Radiation Oncology: A Question-Based Review

Thymoma and Thymic Carcinoma

Steven H. Lin and Melenda D. Jeter

image Background

What is the embryonic derivation of the thymus?

The embryonic derivation of the thymus is the 3rd pharyngeal pouch.

Where is the thymus located, and what is its function?

The thymus is in the ant mediastinum (MN), involved in the processing and maturation of T lymphocytes to recognize foreign antigens from “self” antigens.

What structures are located in the ant, middle, and post MN?

1.     Anterior: LNs, thymus, mesenchymal tissues

2.     Middle: heart and great vessels, trachea, esophagus, most mediastinal LNs, vagus and phrenic nerves

3.     Posterior: paraspinal tissues, sympathetic and peripheral nerves

What proportion of tumors of the MN are malignant?

One third of mediastinal tumors are malignant.

How prevalent is thymoma relative to other mediastinal tumors?

Thymoma comprises 20% of all mediastinal tumors but 50% of all ant mediastinal tumors.

What is the median age and gender predilection for thymomas?

The median age for thymomas is 40–60 yrs. There is no gender predilection (male = female).

Are thymomas common in children?

No. Thymomas are extremely rare in children, but if present they are extremely aggressive with poor survival.

Pathologically, what is the most important defining feature of thymomas?

Coexistence of nonneoplastic lymphoid cells with neoplastic epithelial cells (spindle to polygonal types)

How do thymic carcinomas differ from thymomas?

Much less prevalent (<1% of thymic tumors), very aggressive, with poorer survival (5-yr OS 30%–50%)

What are the WHO designations of thymomas vs. thymic carcinomas?

1.     WHO type is based on shape and the lymphocyte/epithelial ratio.

2.     WHO type A–AB: benign thymoma, medullary, spindle cell

3.     WHO type B1–B3: malignant thymoma, lymphocytic, cortical, epithelial

4.     WHO type C: highly malignant, thymic carcinoma, clear cell/sarcomatoid types

What is the LN metastatic rate of thymomas vs. thymic carcinomas?

1.     Thymoma: ~1%–2%

2.     Thymic carcinoma: ~30%

3.     (Kondo & Monden review of 1,320 pts with thymic tumors [Ann Thorac Surg 2003])

What is the hematogenous dissemination of thymomas vs. thymic carcinomas?

1.     Thymoma: ~1% (mostly to lung)

2.     Thymic carcinoma: 12% (lung > bone, liver)

image Workup/Staging

What is the DDx of a mediastinal mass by location in the ant, middle, and post MN?

1.     Anterior: thymoma, thymic carcinoma, carcinoid, germ cell tumors, lymphomas (Mnemonic TTT: Thymoma, Teratoma, Terrible lymphoma)

2.     Middle: cysts > lymphoma, teratomas > sarcomas (osteosarcoma, fibrosarcoma, angiosarcoma, rhabdomyosarcoma of the heart), granuloma

3.     Posterior: neurogenic tumors (PNET, schwannoma, neurofibroma, neuroblastoma, ganglioneuroma), pheochromocytoma

What clinical presentations are common for pts with mediastinal tumors?

50% are diagnosed incidentally on imaging studies. Sx are caused by a mass effect resulting in cough, shortness of breath, pain, stridor, Horner, superior vena cava (SVC) syndrome, HTN (catecholamine), myasthenia gravis (MG) (thymoma)

How do pts with thymomas or thymic carcinomas usually present?

50% are incidental findings; but if there are Sx, they reflect either locally advanced Dz, metastatic sequalae, or paraneoplastic disorders (in 50%–60% of thymomas but hardly seen in thymic carcinomas).

What paraneoplastic disorders are commonly seen in thymomas?

MG (35%–50% of cases), red cell aplasia (5%), immune deficiency syndromes like hypogammaglobulinemia (5%), autoimmune disorders (collagen vascular, dermatologic, endocrine, renal Dz), and other malignancies (lymphomas, GI/breast carcinomas, Kaposi sarcoma)

What workup should be employed for a mediastinal mass?

Mediastinal mass workup: H&P (ask about B Sx [fever >38°C, drenching night sweats, weight loss >10% in preceding 6 mos] to r/o lymphoma, physical to assess nodal status, MG Sx), basic labs (LDH, ESR, AFP/HCG to r/o germ cell tumor as needed), imaging (PA/lat CXR, CT chest, MRI, PET if lymphoma suspected), Bx (FNA, but preferably Tru-Cut core Bx) or surgical (video-assisted thorascopic surgery, Chamberlain), PFTs to assess lung function

The Dx of thymoma is essentially established clinically if the pt presents in what way?

