Robert F. Gagel
Ana O. Hoff
Gilbert J. Cote
Medullary thyroid carcinoma (MTC) is an uncommon malignant tumor derived from the calcitonin-producing cells (C cells) of the thyroid gland. It is notable for being one of the last forms of thyroid carcinoma to be identified. The first description by Hazard et al in 1959 separated this form of aggressive thyroid carcinoma from other poorly differentiated or anaplastic forms of thyroid carcinoma (1). This description of MTC presaged two periods of intense study over a period of almost a half century that has brought our understanding of the pathogenesis of this unusual thyroid carcinoma to a remarkable level.
The first period, during the 1960s and early 1970s, was initiated by a report from Sipple (2) describing the association of MTC and pheochromocytoma in a single patient and several other examples from the literature, an association we now know as multiple endocrine neoplasia type 2A (MEN2A). Although the thyroid carcinoma was not identified as MTC, others (3, 4, 5) quickly pieced together the association of MTC and pheochromocytoma and correctly hypothesized that MTC was derived from the parafollicular or C cells, a cell type that produced the newly discovered peptide, calcitonin (6, 7, 8). During the next 3 to 5 years this hypothesis was confirmed by the demonstration that the carcinomas produce calcitonin (9,10) and that measurement of serum calcitonin could be used to diagnose MTC (11).
The second intense period of discovery followed the mapping of the gene for MEN2 and the discovery that mutations of the protooncogene (12,13), a tyrosine kinase (TK) receptor (14), were present in the germ line of patients with MEN2 and also as somatic mutations in approximately 25% of sporadic MTCs (15). The recognition of mutations as the major cause of MTC led to the identification of all the major components of the receptor system, an understanding of the role of the receptor system in the development of the sympathetic nervous system, and the beginnings of an understanding of how mutations of cause cell transformation. More importantly, these discoveries created a new paradigm for management of patients with genetic tumor syndromes: identification of gene carriers and removal of the organ containing cells at risk for transformation early in life.RETRETRETRET
The remainder of this chapter chronicles these events and, more importantly, attempts to place the discoveries into a context of clinical usefulness.
THE THYROIDAL C CELL
C cells are neuroendocrine cells that constitute less than 1% of the cells in the thyroid gland. They are distinct and separable from the more ubiquitous thyroid follicular cells. The precursors of the C cells originate in the neural crest and migrate to the ultimobranchial body, a discrete and separable entity in the neck, where they differentiate into C cells. The ultimobranchial body is the repository of C cells in birds and fish (16,17), whereas in mammals these cells migrate into the thyroid gland during development. There they occupy a characteristic central location at the junction of the upper one third and lower two thirds of each lobe. This developmental pattern explains the characteristic location of hereditary MTC (Fig. 71.1).
FIGURE 71.1. Distribution of C cells in the thyroid gland. In the normal human thyroid gland, the concentration of C cells is highest at the junction of the upper one third and lower two thirds of the gland, and this is where most medullary carcinomas of the thyroid () are located. Hereditary medullary carcinoma is almost always bilateral, although the extent of involvement may be asymmetric. Sporadic medullary carcinoma is usually unilateral, and may occur anywhere in the thyroid gland. (Adapted from Grauer A, Friedhelm R, Gagel RF. Changing concepts in the management of hereditary and sporadic medullary thyroid carcinoma. 1990;19:3, with permission.)A:MCTB:C:Endocrinol Metab Clin North Am
C cells are neuroendocrine cells with quasineuronal properties (uptake of biogenic amines and neuronal features when placed in tissue culture) combined with endocrine secretory peptide production analogous to adrenal chromaffin, pancreatic or gastrointestinal neuroendocrine, and certain pituitary cells. C cells secrete calcitonin and other small peptides (somatostatin) directly into blood vessels. In the past, members of this class of neuroendocrine cells were called APUD cells (amine precursor uptake and decarboxylation) (18), but this term has fallen out of favor as the complexity and differences between members of this group of cells have been elucidated.
Unique Features of C Cells
C cells, be they located in the ultimobranchial body (birds and fish) or distributed throughout the thyroid gland (mammals), are characterized by the production of calcitonin. This small 32–amino acid peptide lowers serum calcium concentrations when injected into rodents (6,8). This effect is mediated through binding of calcitonin to a specific calcitonin receptor on osteoclasts (19), causing them to retract and cease bone reso ption (20). Recent studies suggest a more complicated role for calcitonin in bone remodeling. Deletion of the calcitonin/calcitonin gene–related peptide (CT/CGRP) gene in mice leads to a higher bone mass (21), as does deletion of the calcitonin receptor (22). Thus, disruption of the calcitonin signaling pathway leads to increased bone formation, suggesting that calcitonin may inhibit bone formation. The major physiologic effects of calcitonin appear to be short-term inhibition of bone resorption, perhaps to protect against hypercalcemia, and a more subtle and potentially more important long-term effect to regulate bone formation.
Although calcitonin is produced in multiple neuroendocrine cell types in mammals [neuroendocrine cells in the adrenal, pancreas, lung, prostate, and other tissues, and in any cell type during sepsis (23,24)], under normal physiologic conditions the C cells of the thyroid are the predominant source. Removal of the thyroid gland in mammals lowers serum calcitonin concentrations to nearly undetectable levels. Non–C cell production of calcitonin during infection or inflammation may explain the persistent elevations of serum calcitonin concentrations after thyroidectomy in some patients with MTC (25).
