic treatment depends on several factors including the dosage (dose and schedule) and pharmacokinetics of the anticancer agents, the chemosensitivity of the leukaemic clone,14,26 and the tumour cell growth kinetics. Refinements of chemotherapy for childhood ALL have developed immensely over the last decades, and reflect both randomized clinical trials and pharmacokinetic and pharmacodynamic studies. Through the focus on these aspects for every drug used in the treatment of childhood ALL, impressive cure rates have been obtained with the same drugs that were available 30 years ago. Since the leukaemias differ in their chemosensitivity, and as each antileukaemic drug may have a limited tumour-reducing potential, multidrug chemotherapy is necessary to achieve cure, and most protocols will include six to ten anticancer agents. The treatment intensity reflects the risk group assignment based on the combination of clinical presentation and characteristics of the lymphoblasts. Across study groups, the treatment of T-cell and precursor B-cell ALL generally includes four main components (Fig. 12.7):

Fig. 12.5 EFS curves for patients with or without CNS disease at diagnosis. All patients received therapy according to the NOPHO ALL-92 protocol.

Induction treatment

A four- to five-drug induction treatment commonly consists of daily prednisolone or dexamethasone, weekly vincristine, repeated doses of L-asparaginase and/or an anthracycline, and i.t. MTX.1 Whether to use dexamethasone (e.g. 6–10 mg/m2) or prednisolone (e.g. 60 mg/m2) depends on the general strategy of the study group. These glucocorticosteroids are among the most powerful antileukaemic drugs. Compared with prednisolone, dexamethasone has higher in vitro antileukaemic efficacy, longer half-life, and better CNS penetration, but it is also more toxic. Dexamethasone has been shown both to reduce the risk of CNS relapse and to improve the overall event-free survival (EFS). However, dexamethasone has also been suspected of increasing the risk of neurocognitive damage, osteonecrosis, excessive weight, and life-threatening sepsis during induction therapy. Patients with a very high tumour burden at diagnosis (excessively large liver and spleen or WBC >100 109/liter) may be at risk of tumour lysis syndrome at initiation of therapy and should start pretreatment with reduced doses of prednisolone (e.g. 5 mg/m2) with subsequent slow increments to standard doses within 3–5 days (depending on the degree of tumour lysis), after which full-dose antileukaemic treatment can be given (see section on supportive care below). Some collaborative groups, including the Berlin–Frankfurt–Münster (BFM), group give all patients a 1-week prednisone prephase with one i.t. dose of MTX. Subsequently, persistence of more than 1 109/liter lymphoblasts in peripheral blood will identify a small (10 per cent) subset of patients with a very high probability of remission failure or relapse. As with glucocorticosteroids, the choice of brand of L-asparaginase may also have a significant impact on the cure rate.27 Thus both pharmacokinetic studies and randomized clinical trials have shown that the longer half-life of the enzyme derived from Escherichia coli enzyme improves the EFS compared with that derived from Erwinia. Generally, induction therapy will induce morphologic remission (defined as <5 per cent lymphoblasts in the bone marrow) in 99 per cent of children with lower-risk ALL (even with relatively low-intensity regimens) and 95 per cent of the higher-risk patients. Previously the definition of remission also included full haematopoietic recovery, but as therapy has become more intensive, the continuation of therapy will prevent the marrow regenerating up to normal levels. Although failure to achieve remission is a poor prognostic sign, unfortunately the opposite is not the case. Morphologic remission only signifies a reduction of the tumour burden by 2–3 log (with up to 109 leukaemic cells left), and eventually most relapses will occur among those patients who were in remission at the end of induction therapy. Thus a high chance of cure can only be obtained by prolonged and intensive post-remission multidrug programmes.

Fig. 12.6 EFS curves for patients treated during three consecutive protocol periods (July 1981–June 1986, July 1986–december 1991, and January 1992–December 2001). Despite some improvement, infants do poorly during all three periods. The improvement in outcome has been most pronounced for children with standard-risk (SR) and intermediate-risk (IR) ALL, whereas the change in EFS is less impressive for children with high-risk (HR) ALL, which includes WBC ≥50 109/liter, T-cell, mediastinal or CNS, or testicular disease, t(4;11) or t(9;22), or M2/3 bone marrow at treatment day 29.

Fig. 12.7 Treatment strategy for children with intermediate-risk B-cell precursor ALL (one-third of all cases; see Table 12.2). Detailed cytogenetic mapping with G-band karyotyping, high-resolution CGH, RT-PCR and/or FISH for certain translocations, and in vitro sensitivity studies are performed at diagnosis. Pharmacologic studies of l-asparaginase, HD-MTX, and thiopurines are done routinely. Minimal residual disease measurements by PCR or flow cytometry are made after 1, 2, and 3 months of therapy, prior to randomization, and at the end of therapy.