Ant mediastinal mass with Sx of MG, red cell aplasia, or hypogammaglobulinemia

Approximately what % of pts with thymoma present with MG?

35%–50%. Conversely, about 10%–15% of pts with MG have thymoma.

What are the pathogenesis, presentation, Dx, and Tx of MG?

Autoantibody to the ACh receptor at the postsynaptic endplate. Pts present with easy fatigability, ptosis, and diplopia (worse with movement, whereas the opposite is true for Lambert-Eaton). Dx is by the Tensilon test (edrophonium). Tx is by anticholinesterase (pyridostigmine) or thymectomy (reverses in 40% of pts with thymoma).

What is the Masaoka system used to stage thymomas?

1.     Stage I: encapsulated, no microscopic capsular invasion

2.     Stage II: macroscopic invasion into surrounding fat or mediastinal pleura, or microscopic capsular involvement

3.     Stage III: extension to surrounding organs or great vessels

4.     Stage IVA: pleural or pericardial dissemination

5.     Stage IVB: +LN or DM

What are the 5-yr survival rates for thymomas based on the Masaoka staging?

5-yr survival for thymomas (Masoaka staging):

1.     Stage I: 95%

2.     Stage II: 90%

3.     Stage III: 60%

4.     Stage IV: 11%–50%

What is the 5-yr survival rate of invasive vs. noninvasive thymomas?

1.     Invasive: 50%

2.     Noninvasive: 70%

What are the most important prognostic factors for thymomas?

The Masaoka stage and completeness of resection are the most important prognostic factors for thymomas.

Based on modern surgical series, what are the rates of complete resection, the recurrence rate, and 5-yr OS based on Masaoka staging?

Kondo et al. reported outcomes for 1,320 pts. The % of complete resection, % of recurrence, and 5-yr OS, respectively, were as follows:

1.     Stage I: 100%, 1%, 100%

2.     Stage II: 100%, 4%, 98%

3.     Stage III: 85%, 28%, 89%

4.     Stage IVA: 42%, 34%, 71%

(Ann Thorac Surg 2003)

What are the most important prognostic factors for thymic carcinomas?

The completeness of resection and presence of LN mets are the most important prognostic factors for thymic carcinomas.

What is the 5-yr survival rate for thymic carcinoma?

The 5-yr OS for thymic carcinoma is 20%–30%.

image Treatment/Prognosis

What is the most important modality in the management of thymomas?

Surgery is the mainstay of Tx, with complete resection being the primary goal. The outcome is fully dependent on the extent and completeness of the resection, regardless of stage or histology. Surgical clips can help identify areas difficult to resect or possible residual Dz. It may even be reasonable to resect pleural mets since prolonged survival is possible.

What is the usual approach for the surgical management of thymomas?

Median sternotomy, but more extensive resections may be required depending on the stage at presentation, including partial or total pneumonectomy or pericardiectomy.

In a thymoma pt with MG, what should be done preoperatively?

Signs + Sx should be controlled medically prior to undergoing surgical resection.

When is adj radiotherapy a reasonable indication for the management of thymomas?

Adj radiotherapy should be considered with stage III–IVA, + /close margins, or thymic carcinoma.

Is adj radiotherapy necessary for a stage II thymoma after complete resection?

1.     This is controversial. It initially was recommended based on a classic review (Curran W et al., JCO 1988) in 103 pts with thymomas, finding that pts without PORT had ↑LR (6 of 19 pts for stage II) vs. no LR in PORT (0 of 1 pt for stage II, 0 of 4 pts for stage III). More recent data out of the Massachusetts General Hospital (Mangi A et al., Ann Thorac Surg 2002) and Japan (Haniuda M et al., Ann Surg 1996) showed that PORT may not be necessary after complete resection for stage II pts. Haniuda et al. did demonstrate that stage II thymoma with macroscopic adherence to the pleura did benefit from PORT (LR 36% vs. 0%), but PORT was not useful for microscopic invasion of the pleura or pericardium.

2.     Meta-analysis (Korst RJ et al., Ann Thorac Surg 2009): 1981–2008 systematic review of 13 studies, 592 pts, ~42% had surgery + PORT. The LR rate did not benefit from PORT for stage II–III thymoma (OR 0.87 for both stage II–III, p = 0.69).