The CT/CGRP gene () contains 6 exons (Fig. 71.2). Alternative RNA processing of the primary transcript by inclusion of exons 1 through 4 with polyadenylation following exon 4 produces a messenger RNA (mRNA) that encodes the precursor for calcitonin; processing the same transcript to exclude exon 4 produces an mRNA that encodes the precursor for CGRP. The splice signals surrounding exon 4 are weak, particularly an atypical pyrimidine branch point instead of the usual adenine, making inclusion of exon 4 in the final transcript an unfavored event. This suggests that exon 4 exclusion is the default pathway. Therefore, the inclusion of this exon to produce calcitonin requires a mechanism that enhances recognition of the splicing and polyadenylation sequences and is ubiquitous. Selective interaction of transacting factors with repeating sequences within exon 4 by a -like protein and Srp55 (26) increases recognition of the splice site, whereas a large complex of factors bound to a novel intronic sequence downstream of the exon 4 polyadenylation signal increases polyadenylation and simultaneously inhibits splicing to the CGRP exon (27,28). This ultimately leads to selective inclusion of exon 4 in C cells and production of calcitonin. The mechanisms involved in the production of CGRP are less well studied, but are thought to involve the disruption of enhancer complexes (28,29). In MTC there is disordered splicing that causes production of a higher percentage of CGRP; at present there is no evidence that the disordered splicing plays a pathogenic role in transformation of the C cells.CALCAtra
FIGURE 71.2. Alternative splicing of the calcitonin/CGRP () gene. Transcripts derived from the six-exon gene are alternatively spliced to produce messenger RNAs (mRNAs) encoding either calcitonin (CT) or calcitonin gene-related peptide (CGRP). In thyroid C cells, CT is produced by RNA splicing that includes exon 4; in neuronal cells, CGRP is produced by skipping this exon. Studies of the rat and human genes have identified five key sequences that are involved in recognition of exon 4. These sequences include a pyrimidine branch point, which creates a “weak” 3′ splice site, a double-sex repeat element (DSX), an exonic splicing silencer (ESS), an exonic splicing enhancer (ESE), and an intronic splicing enhancer (ISE). The production of calcitonin mRNA requires facilitated recognition of exon 4. The binding of tra2b to the DSX and SRp55 to the ESE is thought to facilitate recognition of the 3′ splice site by splicing factors. The downstream ISE element forms a large complex that binds several factors. The binding of polypyrimidine-binding protein (PTB), SR20, and U1 snRNP is thought to stimulate polyadenylation, whereas binding of U6 snRNP and TIAR is thought to inhibit recognition of the exon 5 splice site. The production of CGRP requires the blocking or disruption of enhancer complexes. A brain-specific candidate splicing repressor (CSR) has been demonstrated to bind the ESS sequence and to prevent the binding of tra2b and SRp55, as discussed in the text. In addition, formation of the ISE complex is disrupted in neural cells due to the presence of a neural-specific PTB (nPTB) and reduced levels of SR20. In the absence of facilitated recognition, exon 4 skipping is thought to be the default pathway.CALCAA:CALCAB:
Calcitonin gene expression is increased by activators of protein kinase A (30), protein kinase C (31), and mitogen-activated protein (MAP) kinases (32). It is inhibited by 1,25-dihydroxyvitamin D, retinoic acid analogues, nerve growth factor, and 5-hydroxytryptamine agonists (33, 34, 35, 36, 37). Glucocorticoids have complex effects on transcription (38, 39, 40). Tissue-specific expression of the calcitonin gene is regulated by two widely separated enhancer regions. The first (distal enhancer) is located 1000 nucleotides (41,42), and the second (proximal enhancer) approximately 250 nucleotides (30) upstream of the transcription start site. The distal enhancer is composed of three functional CANNTG or E-box motifs that bind helix-loop-helix (HLH) transcription factors (41). Members of the USF-1 and USF-2 HLH family of proteins, in combination with other undefined proteins, constitute the major transcription complex regulating the distal enhancer and likely confer tissue specificity (43). The proximal site contains cyclic adenosine monophosphate (cyclic AMP) response elements (CREs) and an overlapping octamer homeodomain-like binding site (CREL/O). These sequences mediate the cyclic AMP/ protein kinase A responsiveness of cells (30). MAP kinase activation of CT/CGRP gene transcription is mediated through a Ras interacting site near the CRE that binds a novel and as yet unidentified zinc-fingered protein (32). The active metabolite of vitamin D, 1,25-dihydroxyvitamin D, inhibits transcription of the calcitonin gene by interfering with the interaction between the transcription complexes that occupy the upstream and downstream enhancers (34).
Calcitonin secretion is regulated primarily by the extracellular calcium concentration. An increase in serum calcium concentration above normal activates the calcium-sensing receptor (CaSR) (44,45), a G protein–coupled receptor that facilitates increased calcitonin secretion. Binding of calcitonin to its receptor on the osteoclast inhibits bone resorption (46). The resulting reduction in serum calcium concentration leads to lowered calcitonin secretion. Other stimuli that increase calcitonin secretion include supraphysiologic concentrations of glucagons, 5-hydroxytryptamine receptor agonists, gastrin (and pentagastrin), alcohol, and exercise (47, 48, 49, 50, 51). A detailed discussion of these stimuli has been published (52).
The recognition that more than 90% of calcitonin is derived from the thyroid (except during periods of inflammation or sepsis) and that serum calcitonin concentrations are high in patients with MTC has made it a useful tumor marker. The sensitivity of calcitonin measurements as an indicator of MTC was further increased by measurements after administration of calcium (11) or pentagastrin (49), and, at a time when serum calcitonin assays lacked sensitivity, these stimulation tests formed the basis for early identification of tumor in patients with hereditary MTC. Serum calcitonin measurements are used less frequently for early diagnosis of MTC today, largely replaced by genetic tests for diagnosis of hereditary MTC, although some have recommended that serum calcitonin be measured in all patients with thyroid nodules (see below and Chapter 73).
Serum calcitonin measurements are also used to follow patients with residual or metastatic MTC. Measurements over extended periods of time are useful for quantitation of tumor mass; there is an almost direct correlation between serum calcitonin concentrations and tumor volume (Fig. 71.3). There are several points to keep in mind regarding serum calcitonin measurements. The first is that in normal subjects (or patients with some normal thyroid tissue) calcitonin secretion may be episodic; it also may be affected by the ambient serum calcium concentration, exercise, eating (gastrin stimulation), or other poorly defined stimuli. Serum calcitonin concentrations may vary by two- or threefold, and therefore direct comparison of any two serum calcitonin measurements provides little useful information. A second consideration, although infrequent, is that more aggressive and dedifferentiated MTCs may lose the ability to transcribe the CT/CGRP gene, and therefore a decreasing serum calcitonin concentration value may be an indicator of a poor prognosis (53). Another pragmatic observation: patients with MTC who have serum calcitonin concentrations greater than 5,000 pg/mL (normal 2–30 pg/mL in most assays) after total thyroidectomy and lymph node dissection usually have metastatic disease outside the neck. In such patients, neck tumors can usually be detected by ultrasonography, and mediastinal, pulmonary, and abdominal disease by computed tomography. In patients in whom these radiographic studies are negative, the most likely site of metastasis is the liver, in which microscopic metastases are difficult to detect.
FIGURE 71.3. Measurements of serum calcitonin (CT) and carcinoembryonic antigen (CEA) in a patient with progressive medullary thyroid carcinoma (MTC) showing a progressive increase in serum calcitonin and carcinoembryonic antigen concentrations. Note the point-to-point variability in the values, although the trend over the 6 years of observation is clear. The patient died from hepatic failure caused by metastatic MTC shortly after the last measurements.
Carcinoembryonic antigen (CEA) is another useful tumor marker for monitoring the growth of MTC (54,55) (Fig. 71.3). There is less day-to-day variation in serum CEA concentrations than in serum calcitonin concentrations. However, CEA is made throughout the gastrointestinal tract and in the liver, and may be secreted in excess in cigarette smokers and in patients with other tumors, making it an insensitive and inaccurate marker for detection of early MTC.
Other substances produced by normal C cells that may be produced in excess by MTC include somatostatin, histaminase, and chromogranin A (56, 57, 58), but none are specific for MTC. Other substances produced by occasional MTCs are propopiomelanocortin (POMC), vasoactive intestinal peptide (VIP), neurotensin, and bombesin (59, 60, 61). Approximately 5% of patients with the ectopic corticotropin (ACTH) syndrome have MTC (62).
HISTOLOGIC FEATURES OF MEDULLARY THYROID CARCINOMA
Medullary thyroid carcinoma develops as a firm, white, almost chalky tumor within the thyroid gland. The transformed C cells are usually polyhedral or polygonal in shape and may be arranged in a variety of patterns (1,63) (see Chapter 21). Amyloid, dispersed between pockets of tumor cells, is commonly seen in slowly growing tumors (64), whereas cellular components predominate in more rapidly growing tumors. Tumor calcification is common and may be detected by x-ray.