Delayed intensification (re-induction or consolidation) therapy

Delayed intensification (re-induction or consolidation) therapy is administered when remission has been achieved. The post-remission re-intensification or consolidation therapy varies across study groups, but generally includes alkylating agents and/or epipodophyllotoxins, repeated doses of asparaginase, and HD-MTX with or without concurrent thiopurines. 1,2,3,23,28 From the late 1970s onwards, the BFM and other groups have demonstrated the efficacy of a second induction and consolidation treatment with, for example, vincristine, anthracyclines, glucocorticosteroids, and L-asparaginase. Such an intensification (or even double intensification) has been shown to be of benefit for both high-risk and non-high-risk patients. Today, many collaborative groups offer such a second (or even third) induction phase to all patients, while others (such as the NOPHO group) reserve it for the higher-risk patients. The children who are eligible for SCT in first remission should optimally be transplanted within 6–12 months of diagnosis, i.e. as soon as a histocompatible donor has been identified.24 If a stem cell donor cannot be identified within a year of diagnosis, the advantages of SCT become less obvious for some high-risk groups (e.g. T-cell leukaemia with a WBC > 200 109/liter at diagnosis or infant ALL) since they are primarily characterized by an increased relapse hazard early in therapy.

CNS-directed therapy

Prior to the introduction of CNS-directed therapy in the 1960s, 50 per cent of children with ALL would develop a CNS relapse. Today, irradiation and/or HD-MTX and/or Ara-C with i.t. MTX or triple therapy (MTX, Ara-C, and glucocorticosteroids) has reduced the risk of CNS relapse to <5 per cent.1 At diagnosis, 5 per cent of all children have CNS leukaemia (defined as >5 106 lymphoblasts/liter), and an even higher number have subclinical involvement. A few studies have indicated that patients with CNS-2 (unequivocal blasts < 5 109/liter) also have an increased relapse rate, but several groups have failed to confirm this. Even though CNS irradiation is one of the most effective prophylactic and therapeutic modalities for CNS leukaemia, the strategy adopted by many collaborative groups has been to restrict prophylactic cranial irradiation to selected subsets of patients and to reduce the dose to as little as 12 Gy to avoid late effects.29 Leucoencephalopathy is one of the most worrisome side effects, and the risk is significantly increased if i.v. MTX is given within the first few months after CNS irradiation. On the other hand, the potential neurotoxicity of the intrathecally administered drugs as well as of HD-MTX should not be underestimated, and the frequency and severity of neuropsychologic late effects of high-dose therapy need to be explored thoroughly in future trials. Cranial irradiation can be omitted at least for the low- and average-risk patients, and probably also for the high-risk patients with an M1/M2 marrow on treatment day 7, provided that intensive systemic therapy and/or i.t. MTX (or triple therapy) is given. In the Nordic countries, 10 per cent of the total cohort of patients will receive cranial irradiation (18–24 Gy) in first remission. This includes children with CNS disease at diagnosis and those patients >5 years of age who are in the highest risk groups, since their risk of CNS relapse justifies the neurologic, neuropsychologic, and endocrine late effects induced by irradiation (Table 12.2). The purpose of high-dose chemotherapy is to overcome cellular drug resistance mechanisms or to increase the penetration of the anticancer agent into pharmacologic sanctuaries (such as the CNS). The ratio of CSF to plasma concentration is acceptable for some systemically administered anticancer agents, such as cyclophosphamide (50 per cent), 6-mercaptopurine (25 per cent), cytarabine (15 per cent), and dexamethasone (15 per cent). In contrast, the ratio is poor for prednisone (< 10 per cent), MTX (1–3 per cent), vincristine (< 5 per cent), and teniposide (< 5 per cent). The pharmacologic barrier to some of these drugs can be overcome by administering high-dose chemotherapy, such as HD-MTX and Ara-C, or by direct administration of MTX, Ara-C, or glucocorticosteroids into the cerebrospinal fluid. Since the cerebrospinal fluid volume (and the brain) grows rapidly in early life and reaches close to adult size when the child is 3 years of age, the doses of intrathecally administered drugs should be based on the age of the child rather than on the body surface area. Thus for children <1, 1–2, 2–3, and >3 years of age the dose of i.t. MTX will be 6 mg, 8 mg, 10 mg, and 12 mg, respectively. HD-MTX (1–5 g/m2 with i.t. MTX and leucovorin rescue) is most commonly given together with 6-mercaptopurine because of the synergistic action of these drugs.28 HD-MTX may reduce the risk of CNS disease, but even more convincingly reduces the risk of bone marrow and testicular failures. At least in the lower dose range of 1–2 g/m2, the interindividual variation in MTX pharmacokinetics may significantly influence the risk of relapse. Thus children with B-lineage ALL who are randomized to receive pharmacokinetically guided doses of MTX during consolidation therapy to achieve a target plasma drug concentration have a significantly better outcome than those who are treated with dosing based on body surface area.30