3.     Forquer JA et al., IJROBP 2009: 901 pts, SEER data 1973–2005; 92% thymoma, 8% TC; localized Dz in 274 pts, regional Dz in 626 pts. 5-yr OS benefited from surgery + PORT for regional Dz (76% vs. 66%, p = 0.01); for localized Dz, surgery alone was more favorable (98% vs. 91% [PORT], p = 0.03).

4.     Current recommendation for adj RT for stage II–III thymoma: +/close margin (<1 mm), gross fibrous adhesion to pleura, or ↑ WHO grade (B3) tumors (Wright CD et al., Hem Onc 2008)

What should the postop target volume include?

The postop target volume should include the entire bed of resection and any involved organs. It is imperative to have a preop CT scan available to help delineate tumor bed volumes. Also, information from operative and pathology reports may help determine areas that might have had adherent, invasive Dz.

When should postop CRT be considered for the management of thymic malignancies?

Per NCCN 2010, thymoma with gross residual Dz or thymic carcinoma with R1-R2 resection

What are some management approaches for unresectable thymic tumors?

Management approaches for unresectable thymic tumor:

1.     RT only

2.     Induction chemo → surgery → RT

3.     Preop RT

What are the results of definitive RT for unresectable thymic tumors?

RT can be used as sole modality, with 5-yr OS 50%–87%. Surgery should be done whenever possible, however, since resectability is still the most important prognostic factor.

Given the good response rates seen with platinum-based chemo, what is the current preferred Tx paradigm for unresectable thymic malignancy?

Unresectable thymic malignancy Tx paradigm:

1.     Chemo → RT (no surgery) (Loehrer PJ et al., JCO 1997). Cisplatin/doxorubicin/cyclophosphamide (PAC) 2–4 cycles → RT to <54 Gy to primary + regional LN. 5-yr OS was 53%.

2.     Chemo → surgery (if possible) → PORT (MDACC: Shin DM et al., Ann Int Med 1998). 3 cycles induction chemo (Cytoxan/Adriamycin/cisplatin [CAP] + prednisone) → max surgery → RT. 7-yr follow-up showed 100% OS and 73% DFS.

What are the 1st-line combination chemo regimens used for the management of thymic malignancies?

CAP +/− prednisone; VP-16/ifosfamide/cisplatin (VIP); cisplatin/VP-16 (EP); carboplatin/Taxol; cisplatin/Adriamycin/vincristine/Cytoxan (ADOC); Cytoxan/Adriamycin/vincristine/prednisone (CHOP)

What are the typical response rates with induction chemo for the management of thymic malignancies?

Typical response rates with induction chemo are 50%–60%.

What is the most common approach for the management of thymic carcinomas?

If possible, max surgery → CRT postoperatively. If inoperable, consider induction therapy with chemo, RT, or combination CRT.

What are the RT doses used for the postop management of thymomas?

Depends on the extent of resection:

1.     If R0: 45–54 Gy

2.     If R1: 55–60 Gy

3.     If R2: 60–70 Gy

What are the RT volumes utilized for high-risk resected thymomas?

Treat at-risk subclinical sites in the surgical bed and LN regions. The entire MN or SCV does not need to be covered unless indicated.

What RT doses can be used for the preop management of unresectable thymomas?

24–30 Gy with chemo → surgery → the consideration for more RT depending on resection status

image Toxicity

What is the follow-up for pts who have had a complete resection of a thymoma?

Completely resected thymoma follow-up: H&P + annual CT chest

Is 5 yrs of follow-up sufficient for a pt treated for thymomas?

No. Late recurrences can occur at >10 yrs. Pts need lifelong follow-up.

What are the expected early and late toxicities after adj RT for the management of thymic tumors?

1.     Early: skin reaction, fatigue, dysphagia/odynophagia, cough

2.     Late: RT pneumonitis/fibrosis, pericarditis, esophageal stricture, myelitis

What are the dose limiting structures and dose limits when the MN is irradiated?

1.     Lung: RT alone → V20 <40%; CRT → V20 <35%. For DVH analysis, keep V5 <40%, V20 <30%, and V30 <8% (NCCN 2010).

2.     Heart: V40 <50% (V40 <40% if CRT)

3.     Spinal cord: ≤45 Gy

4.     Esophagus: Ideally, the mean dose of RT to the esophagus should be <34 Gy. Try to minimize the V60 as much as possible (V60 <33%, V50 <50%, ≤45 Gy to the entire esophagus, max dose point <70 Gy).