The derivation of MTC from C cells provides its most characteristic feature, the presence of calcitonin, which can be detected by immunohistochemical staining (65). The combination of amyloid and immunohistochemical staining for calcitonin are the most characteristic features of MTC. Rare, poorly differentiated MTCs lose the ability to produce calcitonin, a finding that generally indicates a poor prognosis. Other tumors that produce calcitonin and should be considered in the differential diagnosis in a patient found to have a high serum calcitonin concentration include breast, prostate, and small cell lung carcinomas and other neuroendocrine tumors, including pheochromocytomas, islet cell carcinomas, and carcinoid tumors (65). The diagnosis of MTC can be made easily by fine-needle aspiration biopsy, based on immunohistochemical staining for calcitonin and the presence of amyloid (66).
CLINICAL PRESENTATION OF MEDULLARY THYROID CARCINOMA
The most common presentation of MTC is as a single or multiple thyroid nodules, with or without palpable lymph nodes. This tumor may also present in the context of a known kindred with hereditary MTC. In most such patients, early screening by genetic testing will result in a thyroidectomy before the presence of any detectable disease. Rare patients with hereditary MTC may present with clinical features of a pheochromocytoma (MEN2A or MEN2B), Hirschsprung's disease with intestinal obstruction or pseudoobstruction (MEN2A or MEN2B), or hyperparathyroidism (MEN2A). A few patients, generally with widespread disease and hepatic metastases, may present with diarrhea (67, 68, 69). Perhaps the most unusual example was a patient with breast cancer who had a high serum CEA concentration for years, and was presumed to have metastatic breast cancer, who then presented with diarrhea and was belatedly discovered to have MTC (70).
CLINICAL SYNDROMES ASSOCIATED WITH MEDULLARY THYROID CARCINOMA
Sporadic Medullary Carcinoma
Sporadic MTC is the most common form of this tumor. It arises, , as a result of a somatic mutation of the protooncogene in a single C cell or by other undefined mechanisms. Sporadic MTC is most commonly unicentric, but a small percentage are multicentric. Six percent to 10% of patients with apparently sporadic MTC are subsequently found to have a germline mutation indicative of hereditary disease (discussed in detail below) (15).de novoRETRET
Most patients with sporadic MTC present with a thyroid nodule. MTCs have no distinguishing ultrasound features, and they do not concentrate iodide, and therefore they are hypofunctioning on 131I, 123I, or 99mTcO4 imaging. The diagnosis is most commonly made by fine-needle aspiration biopsy and confirmed by measurement of serum calcitonin. The cytologic diagnosis of MTC may be difficult. MTCs are rare and may be diagnosed as a parathyroid tumor, poorly differentiated carcinoma of unknown etiology, or rarely anaplastic carcinoma. Any biopsy that reveals cells that are not obviously of thyroid follicular origin should be immunostained for calcitonin, CEA, or chromogranin A.
At the time of diagnosis, 80% of patients with an MTC greater than 1 cm in diameter have lymph node metastases (71, 72, 73). Lymph node metastases are often not apparent to the surgeon, and may be missed by the pathologist unless each node removed is carefully examined. The most common pattern of metastasis is to ipsilateral lymph nodes in levels II through VI (see Chapter 17); metastases to contralateral nodes occur in approximately 40% of patients in whom the primary MTC is palpable (71,72). Another common pattern of metastasis is to mediastinal lymph nodes. Hepatic and pulmonary parenchymal metastases are commonly vascular. Clinicians should be aware that even experienced radiologists often report a small focus of hepatic metastasis, as defined by contrast-enhanced computed tomography, as a hemangioma. Another pattern of metastasis is miliary spread to the liver. This pattern is difficult to identify by any currently available imaging technique; it is usually identified by laparoscopic biopsy (74). Bone metastases are most commonly lytic; spinal metastases are uncommon, but of particular concern because of the potential for neurologic deficit.
Hereditary Medullary Carcinoma
Sipple's description in 1961 provided the first compilation of the association between thyroid carcinoma and pheochromocytoma (now known as MEN2A) (2,75). The subsequent separation of MEN1 from MEN2 (76) and the separation of MEN2A and MEN2B (77, 78, 79, 80, 81) provided further clarification of the distinctions between these clinical syndromes. The addition of several variants: familial MTC (without other manifestations of MEN2A) (82), MEN2A with Hirschsprung's disease (83), and MEN2A with cutaneous lichen amyloidosis (84,85) have added further complexity to the classification system. The discovery of mutations and insight gained from the correlation of clinical phenotype with specific molecular changes has led to the current classification system (86) (Table 71.1).RET
TABLE 71.1. HEREDITARY MEDULLARY THYROID CARCINOMA
Multiple endocrine neoplasia type 2 (MEN2)
Multiple endocrine neoplasia type 2A (MEN2A or Sipple's syndrome)
Medullary thyroid carcinoma
Variants of MEN2A
Familial medullary thyroid carcinoma (FMTC)
MEN2A with cutaneous lichen amyloidosis (MEN2A/CLA)
MEN2A with Hirschsprung's disease
Multiple endocrine neoplasia type 2B
Medullary thyroid carcinoma
Absence of parathyroid disease
Marfanoid habitus and tall stature
Multiple Endocrine Neoplasia Type 2A
The association of MTC, pheochromocytoma, and hyperparathyroidism is classified as MEN2A (Table 71.1). The complete syndrome develops most often in the third and fourth decades of life. It is characterized by bilateral and multicentric MTC in more than 90% of gene carriers, unilateral or bilateral multicentric pheochromocytomas in approximately 50%, and hyperparathyroidism in 5% to 10%. MTC is nearly always the first manifestation of the syndrome. Before this syndrome was recognized, death was as likely to be sudden, caused by a pheochromocytoma, as by metastatic thyroid carcinoma.
The recognition of the clinical syndrome and its genetic nature has led to its earlier recognition in affected families. It is transmitted as an autosomal-dominant trait. The fact that both MTC and pheochromocytoma may occur late in the second decade or later ensured transmittal of this gene to the next generation and the existence of multigenerational kindreds; death from either MTC or pheochromocytoma most commonly occurred in the mid-fifties. Because the average life expectancy at the beginning of the twentieth century was 50 to 55 years, it is not surprising that the hereditary nature of this syndrome escaped detection then.
After or concurrent with Sipple's description in 1961, there followed the discovery of calcitonin (6,8), followed by recognition that it is produced by the parafollicular cells of the thyroid, that MTC is composed of transformed C cells (87), and that calcium and peptides such as glucagon and gastrin (or pentagastrin) stimulate release of calcitonin from C cells (49,88,89,90). These observations led to the use of calcium or pentagastrin in conjunction with measurements of serum calcitonin to define the spectrum of C-cell abnormalities in MTC.
The Natural History of Medullary Carcinoma in Multiple Endocrine Neoplasia Type 2A
The earliest histologic abnormality in thyroid glands removed from children and young adults with MEN2A is one or more foci of C-cell hyperplasia (91) (Fig. 71.4). These foci become nodular, so named because the growing mass of C cells appears to displace a thyroid follicle, creating the appearance of a nodule (65), then appear as microscopic MTC, and finally as a visible focus of MTC. Although the progression time is unclear, foci of microscopic MTC have been observed in children as young as 3 years of age, implying progression from normal to carcinoma in a period of less than 4 years. The variability of the latent period for development of MTC (4–50 years) suggests that the presence of a mutant TK receptor is necessary but not sufficient for transformation.RET
FIGURE 71.4. Progression of transformation in hereditary medullary thyroid carcinoma. A histologic section from a 3-year-old child with a germ line codon 634 mutation. The C cells were immunostained for calcitonin. The section shows a slight increase in number of C cells, which are clustered around the thyroid follicles. Nodular C-cell hyperplasia. In this section the C cells displace the thyroid follicle, creating the appearance of a nodule. A microscopic focus of medullary thyroid carcinoma, which obliterates the thyroid follicles.A:B:C:
When metastasis occurs in the evolution of MEN2A is not known. The youngest reported patient with metastatic disease to local lymph nodes was a 6-year-old child (92,93), and unpublished reports describe metastatic MTC in 3-year-old children. Also unknown is when metastases progress beyond local lymph nodes, but it is known that metastases may be confined to local lymph nodes. Reports from the early 1970s and more recently described MEN2A patients with MTC and local lymph node metastases who underwent extensive lymph node dissection and remained free of disease for up to 30 years thereafter (73,94,95,96,97,98). These results provide a compelling argument for a thorough lymph node dissection at the time of initial surgery.