Maintenance therapy

Following consolidation and re-intensification treatment, maintenance therapy to eradicate low-level residual leukaemia is initiated starting with oral 6-mercaptopurine (6MP) 50–75 mg/m2/day and oral or parenteral MTX 20–40 mg/m2 at 1- to 2-week intervals. Although several studies have shown that 50 per cent of patients are cured even if all therapy is truncated at 1 year from diagnosis, it has so far not been possible to identify the patients within each risk group for whom this short treatment is sufficient. Thus maintenance therapy is given until a total duration of therapy of 2–3 years, with the lower-risk groups and boys needing longer therapy. No studies have demonstrated an advantage of extending treatment beyond this duration. During the first year of maintenance therapy (or longer), many study groups give alternate pulses of vincristine–glucocorticosteroid re-inductions, HD-MTX (2–5 g/m2), or other drug combinations (Fig. 12.7). After cessation of therapy, the relapse rate is 10–20 per cent even if therapy has lasted for more than 2–3 years. Thus the intensification of antileukaemic therapy over recent decades has had most impact on the risk of relapse during therapy, whereas the risk of relapse after cessation of treatment has changed little. According to most protocols, the doses of 6MP and MTX during maintenance therapy are targeted to maintain a WBC of 1.5 109 to 3.0–3.5 109/liter or a low neutrophil count. A neutrophil level < 2.0 109/liter during maintenance therapy is associated with a superior cure rate, but whether this reflects treatment intensity or the endogenous bone marrow activity is uncertain.31 A rise in amino transaminase levels is common during MTX–6MP maintenance therapy, but rarely necessitates treatment withdrawal if the liver function parameters (bilirubin and coagulation factors 2, 7, and 10) are normal. A rise in amino transaminase levels primarily reflects the cellular levels of methylated 6MP metabolites, some of which are strong inhibitors of the purine de novo synthesis.32 In addition, the presence of increased amino transaminase levels has been related to a lower risk of relapse.33 Maintenance therapy has been related to a superior antileukaemic effect if oral MTX and 6MP are given in the evening, but this effect has so far not been explained biologically or pharmacologically.34 Although interindividual variations in the pharmacokinetics of 6MP and MTX seem to have significant impact on the chances of cure, it remains to be demonstrated that pharmacokinetically guided dose adjustments will improve the cure rate.31 One of the most important factors that determine the efficacy of 6MP therapy is the genetic polymorphism in the activity of the enzyme thiopurine methyltransferase (TPMT). 6MP primarily exerts its antileukaemic effect through conversion into 6-thioguanine nucleotides (6TGN) that are incorporated into DNA, and TPMT competes with this process by methylating 6MP and some of its metabolites. Because of TPMT mutations, one in 300 patients will be TPMT deficient with very high 6TGN levels and at risk for lifethreatening bone marrow suppression unless treated with only 10–20 per cent of the normal 6MP dose. Eleven per cent of all patients will be heterozygous because of mutations in one of the TPMT alleles, and they will have 50 per cent of the TPMT activity present in wild-type patients. The TPMT-heterozygous patients will experience more bone marrow toxicity when treated with standard 6MP doses, and they will have a reduced risk of relapse but a higher risk of second cancers.31,35,36 Some studies have indicated that substituting 6MP with 6TGN will improve the EFS (although others have failed to confirm this), but 6TGN therapy also seems to induce a high risk of veno-occlusive disease.

For several decades, standardization of therapy by surface-area-based drug dosing was the mantra. Now the scenario is changing with the availability of new surrogate parameters for predicting tumour response by in vitro sensitivity testing,14,26 precise quantification of residual disease in the blood or bone marrow through PCR or flow cytometric measurements,11,21,22,37 determination of pharmacogenetic polymorphism (e.g. cytochrome P- 450, TPMT, and methylenetetrahydrofolate reductase), and options for individual therapeutic drug monitoring.30,31 Today, many collaborative study groups adapt this biologic approach. An example is the NOPHO ALL-2000 protocol (Fig. 12.7), where detailed cytogenetic mapping and in vitro sensitivity studies at diagnosis, pharmacokinetic studies of L-asparaginase, HD-MTX, and thiopurines during consolidation, and re-induction and maintenance therapy combined with PCR or flow-cytometric-based measurements of MRD after 1, 2, and 3 months of therapy allow evaluation of the effect of drug sensitivity and pharmacokinetics on treatment response within well-defined cytogenetic subsets. Such pharmacokinetic–dynamic studies are likely to increase our understanding of treatment failures and to optimize the efficacy of treatment.

Supportive care

Although the impressive improvement in the cure rate of childhood ALL reflects a significant intensification of therapy in recent decades, this has not lead to a parallel increase in the risk of life-threatening complications during therapy.1,38 Only about 2 per cent of patients will die in first remission because of treatment-related complications. This reflects a comparable impressive improvement in the nursing and supportive care of these patients which includes improved treatment of hyperleucocytosis and tumour lysis syndrome, transfusions of blood products, emphasis on adequate nutrition, better imaging of organs for infections or other complications, improved detection of pathogenic microorganisms in tissue, including blood and lungs, and more effective antimicrobial agents (see Chapter 8).

Hyperleucocytosis and tumour lysis syndrome

Tumour lysis syndrome is a life-threatening complication preferentially seen among cases with mature B-cell ALL or WBC >400 109/liter at diagnosis. It is characterized by the triad of hyperkalaemia, hyperphosphataemia, and hyperuricaemia caused by rapid cell lysis, and frequently complicated by hypocalcaemia (with tetany) and renal failure requiring dialysis. It may be present at diagnosis or be precipitated by the introduction of chemotherapy. Thus at diagnosis close monitoring of blood counts, serum electrolytes (not least calcium, potassium, and phosphate), creatinine, uric acid, vital signs including blood pressure, intake/output, and body weight is mandatory. Tumour lysis syndrome should be prevented and treated by intravenous hydration (3000–4500 ml/m2/24 h), allopurinol (10–15 mg/kg/24 h) or recombinant urate oxidase (e.g. rasburicase 0.2 mg/kg i.v. once daily), urine alkalization to pH 7.0–7.5 with systemic sodium bicarbonate or acetazolamide, and forced diuresis with furosemide (1–2 mg/kg single dose) and/or mannitol 15 g/m2 at 6-h interval. Dialysis may be indicated by severe hyperkalaemia, hyperphosphataemia with or without symptomatic hypocalcaemia, and falling urinary output with rising creatinine. Although leucopheresis may substantially reduce a very high WBC, it is not known whether this approach improves the survival of patients at risk of tumour lysis syndrome. Treatment with very reduced doses of glucocorticosteroids (e.g. 5mg/m2/24 h) should be started when the patient is stabilized, urine pH >7.0, and the urinary output is adequate. In rare life-threatening cases of hyperleucocytosis, the diagnostic workup may occasionally be based on samples of peripheral blood, and cytoreductive therapy may be started before the results of the diagnostic studies are available.

Stem cell transplantation

SCT in first, second, or later remission is a relevant treatment option for certain subsets of ALL, but the definition of these subsets is a moving target which reflects the balance of the likelihood of cure by conventional chemo- and radiotherapy and the risk of SCT-induced morbidity and mortality.24 At present, the relatively well-established indications for SCT include late remissions (M2/3 bone marrow after conventional induction therapy), t(9;22)-positive ALL, patients with a very high WBC at diagnosis (> 200 109/liter in the NOPHO ALL-2000 protocol), and patients who relapse within 3 years of diagnosis.16,24In contrast, it is uncertain whether children with 11q23-translocation positive ALL (most of whom are infants) will gain from SCT.15 Patients who are unresponsive to chemotherapy are probably only candidates for SCT as part of clinical trials. The value of SCT in patients with hypodiploid ALL has currently not been established.3

As the prophylaxis of and treatment for graft-versus-host disease (GVHD) improves, SCT using unrelated matched donors has become increasingly relevant.25 Parallel to this development, the donor registries have expanded considerably within the last 5–10 years and now contain HLA information on more than six million potential donors. Since many of the registered donors are of European origin, the chances of finding a good matching donor is especially good for White children, being close to 80–90 per cent. In contrast, there is an unsatisfactory lack of donors from other ethnic groups. Because of the higher availability of unrelated donors and the improvements in tissue-type matching on a genetic level (high-resolution HLA-typing), the percentage of unrelated transplants has increased worldwide, and in many centers unrelated SCT now constitute more than half of the transplants since the benefit of the graft versus leukaemia (GVL) effect outweighs the negative effect of GVHD.