Pheochromocytoma in Multiple Endocrine Neoplasia Type 2A
Approximately one half of patients with MEN2A develop clinical evidence of a pheochromocytoma. These tumors evolve from adrenal medullary hyperplasia and may be multicentric. A detailed discussion of pheochromocytoma in MEN2 is beyond the scope of this chapter, and the reader is referred to other reviews (99, 100, 101, 102). Several points are of relevance to the management of patients with MEN2A in this context. First, all patients should be screened for pheochromocytoma before undergoing thyroid surgery. Measurements of plasma or urinary metanephrines and catecholamines are sufficient for screening purposes. More often the question of pheochromocytoma is raised in the context of a planned thyroid surgical procedure, when the patient or surgeon opposes delay. In this situation, demonstration of normal adrenal glands by computed tomography will suffice, because in 99% of patients with MEN2A who have a pheochromocytoma the tumor is intraadrenal (the remainder are intraabdominal). If a pheochromocytoma is identified, the thyroid surgical procedure should be deferred until the pheochromocytoma is removed. The absence of hypertension does not exclude a MEN2-related pheochromocytoma. Unlike the clinical picture in sporadic pheochromocytoma or other hereditary pheochromocytoma syndromes (von Hippel-Lindau disease or hereditary paraganglioma), the disproportionate production of epinephrine associated with MEN2-related pheochromocytomas is more likely to cause palpitations than hypertension (Fig. 71.5).
FIGURE 71.5. Plasma concentrations of normetanephrine, norepinephrine, metanephrine, and epinephrine and urinary excretion of norepinephrine, epinephrine, metanephrines, and vanillylmandelic acid . The values are expressed as percentages of the upper reference limit for each test. Data on individual patients are shown for three groups of patients with von Hippel-Lindau disease and multiple endocrine neoplasia type 2 (MEN2), as follows: patients with von Hippel-Lindau disease or MEN2 in whom a pheochromocytoma was ruled out on the basis of normal computed tomography (CT-negative), patients with von Hippel-Lindau disease who had histologically verified pheochromocytomas (VHL), and patients with MEN2 who had histologically verified pheochromocytomas (MEN2). The values for patients with pheochromocytoma were determined when the tumors were first identified by computed tomography. The dotted horizontal line represents the upper reference limit for each test. The y-axes are logarithmic. (Data from Eisenhofer G, Lenders JWM, Linehan WM, et al. Plasma normetanephrine and metanephrine for detecting pheochromocytoma in von Hippel-Lindau disease and multiple endocrine neoplasia type 2. 1999;340:1872.)(top)(bottom)N Engl J Med
Primary hyperparathyroidism is an uncommon manifestation of MEN2A, occurring in 5% to 10% of patients. Serum calcium should be measured preoperatively in patients with MTC, and the possibility of MEN2 should be considered in all patients with primary hyperparathyroidism.
Variants of Multiple Endocrine Neoplasia Type 2A
In addition to classic MEN2A, there are three distinct variants of MEN2A (Table 71.1). The first, familial MTC (82), is characterized by the presence of MTC without other manifestations of MEN2A. The reports describing this variant have usually encompassed three or four generations with 20 to 30 affected members with no evidence of pheochromocytoma or hyperparathyroidism. In these families, MTC tends to develop later and be less aggressive, as compared with MEN2A families. Before categorizing a particular kindred as having familial MTC, it is important to consider the fact that the penetrance of pheochromocytoma in MEN2A is only 50%. In a small kindred with predominantly young members, in whom pheochromocytomas may not have yet developed, miscategorization is possible. The important point is that screening for pheochromocytoma should continue in affected members unless there is multigenerational experience to exclude this possibility.
MEN2A with Hirschsprung's disease is also an uncommon variant in which affected family members develop Hirschsprung's disease (83,103). The penetrance varies considerably; in some large kindreds a high percentage have the disease, whereas in other equally large kindreds only one or two family members are affected (15,104).
The association of a cutaneous form of lichen amyloidosis with MEN2A is the most recently discovered variant (84,85). In a few kindreds, estimated to be fewer than 25 worldwide, affected people have pruritic papular lesions superimposed on a well-demarcated plaque, invariably over the upper back. The rash may be unilateral or bilateral, and may extend from C6 to approximately T5. Current evidence suggests that the rash results from continuous scratching and irritation of the skin, a condition analogous to “friction” amyloidosis, and is caused by deposition of amyloid in the skin (105,106). In some kindreds, the presence of localized pruritus is a phenotypic indicator of MEN2A (85).
Multiple Endocrine Neoplasia Type 2B
The association of MTC, pheochromocytoma, and ganglioneuromatosis has been classified as MEN2B. Williams and Pollock (107) were the first to piece together the components of this syndrome, although earlier reports exist. It is transmitted as an autosomal-dominant trait (78,108,109), although most identified cases represent mutations (110). The presence of mucosal neuromas on the distal tongue, conjunctiva, and throughout the gastrointestinal tract give this clinical syndrome its most distinctive feature. Hyperparathyroidism is almost never found (111), but unilateral or bilateral pheochromocytomas occur in approximately half of affected patients.de novo
This syndrome is most commonly identified in childhood as a result of gastrointestinal manifestations that may include colic, abdominal cramping, intestinal obstruction (pseudo or real), or diarrhea. These symptoms are most commonly related to intestinal dysfunction caused by neurologic dysfunction of the gastrointestinal tract (Hirschsprung-like) or intermittent physical obstruction caused by neuromas (80). Marfan-like features include long, thin arms and legs, a reduced upper/lower body ratio, pectus excavatum, and slipped femoral capital epiphyses (78). The lens and aortic manifestations of Marfan's syndrome are never seen. Approximately 5% of patients have a more subtle form of the disorder, with normal stature or barely detectable neuromas, making clinical diagnosis challenging.
MTC in patients with MEN2B is bilateral and multicentric, metastasizes early, and is usually more aggressive than in MEN2A (112,113). Metastasis to local lymph nodes during the first few years of life is common. Despite the high level of aggressiveness, there is considerable variability in outcome. Death from metastatic MTC most commonly occurs during the second or third decade, but some patients with metastatic disease have survived for long periods (109,114).