The purpose of SCT is the elimination of the malignant clone by chemotherapy, irradiation, and the alloreactive antileukaemia effect (GVL). Unfortunately, the GVL effect is closely related to donor–recipient incompatibility and, thus to the risk of GVHD. So far attempts to separate the GVL effect and GVHD-induced morbidity have failed. Although T-cell depletion can reduce the risk of acute and chronic GVHD, it increases the risk of graft failure and the relapse rate, and it severely reduces the immune function for a long period. Another alternative is the use of a haplo-identical parent. Although such donors in principle are always available and the graft processing procedures have improved in recent years, there is a significant risk of graft rejection, life-threatening toxicities, and relapse of the leukaemia. Syngeneic HLA-identical twins are not a donor option since they lack both major and minor histocompatability mismatch, and thus also the GVL effect.

Traditionally most transplantations have been performed with bone marrow stem cells. However, in recent years, peripheral stem cells and cord blood stem cells have been used with increasing frequency in children. Bone marrow stem cells cause chronic GVHD less frequently than peripheral stem cells, which on the other hand lead to more rapid engraftment and possibly more GVL effect.

The purposes of the conditioning regimen given in the days before the infusion of stem cells is myeloablation, immunosuppression/ablation to avoid rejection, and reduction of the residual tumour burden. The conditioning regimen most commonly consists of cyclophosphamide (120–200 mg/kg) or VP16 (60 mg/kg) with 12 Gy total body irradiation (TBI) or alternatively busulfan plus cyclophosphamide; the latter is used mainly in children <3–4 years of age because of the risk of TBI-induced leucoencephalopathy. The non-myeloablative conditioning regimens (‘mini-transplantation’), which predominantly provide immunoablation, are still not widely used in childhood leukaemia.

The primary causes of death following SCT are recurrence of the leukaemia, viral, bacterial and fungal infections, GVHD, interstitial pneumonia, and multi-organ failure. Although most long-term survivors lead a normal life, a significant fraction of patients are burdened by a varying degree of late effects including second cancers (not least after TBI), endocrinopathies, precocious (most common in girls) or delayed puberty, infertility (predominately after TBI) due to gonadal damage and uterine fibrosis, reduced lung and cardiac function, cataract in at least 20 per cent of the patients within 5 years of TBI, and leucoencephalopathy, which is most common in young children exposed to TBI.

Treatment failures

When the diagnosis has been established and treatment has been initiated, children with ALL are at risk of four different types of treatment failure.

First, 2–3 per cent of all patients fail to achieve remission following a standard induction regimen. These patients are difficult to identify at diagnosis, but features associated with remission failure include older age, T-cell disease with a high WBC at diagnosis, Ph+ ALL, undifferentiated leukaemia, a poor response to a prednisone prephase, and an M3 bone marrow after 2–3 weeks of therapy. Remission may be achieved for some of these patients by treatment intensification by, for example, cyclophosphamide and vincristine, high-dose Ara-C with VP16, intensive relapse protocols, or other established or experimental drug combinations. If remission is achieved, such patients are candidates for allogenic SCT.

Secondly, 2–3 per cent of all patients will die during induction or in remission, most frequently due to leucostasis, bleeding, or opportunistic infections such as Gram-negative septicaemia and Pneumocystis carinii pneumonia (see Chapter 8).1,38

Thirdly, 1–2 per cent of all patients will develop a second cancer, with a higher risk for those who have received cranial irradiation or epipodophyllotoxins, or who have a TPMT heterozygous phenotype (see section on late effects).

Finally, 15–20 per cent of all children with ALL will develop a bone marrow (15 per cent), CNS (3–5 per cent), testicular (2 per cent), or other focal relapse within the first 5 years after diagnosis. Because of the genetic instability of the cancer cells, subclones may develop with randomly occurring differences in drug sensitivity. The risk of developing chemoresistant subclones is correlated with the tumour burden, and therefore is most likely to occur prior to or during the early phases of therapy. Through the use of anticancer agents, resistant subclones will be selected. The most resistant of these may progress to overt relapse during chemotherapy, whereas the less resistant subclones will recur when therapy is stopped. This supports the clinical observation that recurrent ALL during chemotherapy is generally far more resistant to second-line therapy than relapses that occur after a certain interval from cessation of treatment (Fig. 12.8).

It is unlikely that early morphologic identification of relapses by routine bone marrow or spinal fluid examinations will improve the chances of cure for these patients. However, it remains undetermined whether flow-cytometric- or PCR-based detection of emerging disease reflected by low but increasing MRD levels during treatment or at the end of therapy will allow eradication of such residual disease by intensified chemotherapy.

Previously, the occurrence of relapses more than 5 years after diagnosis was not uncommon, but with modern intensive multidrug chemotherapy such late occurrences are now rare. At relapse, the leukaemia must be explored as extensively as at initial diagnosis, including by morphology and histochemistry, immunophenotyping, karyotyping, and molecular biology. Some biologic features, including the WBC level, are often very similar to the presentation at diagnosis, but cytogenetic evolution is common. Approximately 85–90 per cent of the patients who relapse will achieve a second remission, but less than half of these will be long-term survivors (Fig. 12.8). The prognostic factors at relapse include age <1 year at diagnosis (Fig. 12.8), the duration of first remission, T-cell disease (poor risk), t(12;21) (good risk), extramedullary relapse (good risk), and a low to undetectable MRD level following the first course of relapse therapy (good risk). The outcome is worst for those with T-cell disease (cure rate around 10 per cent) and those who relapse while on therapy, for whom the chance of cure is 20 per cent (Fig. 12.8). The small number of patients who develop a bone marrow relapse within the first year of treatment have an extremely poor outcome (Fig. 12.8). In contrast, patients with B-lineage ALL who relapse >3 years from diagnosis have a chance of cure, which may exceed 50 per cent even without SCT. These late recurrences include patients with intermediate chemosensitivity, who will benefit from intensified second-line therapy, patients with poor compliance to oral maintenance therapy, and patients with a true second ALL that has occurred as a consequence of a second hit among the first-hit ‘preleukaemic’ cells,9 as has been indicated in cases of t(12;21)-positive relapses. Patients with late relapses tend to have a longer duration of their second remission than those with early relapse, and thus require longer follow-up for judgement of their ultimate outcome.

CNS relapse

The CNS is a well-known sanctuary for leukaemic cells and, before the introduction of CNS-directed therapy in the 1960s, as many as 50 per cent of all patients developed a CNS relapse. With modern therapy this will occur in <5 per cent of patients. Half of these will be combined with other sites of relapse, most often bone marrow, but the remainder will be so-called isolated CNS relapses, although many of these will also have subclinical bone marrow involvement. The risk of CNS relapse seems to be increased in patients with T-cell ALL and additional high-risk features (e.g. a high WBC or a mediastinal mass). In addition, the risk of CNS relapse is increased in patients with > 5 106/liter lymphoblasts (CNS-3) at diagnosis, and possibly also in those with a lower number of unequivocal blasts (CNS-2), and this feature has also been associated with traumatic lumbar punctures at diagnosis (> 10 106/liter erythrocytes). Only a few groups have been able to demonstrate an inferior outcome for patients with CNS-2. Early combined CNS and bone marrow relapse is most frequently seen in T-cell disease with a high WBC at initial diagnosis, and is associated with a very poor prognosis. Today, most CNS relapses occur during treatment. Later on, CNS recurrences are rare and are most commonly combined with bone marrow relapse. Generally, patients with a CNS relapse should receive craniospinal irradiation, but the radiation dose must be reduced according to CNS irradiation given previously. In addition, all patients must receive intrathecal as well as systemic chemotherapy, which should be at least as intensive as the first-line treatment and should include high-dose therapy. Whether or not to offer SCT will depend on the time of relapse and whether or not it is combined with bone marrow disease.

Fig. 12.8 Second EFS curves for patients diagnosed between 1992 and 2001 who developed a relapse. Overall the prognosis is very poor for infants and children with T-cell ALL. The prognosis for B-cell precursor ALL is strongly correlated to the duration of first remission.

Testicular relapse

A testicular relapse presents as a non-tender swelling of one or both testes. After the introduction of intensive systemic chemotherapy, and not least HD-MTX, testicular relapses have become rare and now occur in <3 per cent of boys. The testes are probably a pharmacologic sanctuary from where leukaemic cells may seed. Accordingly, patients with combined bone marrow and testicular relapse do better than those with isolated bone marrow relapse, since the former is believed to reflect seeding of chemosensitive leukaemic cells from the testicular sanctuary.39 In clinically unilateral disease, the affected testis may be removed and the other testis irradiated, whereas in bilateral testicular relapse both testes are irradiated. In addition, intensive systemic chemotherapy is required.

Other sites

More rarely, the recurrence of ALL is located in a lymph node, skin, eye, ovary, kidney, or bone. No standard approaches for these leukaemias exist, but they should always receive intensive systemic chemotherapy followed by oral antimetabolite-based maintenance therapy. Whether or not to offer SCT will depend on the time of their recurrence.

Special subsets

Certain subsets of ALL need special consideration. Some have a very poor prognosis because of inherent chemoresistance and may be eligible for SCT in first remission. Others have a more favourable outcome as long as certain therapeutic precautions are taken (see also the section on cytogenetics).