THE GENETICS AND MOLECULAR CAUSATION OF MEDULLARY THYROID CARCINOMA
The availability of large and well-characterized kindreds with hereditary MTC made it an ideal candidate for genetic linkage analysis. The causative gene was mapped to chromosome 10q in 1987 (115,116), and mutations of the protooncogene were identified in 1993 (12,13). Since then, mutations of have been identified for each of the variants of MEN2. Current understanding of the genotype-phenotype relationships are shown in Fig. 71.6 and Table 71.1. Importantly, mutations of have been identified in greater than 98% of kindreds with hereditary MTC.RETRETRET
FIGURE 71.6. Common germ line protooncogene mutations in hereditary medullary thyroid carcinoma. The figure shows genotype-phenotype correlation for the most common mutations found in patients with multiple endocrine neoplasia type 2. Each of the specific mutations is described in Table 71.2. , multiple endocrine neoplasia type 2A and cutaneous lichen amyloidosis; , the association of MEN2A with Hirschsprung's disease in affected kindreds; , familial medullary thyroid carcinoma with no other manifestations of MEN2A; , multiple endocrine neoplasia type 2B.RETMEN2A/CLAMEN2A/Hirschsprung's diseaseFMTCMEN2B
The (arranged during ransfection) protooncogene is a transmembrane TK receptor (14). It has an extracellular domain with a cadherin-like portion and cysteine-rich region just external to the plasma membrane spanning region. The intracellular component is analogous to other TK receptors, particularly the epidermal growth factor receptor, and links to multiple signaling pathways (Fig. 71.7).RETRET
FIGURE 71.7. The tyrosine kinase receptor/glia cell–derived neurotrophic factor- (GFR) receptor system. The receptor forms the backbone for a complex signaling system. partners with one of four coreceptors (GFRα-1, GFRα-2, GFRα-3, or GFRα-4) to form a receptor for one of four ligands (GDNF, glial cell-derived neurotrophic factor; NTN, neurturin; ART, artemin; PSP, persephin). The GDNF family functions as neuronal survival factors and is intimately involved in the developmental embryology of the sympathetic nervous system. Activation of the /GFRα receptor system in the C cell leads to autophosphorylation and phosphorylation of downstream substrates from at least four different signaling pathways: phosphatidyl inositol-3-kinase (PI-3-K), jun activated kinases (JNK), mitogen-activated kinase (MAPK), and phospholipase CRETααRETRETRETγ, (PLCγ). Phosphorylation of tyrosine 1062 is required for transformation of C cells; mutation of this residue abrogates transformation. The activating mutations of lead to dimerization of the receptor in the absence of ligand (extracellular domain mutations) or modify the tyrosine kinase domain (intracellular domain mutations) to facilitate autophosphorylation and activation of downstream signaling pathways.RET
The gene occupies a unique place in thyroid oncogenesis. Not only are germ line mutations of this gene involved in the development of MTC, but rearrangements of that bring the TK domain under the control of one of several promoters expressed in thyroid follicular cells have been implicated in the causation of approximately one third of papillary thyroid carcinomas (PTCs) (see section on oncogenes in Chapter 70). With the exception of a few reports of involvement in acute myeloid leukemia (117, 118, 119), papillary carcinoma and MTC are the only tumors caused by mutation/rearrangement.RETRETRETRET
What is unique about RET is that it is the backbone for a family of ligands and coreceptors that are involved in the development of specific components of the nervous system. RET partners with one of four proteins tethered to the extracellular membrane by a glycosyl-phosphatidyl-inositol (GPI) linkage (Fig. 71.7). These four coreceptors (GFRα-1, GFRα-2, GFRα-3, GFRα-4) together with the RET TK receptor form a receptor for one of four ligands (glial cell–derived neurotrophic factor (GDNF), neurturin (NTN), persephin (PSP), and artemin (ART) (120, 121). GFRα-4 and may play a role in the normal development of C cells (22). These two proteins are expressed in C cells, adrenal chromaffin cells, and certain pituitary cells, and homologous inactivation of RET in mice causes a reduction in the numbers of C cells and adrenal chromaffin cells (122). This suggests that persephin, a ligand for the GFRα-4 and RET complex, is involved in normal differentiation of C cells and adrenal chromaffin cells (123).RET
There appear to be two fundamentally different mechanisms by which mutations of the protooncogene cause activation of this receptor system. Mutations of the extracellular domain, affecting predominantly cysteine residues at codons 533, 609, 611, 618, 620, 630, and 634, cause dimerization of the receptor in the absence of ligand. This leads to autophosphorylation of RET and activation of downstream signaling pathways (124, 125, 126) (Fig. 71.7). Mutations of the intracellular domain change the conformation of the TK domain, leading to activation of catalytic activity in the absence of receptor dimerization (124,125,127).RET
Several different intracellular signaling pathways are activated by these mutations. There is general consensus that tyrosine 1062 (Fig. 71.7) is an important mediator of transformation and mediates activation of MAP kinase pathways through binding of shc (128, 129, 130). Tyrosine 1062 also mediates phosphatidyl inositol-3-kinase (PI-3-K) and activation of AKT (Fig. 71.7) (131). The nature of the complex that forms at tyrosine 1062 is still being investigated; there is some evidence that SNT/FRS2 is involved in MAP kinase but not PI-3-K activation (127,132). Most importantly, mutation of codon 1062 abrogates the transforming capability of RET (133). There is also evidence that mutations activate JNK through interaction with the p85 subunit; this effect also may be mediated through the tyrosine at codon 1062 (131,134).RET
The importance of tyrosine at codon 1062 for transformation is documented by the fact that protooncogenes in which this residue is deleted are incapable of causing transformation. However, this does not exclude aRET contribution by other tyrosine residues to the transformation process or some of the other manifestations found in the variant forms of MEN2.
Indirect evidence from studies of the / rearrangements found in papillary carcinoma indicates that MAP kinase activation is important. Nearly all of these carcinomas have mutations of or one of the linker proteins (RAS or RAF) that activate MAP kinase (135, 136, 137). These observations have led to studies of small molecules that inhibit RET-TK activation (138,139), and several of these small molecules are now being investigated in phase I clinical trials.RETPTCRETin vitro
Somatic RET Protooncogene Mutations in Sporadic Medullary Carcinoma
Approximately 25% of sporadic MTCs have a somatic protooncogene mutation, most commonly a Met918Thr mutation (140,141), identical to the germ line mutation that causes MEN2B. Furthermore, there is compelling evidence that codon 918 mutations are associated with a more aggressive form of MTC. Reports from several groups indicate that this mutation is associated with a greater extent of disease and a significant reduction in survival (142,143). Indeed, the characteristic phenotype associated with a somatic codon 918 mutation is the presence of extensive lymph node metastases in the neck, extension of disease into mediastinal and perihilar lymph nodes, and pulmonary, hepatic, and bony metastases. In one study, the 10-year survival in patients with a codon 918 mutation was 50%, as compared with 85% among patients without this mutation (143). Findings at the University of Texas M.D. Anderson Cancer Center are nearly identical (unpublished observations). Other codons mutated in sporadic MTC include 631, 634, 766, 768, 876, 804, 883, 884, 901, 922, and 930 (140,144,145,146,147,148,149).RETRETRET
There may be heterogeneity of somatic mutations. For example, there are reports that some metastatic foci had a codon 918 mutation, whereas the primary MTC or another metastatic focus did not. These results suggest that acquisition of mutations may be but one element in the progression of metastatic potential (150,151).RET
There are reports that polymorphism at codon 836 of is more frequent in patients with sporadic MTC (152,153). In addition, a codon 834 polymorphism has been associated with a codon 804 mutation, but does appear to affect the aggressiveness of an 804 mutation (154). A relationship between the 836 polymorphism and sporadic MTC is not proven, but is not unrealistic given the role of polymorphisms of in Hirschsprung's disease (155).RETRET Other Molecular Abnormalities in the Progression of Medullary Thyroid Carcinoma
MTC is one of a small number of carcinomas in which an imbalance between the normal and mutant copy of the activated oncogene contributes to progression of the tumor. In MTC there are three identified mechanisms by which this occurs. The first is through a chromosomal duplicative event that results in a 2:1 ratio of mutant to normal RET receptors (156,157). A second variant is found in the TT cell line, derived from a human MTC with a codon 634 mutation. In this cell line there is a tandem duplication of the mutant locus, resulting in twice the level of expression of the mutant sequence (158). Finally, in some MTCs there is loss of part or all of the normal allele, creating a situation in which there is homogeneous expression of mutant allele (156,157,159). The implication of these findings is that the normal copy of functions as a tumor suppressor gene, and that its loss results in a greater mutant effect, analogous to earlier observations for the and oncogenes (160,161).RETRETRETRETRETMETRAS
Although no other specific causative loci have been identified in either hereditary or sporadic MTC, the finding of loss of heterozygosity at several chromosomal locations (chromosomes 1p, 3p, 9q, 17p, and 22q) suggests there are other tumor suppressor genes that are likely to be involved in the genesis of medullary thyroid carcinoma (162, 163, 164, 165).