Down syndrome

Nearly all cancers in children with Down syndrome are leukaemias.40 Interestingly, the leukaemic clone in >50 per cent of non-Down ALL patients also harbours trisomy 21.13 Translocation t(12;21)(p13;q22) and high hyperdiploidy (with trisomies 4, 10, and 17) seem to be less common in ALL patients with Down syndrome, but their cure rate is equal to that of non-Down patients without these favourable prognostic genetic aberrations.41 Leukaemic cells from Down and non-Down patients do not differ in their in vitro sensitivity to the most commonly used anticancer drugs. Their risk of severe toxicity following MTX and APA-C therapy is significantly increased, probably because of drug disposition gene dose effects and a propensity for apoptosis.

Infant ALL

It is likely that infant ALL with translocations involving 11q23 reflect in utero exposure to carcinogens that interact with the topoisomerase II enzyme.42 Children aged <1yearat diagnosis are a therapeutic challenge because of their high frequency of adverse clinical features such as hyperleucocytosis and CNS leukaemia at diagnosis. They rarely have a T-cell phenotype, harbour translocations t(9;22) or t(2;21), or have a high-hyperdiploid karyotype. Two-thirds carry chromosomal aberrations that involve the MLL gene at chromosome region 11q23, most frequently t(4;11), have a CD10–N3 meaning negative immunophenotype, and carry myeloid markers.15 They are inherently drug resistant, most significantly to prednisolone and L-asparaginase, but have a greater sensitivity to Ara-C compared with children aged 1.5–10 years.43 It is frequently recommended that doses of anticancer agents to infants should be reduced and calculated by body weight rather than body surface area (30 kgL4 1m 2). However, a relationship between these dosing principles and systemic drug exposure has not generally been supported by clinical pharmacokinetic studies. The infant leukaemias require intensive ALL-like treatment protocols with inclusion of high-dose Ara-C. It is doubtful whether SCT improves the clinical outcome, at least for the 11q23 aberrant leukaemias.15 Overall, the prognosis of 11q23 aberrant patients is poor with a cure rate less than 30 per cent (Fig. 12.6), and the outcome is even worse in those aged <6 months. Nearly all relapses occur within 2 years of diagnosis, and the chance of cure once the disease has recurred is poor.

Mature B-cell ALL

This subset, which includes 2 per cent of children with ALL, is characterized by a male predominance (3:1), a median age of 10 years, a median WBC that is similar to that in non-B ALL (although WBC > 100 109/liter is rare), frequent renal involvement, only moderate hepatosplenomegaly, a normal haemoglobin, and FAB L3 morphology in a less hyperplastic bone marrow compared with what is usually encountered with B-cell precursor ALL. In the vast majority, c-myc on chromosome 8 is fused with antigen receptor regulatory sequences either on chromosome 14 in 80 per cent of cases [t(8;14)(q24;q32.3)] or, more rarely, on chromosomes 2 or 22 which leads to a deregulated increased cell proliferation. Previously, mature B-cell ALL (as well as disseminated and/or bulky mature B-cell non-Hodgkin lymphoma) had a very poor outcome, but these patients now have a cure rate of at least 80 per cent with 5–6 months of intensive block therapy that frequently includes HD-MTX, cytarabine, cyclophosphamide, epipodophyllotoxins, glucocorticosteroids, vincristine, and i.t. triple chemotherapy (see Chapter 14). The chance of cure is substantially lower for patients with CNS involvement at diagnosis than for those without this feature. Nearly all relapses occur within the first year of diagnosis, and the outcome after relapse is dismal.

T-lineage ALL

Cases of T-lineage leukaemia differ from B-cell precursor ALL by a higher WBC at diagnosis (median 70.0 109 versus 8.0 109/liter), predominately male sex (2.4:1), a mediastinal mass (62.1 versus 1.1 per cent), and CNS leukaemia at diagnosis (8 versus 2 per cent). Although the overall risk of relapse for these patients is significantly higher than that of B-precursor ALL, especially during the first 2 years of diagnosis, the impact of WBC at diagnosis on the cure rate is less pronounced than for B-cell precursor ALL (Fig. 12.3). The worse overall outcome primarily reflects the higher frequency of the risk factors discussed earlier. Thus, if adjustment is made for these and intensive therapy for T-lineage leukaemia is applied, the cure rates are very similar to those of B-cell precursor ALL (Fig. 12.3). Compared with B-lineage cell ALL, T-cell disease seems to have a poorer sensitivity to many anticancer drugs and have less propensity for the formation of MTX polyglutamates. HD-MTX 5-8 g/m2/24 h to overcome this feature and prolonged use of L-asparaginase seem to improve the outcome of these patients.

Late effects

Even when cure has been obtained, many patients need medical attention for late effects imposed by the leukaemia or its treatment. Those that affect the CNS after CNS irradiation are among the most troublesome and include endocrine disturbances, neuropsychologic deficits (seemingly most prominent in girls), and obesity. Growth hormone replacement may be indicated because of the risk of short stature, and there is no convincing evidence that this treatment increases the risk of relapse. In addition, patients cured of ALL may be at risk of reduced fertility (mostly because of alkylating agents or SCT with TBI), cardiomyopathy (due to high doses of anthracyclines, especially in girls), reduced lung function, osteoporosis, and second cancer. The last of these affects 2–3 per cent of all patients. The most common second cancers are brain tumours, generally related to previous CNS irradiation, especially at a young age, myelodysplastic syndrome, or overt myeloid leukaemia. The risk of second cancers is increased manyfold in children who have undergone SCTwith TBI and high doses of alkylating agents. The risks of myelodysplastic syndrome or AML have also been related to intensive exposure to epipodophyllotoxins (the second cancers commonly harbouring 11q23 aberrations) or thiopurine metabolites in patients with TPMT heterozygocity (with a high frequency of monosomy 7 myelodysplastic syndrome or AML).36 The risk of brain tumours has also been related to a combination of TPMT heterozygocity and CNS irradiation.35 So far there are no indications of an increased risk of cancers among children of patients cured of childhood ALL.

Palliative chemotherapy

Despite intensive first- and second-line therapy, including allogenic SCT, at least 15 per cent of the patients will eventually die of their disease.1 These will include patients with B-lineage leukaemia with recurrent late relapses, for whom the disease continues to have some chemosensitivity but still fails to be eradicated. There is no standard approach for these patients, many of whom will have received SCT, and for each case the decision to treat or not to treat demands detailed knowledge of the disease, the previous treatment, the patient, and the family as a whole. For some, a second allogenic SCTwith the goal of cure may be a relevant consideration. For others, only palliative care seems reasonable. In the latter case, palliative chemotherapy should not steal the time that is left; in other words, the short survival time gained by intensive treatment should not equal the time spent in hospital. If chemotherapy is chosen, regimens of low toxicity that are applicable on an outpatient basis should be the choice. This may include oral MTX and 6MP/6TG. In selected cases this can be supplemented with vincristine–prednisolone re-inductions, PEG-asparaginase for non-allergic patients, oral cyclophosphamide, and/or even HD-MTX. Some patients with CNS disease may benefit from palliative intrathecal chemotherapy. When to stop all chemotherapy, and even blood transfusions, can only be decided on an individual basis. Both patients and their families will always need other medical and psychosocial support to be able to stay at home, where most parents and patients will choose that the child should die.

Future considerations

In developed countries the cure rate for some subsets of childhood ALL exceeds 90 per cent. Thus it is likely that future therapy programmes will include a reduction of the treatment intensity for patients with favourable cytogenetic aberrations (e.g. high-hyperdiploid ALL), high in vitro drug sensitivity, favourable drug metabolism profiles, and/or a low level of residual leukaemia within the first months of therapy.9,10,11,21,22,26,30,31 Clinical studies in the 1960s and 1970s showed that a certain proportion of patients can be cured with treatment of very low toxicity (vincristine, steroids, asparaginase, and antimetabolites), and the aforementioned characteristics of good-risk patients indicate that many of these can be identified at diagnosis or during the first months of therapy. In contrast, some patients with adverse cytogenetic findings, high in vitro or in vivo drug resistance, or who relapse during therapy have such a disappointing prognosis that novel approaches are warranted using new drugs, immune therapy, new SCT procedures, and molecular therapy targeted towards tumour suppressor genes or chimeric proteins generated by chromosomal translocations. To explore the toxicity and efficacy of such approaches extensive multicenter collaboration is warranted, such as that established by the US Children's Oncology Group and the European Consortium for the Cure of Childhood Acute Lymphoblastic Leukaemia.

In addition, there is a global need for the development of easily applied tools for risk group assignment and inexpensive effective chemotherapy that could increase the cure rate of the 80 per cent of children with ALL who live in countries with limited resources.

The attempts to unravel the aetiology and pathogenesis, most notably those headed by Mel Greaves, have yielded promising results in recent years.6 Ongoing large-scale studies of the frequency of leukaemia-related cytogenetic aberrations among healthy newborns, and the correlation of these results with certain characteristics of their mothers and their in utero exposures, as well as epidemiologic studies of factors that influence the disappearance or control of such preleukaemic cells during the first years of life, may within the next 10 years suggest measures by which the occurrence of the most common forms of childhood ALL may be prevented or detected at an early and more easily treated stage.

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