GENOTYPE-PHENOTYPE CORRELATION IN HEREDITARY MEDULLARY THYROID CARCINOMA
Since the discovery of mutations in MEN2, there has been a gradual refinement of the phenotypes associated with particular activating mutations of (Fig. 71.6 and Table 71.2).RETRET
TABLE 71.2. MUTATIONS OF THE PROTOONCOGENE HEREDITARY MEDULLARY THYROID CARCINOMARET
HGMD Accession No.
Amino Acid Change Normal → Mutant
Nucleotide Change Normal → Mutant
Percentage of all MEN2 Mutations
cys → arg
TGC → CGC
cys → gly
TGC → GGC
cys → ser
TGC → TCC
cys → tyr
TGC → TACa
cys → arg
TGC → CGC
cys → gly
TGC → GGC
cys → phe
TGC → TTC
cys → ser
TGC → AGCb
cys → trp
TGC → TGG
cys → tyr
TGC → TAC
cys → arg
TGC → CGCc
cys → gly
TGC → GGCd
cys → phe
TGC → TTC
cys → ser
TGC → AGC
cys → ser
TGC → TCC
cys → tyr
TGC → TAC
cys → arg
TGC → CGC
cys → gly
TGC → GGC
cys → phe
TGC → TTC
cys → ser
TGC → TCC
cys → ser
TGC → AGC
cys → trp
TGC → TGG
cys → tyr
TGC → TAC
cys → phe
TGC → TTC
cys → ser
TGC → TCC
cys → tyr
TGC → TAC
cys → arg
TGC → CGC
cys → gly
TGC → GGC
cys → phe
TGC → TTC
cys → ser
TGC → AGC
cys → ser
TGC → TCC
cys → trp
TGC → TGG
cys → tyr
TGC → TAC
glu → asp
GAG → GAT
glu → asp
GAG → GAC
leu → phe
TTG → TTT
leu → phe
TTG → TTC
tyr → phe
TAT → TTT
val → leu
GTG → TTG
val → met
GTG → ATG
ser → val
TCG → GCG
met → thr
ATG → ACG
Rare RET Mutations (< 0.1%)
HGMD Accession No.
Amino Acid Change Normal → Mutant
Nucleotide Change Normal → Mutant
gly → cys
GGC → TGC
9 bp duplication
lys → gln
AAA → CAA
TGC ↑ GACGAgctgtGCC
GAG ↑ CTGTGccGCA
ser → leu
TCG → TTG
Primary hyperparathyroidism (1 case)
lys → glu
AAG → GAG
val → ile
GTC → ATC
gln → arg
CAG → CGG
arg → leu
CGG → CTG
GTG → ATG/
CTG ↑ GTAgcTGC
ala → phe
GCT → TTT
a, c Mutations of these two codons have been reported in Hirschsprung's disease.
a, b, d Reported cases of MEN2A/Hirschsprung's disease variants have these mutations.
eA codon 634 cys ↑ arg (TGC ↑ CGC) mutation accounts for approximately 50% of all mutations associated with MEN 2A.
ACTH, corticotropin; MEN, multiple endocrine neoplasia; FMTC, familial medullary thyroid carcinoma.
Adapted from information available at the Human Gene Mutation Database (www.HGMD.org ) and from Online Mendelian Inheritance in Man (www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=search DB=omim ). In addition, the authors have added to this list from meeting abstracts and personal experience. This table describes mutations identified in 99.9% of germ line carriers of MEN2. In addition there are rare mutations that have been identified in individual patients or in sporadic tumors. In some cases the physical location of these rare coding changes adjacent to a sequence known to be mutated gives credibility to the role of the mutant sequence; in others the relevance is unclear. Further clarification of their relevance will await examination of their transforming abilities or additional genotype/phenotype correlation. It is likely there are other rare mutations.RET
The most commonly identified mutations in hereditary MTC are located in the extracellular cysteine-rich region of the receptor. Mutations of the cysteine at codon 634 account for approximately 80% of germ line mutations; substitution of arginine for cysteine accounts for more than one half of the mutations at this codon. The invariable clinical phenotype associated with any codon 634 mutation is the MEN2A syndrome (166). The next largest clustering of mutations, accounting for 10% to 15% of mutations, involves substitution of one of several amino acids for cysteine at codons 609, 611, 618, 620, and 630. Mutations at these codons may be associated with either MEN2A or familial MTC. There are clearly defined kindreds with either familial MTC or MEN2A with each of these substitutions. All patients with the MEN2A/CLA syndrome have been found to have one of several substitutions at codon 634 (166). Patients with the MEN2A/ Hirschsprung variant have been found to have codon 609, 618, or 620 mutations; investigators studying Hirschsprung's disease have identified codon 609 or 620 mutations among kindreds with hereditary Hirschsprung's disease (167).RETRETRET
The remainder of mutations, mostly intracellular, account for no more than 5% to 10% of all mutations. Mutations at codons 883 and 918 are invariably associated with MEN2B (168, 169, 170, 171, 172). Mutations at codons 768 and V804M are invariably associated with familial MTC (166,173,174,175). Intracellular domain mutations that may be associated with either familial MTC or MEN2A include codons 790, 791, V804L, and 891 (176, 177, 178).
Although it is unlikely that there will be substantial change in the genotype-phenotype correlations described, there will be refinement. Two types of changes seem likely. The first is a gradual expansion of the phenotype associated with rare mutations. An example of this is the reclassification of the phenotype associated with a codon 891 mutation during the past year. Early reports of this mutation described five kindreds (175,179,180,181,182), and a single patient (181) in which the only manifestation was MTC. The identification of a kindred with this same mutation and a single member with a pheochromocytoma has necessitated reclassification of this mutation from familial MTC to MEN2A (178).
A second phenomenon has been the identification of rare germ line mutations or polymorphisms of the gene associated with hereditary MTC. Examples include germ line mutations at codon 666 associated with MTC in one kindred and an ACTH-secreting pheochromocytoma in a single person from another kindred (Table 71.2), a codon 912 mutation associated with MTC in a single kindred (183), a codon 778 mutation associated with MTC in a single patient (184), and an unusual variant of MEN2B in a kindred, namely codon 804 and 904 mutations on the same allele (185). The two largest compilations of genotype–phenotype information describing these rare mutations are at Online Mendelian Inheritance in Man (185a) and the Human Gene Mutation Database (185b), although neither is complete. Many of the rare mutations are listed in Table 71.2, and there are others scattered throughout the literature. It may take another decade or more before a complete understanding of the phenotypes associated with these rare mutations emerges.RET
UNUSUAL DIAGNOSTIC ISSUES IN THE EVALUATION OF MEDULLARY THYROID CARCINOMA
The Use of Serum Calcitonin Measurements in the Evaluation of Nodular Goiter
Several studies have described the results of measurements of serum calcitonin in patients with thyroid nodules, as noted above. A consistent finding in these studies was the identification of MTC in a small number of patients, based on a high serum calcitonin concentration. In an early study, of 1385 patients, all 8 (0.5) who had a high serum calcitonin value had MTC (186). Overall, there have been eight studies of over 6000 patients (186, 187, 188, 189, 190, 191, 192, 193). All patients in these studies who had a serum calcitonin value of greater than 100 pg/mL, with the exception of a patient with pancreatic cancer, had MTC; in many of these patients the diagnosis of MTC was apparent by other clinical criteria or biopsy.
In these same studies there were 130 patients (2%) who had serum calcitonin values between 10 and 100 pg/mL; only 23 of these 130 patients (18%) were found to have MTC at surgery. The basal serum calcitonin concentrations plotted as a function of the size of the MTC in 21 of these patients are shown in Fig. 71.8. It is clear that in most of these patients the MTC was very small, and not likely to have been identified by any diagnostic technique. Furthermore, using a serum calcitonin cutoff value of 25 pg/mL, only 3 of the 21 MTCs would have been missed.
FIGURE 71.8. Basal serum calcitonin (CT) values plotted against size of the medullary thyroid carcinoma in 21 patients with a multinodular goiter who had basal serum calcitonin concentrations of 10 to 100 pg/mL. Note the limited correlation between serum calcitonin values and tumor size. Serum calcitonin concentrations ranged from less than 10 pg/mL to as high as 30 pg/mL in the normal subjects. All but three of the patients with goiter with MTC had values greater than 25 pg/mL (horizontal line).
In one of these studies 1167 patients with one or more thyroid nodules underwent surgery, and the resected thyroid tissue was examined for the presence of MTC (191). Of the 1167 patients, 34 (3) had a high basal serum calcitonin concentration. Fourteen of these 34 patients (41%) had MTC. These findings indicate that a high serum calcitonin value has predictive value for the presence of MTC, but on the other hand high values are not specific for MTC. High values have been found in patients with all types of thyroid nodules, and perhaps most often in patients with autoimmune thyroiditis (193).
These collective results have led a German-based consensus group to recommend total thyroidectomy for patients with a pentagastrin-stimulated serum CT value greater than 100 pg/mL, and total thyroidectomy and lymph node dissection for patients with pentagastrin-stimulated serum CT values greater than 200 pg/mL (194). Pentagastrin is not available in the United States.
Despite the fact that approximately one in five patients with thyroid nodules who have a high basal serum calcitonin value will be found to have MTC, measurement of serum calcitonin in these patients has not been widely accepted in the United States. There are several reasons for this. First is the lack of specificity of measurements of basal serum calcitonin (23,51,195,196). The finding that 1.7% of normal adults without evidence of thyroid disease had a serum calcitonin concentration of greater than 10 pg/mL (193) highlights the difficulty of differentiating between normal subjects and the 3.5% of patients with thyroid nodules who have similarly high values. Secondly, in none of the reported series was an attempt made to assess the risks (unnecessary surgery, hypoparathyroidism, recurrent laryngeal nerve injury, hypothyroidism) versus benefit (cure of MTC) of performing a thyroidectomy in a patient with a slightly high serum calcitonin concentration (197). Third, other than the axiom that “all carcinomas start small,” there is little evidence that the small MTCs detected incidentally will in fact progress (198,199).
From a practical point of view, once the possibility of MTC has been raised, based on the finding of a high serum calcitonin value, however minimal the elevation may be, it is inevitable that the patient will eventually have a thyroidectomy. It is very difficult to reassure such a patient that observation is a rational approach. The lack of availability of pentagastrin in the United States makes it necessary to base a decision on a basal serum calcitonin or a calcium-stimulated value. It is paradoxic that in an era when fine-needle aspiration biopsy is used routinely to reduce the need for thyroid surgery, use of a less specific test may increase the number of thyroid surgical procedures, without clear evidence of benefit. A reasonable course of action, based on the data in Fig. 71.8, is to follow patients with a serum calcitonin value of less than 25 pg/mL, particularly if the value is less than 100 pg/mL after calcium or pentagastrin stimulation. Those with basal values greater than 25 pg/mL or a stimulated value greater than 100 pg/mL could be followed or considered for surgery, dependent on other clinical factors. Finally, any patient with an unexplained elevation of serum calcitonin should have a protooncogene analysis.
Testing for Germline RET Protooncogene Mutations in Sporadic Medullary Carcinoma
A few patients with apparently sporadic MTC have a germ line mutation indicative of hereditary MTC (15,200,201,202,203,204). These findings have led to a consensus recommendation that all patients with apparently sporadic MTC have a DNA analysis for mutations (86). The rationale for this recommendation is based on the fact that patients with palpable MTC have a high rate of local or distant metastatic disease. Identification of a germline mutation in these patients, thereby mandating mutation analysis in their offspring or relatives, should allow earlier identification of family members at risk, thereby improving their outcome. Hence, all patients with apparently sporadic MTC should have a germ line analysis. If a mutation is identified, first-degree relatives should also have a analysis, followed by appropriate evaluation and treatment for those with an identified mutation.RETRETRETRETRET
TREATMENT OF MEDULLARY THYROID CARCINOMA
Medullary Thyroid Carcinoma Presenting as a Thyroid Nodule
Patients with MTC can be classified into one of three groups: those with localized MTC in whom cure is possible, those with metastatic disease limited to the neck in whom cure may be possible, and those with evidence of metastases outside of the neck. Whenever possible, the diagnosis of MTC should be established preoperatively by biopsy, and the extent of disease evaluated by measurements of serum calcitonin and CEA and ultrasonography of the neck. In addition, a germ line analysis of the protooncogene should be performed, although it is unusual to have the result of this analysis before the primary surgical procedure. Consideration should also be given to the possibility of a pheochromocytoma, as discussed in an earlier section.RET
The appropriate surgical procedure for a patient with localized MTC without obvious metastatic disease (no abnormalities outside the neck on imaging studies and serum calcitonin values < 500 pg/mL) is a total thyroidectomy with central (levels VI–VII) and bilateral lateral (levels II–V) node dissection (see Chapter 17). This is an extensive surgical procedure, but one that affords the highest probability of surgical cure, defined as an undetectable serum calcitonin concentration (< 2 to 5 pg/mL). From 10% to 25% of patients can be cured by this approach (73,90,205,206,207).
The second group of patients, those with documented local metastatic disease, should be evaluated more carefully. Although surgical cure in these patients is possible, the probability is lower. Most patients in this group should be treated by total thyroidectomy and central (levels VI–VII) and bilateral lateral (levels II–V) node dissection. A question that arises in these patients is the extent of search for distant metastatic disease. If the patient's serum calcitonin value is greater than 500 pg/mL and there is no evidence of metastases in cervical lymph nodes, as assessed by ultrasonography, chest and abdominal computed tomography scans or positron-emission tomography (PET) should be performed (208,209). The high frequency of hepatic me tastases has led to the recommendation that laparoscopic evaluation of the liver be performed as well (74,210).
The challenge is to differentiate between patients in whom there is a reasonable possibility of surgical cure and those with distant metastasis, in whom extensive neck surgery will not affect long-term outcome. The sensitivity of serum calcitonin measurements for detection of tumor is far greater than current imaging sensitivity, and as a result the number of patients with distant metastases is likely to be underestimated.
In those patients in whom cure by thyroidectomy and neck dissection is not possible, the goals change. In them, it is appropriate to perform a total thyroidectomy with surgical excision of identifiable disease, particularly if the disease is located so that it could impair the airway. There is little evidence that a mediastinal node dissection has curative value, and it is rarely performed as a part of a curative thyroidectomy.
The last group, those patients with clear evidence of distant metastases, is usually relatively easily identified. These are patients who have very high serum calcitonin concentrations (>5000 pg/mL), they may have diarrhea or flushing, and they have metastatic disease easily identified by imaging studies. In these patients, the goal of thyroid surgery is to remove all identifiable tumor in the neck and to protect the airway. Lymph nodes that obviously contain tumor should be resected, but extensive lymph node dissection probably has little value. Treatment of patients with mediastinal metastases should be individualized. In patients who have metastases in lymph nodes in the upper mediastinum or perihilar area that are likely to affect the airway, consideration should be given to a mediastinal lymph node dissection.
Surgical Treatment of Hereditary Medullary Thyroid Carcinoma
The identification of protooncogene mutations has provided a molecular basis not only for predicting a specific MEN2 phenotype, but also for providing insight into the clinical behavior of MTC. Some idea of the relative aggressiveness of MTC associated with specific mutations is provided by an examination of the earliest age at which MTC has been identified in patients with a specific mutation (211) (Fig. 71.9). Correlative studies indicate that the presence of a codon 918 mutation is indicative of an aggressive disease phenotype, whether the mutation is hereditary (MEN2B) or acquired as a somatic mutation. Children with MEN2B should have a total thyroidectomy and lymph node dissection performed in the first month of life or as soon as the germ line mutation is identified. Because most patients with MEN2B have mutations, they are most commonly identified in the 3- to 10-year age period; in such children surgical cure is uncommon.RETRETde novo
FIGURE 71.9. Plot showing the earliest age of identification of medullary thyroid carcinoma in patients with different germ line mutations in , derived from data presented in the EUROMEN study (referenced in text) and other reports. Note that the mutations are located in both the extra- and intracellular regions of the RET protein. (Modified from Cote GJ, Gagel RF. Lessons learned from the management of a rare genetic cancer. 2003;349:1566, with permission.)RETN Engl J Med
The next most aggressive phenotype is associated with a codon 634 mutation; this accounts for approximately 80% of hereditary MTC. Metastasis has been described in a 6-year-old child (92,93), and recent unpublished reports have described microscopic MTC with metastasis to at least one local lymph node in 3-year-old children. The EUROMEN consortium report described 196 children with codon 634 mutations; no one in this group under 14 years of age had metastasis (212). There is now consensus that thyroidectomy, with or without central lymph node dissection, should be performed by the age of 6 years (86), but some advocate earlier thyroidectomy, particularly in children with abnormal serum calcitonin responses to pentagastrin. Most clinicians would include children with mutations of codons 609, 611, 618, 620, 630, and 891 in this group, because of evidence of aggressive disease in a subset of these patients.RET
The final groups of children include those with mutations of codons 768, 790, 791, and 804. In some large kindreds with these mutations there has never been a death caused by MTC; in others metastases and death attributable to MTC occurred infrequently (213). If there has never been a death attributable to MTC in one of these kindreds (and the kindred is of sufficient size and the follow-up encompasses multiple generations), delaying thyroidectomy to a later age may be a reasonable course (213).
Twenty years ago, John Sipple, addressing the First International Workshop on Multiple Endocrine Neoplasia type 2, marveled at the progress toward understanding the syndrome he described some 22 years earlier (75). That morbidity and mortality would be eliminated for more than 90% of patients with disorder during his lifetime likely never entered his mind. Remarkably, it has happened for those kindreds with hereditary MTC who are prospectively screened and appropriately treated.
Medullary Thyroid Carcinoma and Persistent Elevations in Serum Calcitonin Concentrations
A common situation in MTC referral centers is a patient who had a total thyroidectomy with or without a neck dissection and is referred for evaluation of a persistently high serum calcitonin or CEA concentration. In evaluating such a patient, it is important to allow 3 to 4 months to elapse after thyroidectomy before concluding that the high serum value is related to the presence of MTC and not to a postoperative inflammatory stimulation of calcitonin production (23) or slow clearance of serum CEA due to its prolonged half-life (214,215).
The most common scenario is a patient who had a total thyroidectomy and a limited (or no) lymph node dissection. Among these patients, from 10% to 15% will in time have normal serum calcitonin concentrations (71,72,73). Because the patient has received primary therapy (thyroidectomy) for MTC, the decision to perform a thorough lymph node dissection should be based on a realistic estimate of the probability of a surgical cure. Reasonable evaluation would consist of ultrasonography of the neck, chest and abdominal computed tomography, and nuclear imaging [PET, octreotide, or meta-iodobenzylguanidine (MIBG)]. Patients who have basal serum calcitonin concentrations of less than 100 pg/mL are unlikely to have any imaging abnormalities. Some clinicians would, in this situation, perform laparoscopy to examine the liver before proceeding to neck exploration (74). The major complication of reoperation in these patients is hypoparathyroidism (73); in addition, the operation is unlikely to result in normalization of serum calcitonin. In patients with obvious distant metastases, there is little if any reason to perform a detailed neck dissection; in such patients reoperation is appropriate to remove lymph nodes that may become problematic to patency of the airway if allowed to remain in place.
OTHER FORMS OF THERAPY FOR MEDULLARY THYROID CARCINOMA
Radiotherapy is routinely used as adjunctive palliative therapy for patients with extensive neck or mediastinal disease or a localized bony metastasis. It provides effective amelioration of MTC in both of these situations; but there is no evidence that it has any effect on survival (216,217). Targeted radiotherapy using radiolabeled somatostatin analogues (218,219), radiolabeled monoclonal antibodies directed against CEA (220), and radiolabeled MIBG (221) have been used with limited success.
Cytotoxic chemotherapy has a place in the treatment of patients with metastatic MTC. Approximately 30% of patients treated with dacarbazine (DTIC)-based chemotherapy have reductions in tumor size (222, 223, 224), and there are few reports of complete remission. Because MTC is most commonly a slowly progressive tumor, the challenge is to initiate therapy at an appropriate time. In patients with codon 918 mutations and rapidly progressive disease, early intervention with chemotherapy may delay tumor progression; in patients with more slowly progressive disease, intervention is advised at a point when there is substantial tumor mass, but before the patient's performance status has decreased appreciably.RET
Finally, the introduction of a variety of small molecules that inhibit signal transduction has opened a new era of investigation. Many of these drugs have limited side effects; patients with slowly growing MTC are being encouraged to participate in trials of selected drugs that target signal transduction pathways activated by the RET receptor.
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