Thymoma

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Thymoma is a common tumour of the thymus.

General

  • Strong association with autoimmune disease, esp. myasthenia gravis.

Classification

The WHO published a widely used system - WHO classification:[1]

Type A

  • AKA Spindle cell or medullary.
  • Arise from medullary epithelial cells.
  • Good prognosis.

IHC:

  • Usu. keratin+.

Type AB

  • Like Type A... but with foci of lymphocytes.

Type B1

  • Near normal, expanded cortex.

Lesion consists of:

  • >2/3 lymphocytes, <1/3 cortical epithelial cells.

Type B2

  • Neoplastic cells with some resemblance to cortical epithelial cells.
    • Epithelioid cells with distinct nucleoli.
    • May be perivascular.
  • Large population of lymphocytes.

Lesion consists of:

  • <2/3 but >1/3 lymphocytes, >1/3 but <2/3 cortical epithelial cells.

Notes:

  • Most common B type.

Type B3

  • Neoplastic cells with some resemblance to cortical epithelial cells.
    • Polygonal/round shape.
    • Form sheets (of cells) - key feature.
  • Lymphocytes - less than in Type B2.
  • AKA well-differentiated thymic carcinoma.

Lesion consists of:

  • <1/3 lymphocytes, >2/3 cortical epithelial cells.

Note:

  • Neoplastic cells derived from the thymus with cytologic features of malignancy are thymic carcinomas.

Images:

  • Thymoma Type B1. (WC/KGH)

  • Thymoma Type B1. (WC/KGH)

  • Thymoma Type B1 - CAM5.2. (WC/KGH)

Gross

  • Light brown/tan.
  • Encapsulated.

Image:

Microscopic

Features:

  • Lymphocytes.
  • Epithelial cells.
    • Spindle cells - Type A.
    • Epithelioid cells - Type B.

DDx:

Images:

Staging

There is a system by Masaoka and colleagues[2] that was subsequently modified, and is known as the modified Masaoka staging system.[3]

Based on CAP protocol

Staging as per Butnor et al.:[4]

Stage Characteristics
I encapsulated lesion, tumour does not penetrate capsule
IIa microscopic penetration of the capsule
IIb macroscopic penetration of the capsule
III macroscopic invasion of adjacent organs
IVa pleural or pericardial spread
IVb lymphatic or hematogenous spread

Modified Masaoka as per Masaoka et al. (1999)

T-stage - based on Masaoka et al. (1999):[5]

Stage Features
T1 macroscopically and microscopically encapulated
T2 macroscopic invasion or adhesion to surrounding tissue (fat or pleura) or microscopic invasion into the capsule
T3 Spread to adjacent organs, e.g. pericardium, lung, great vessels.
T4 pericardial or pleural spread

N-stage - based on Masaoka et al. (1999):[5]

Stage Features
N0 no lymph node spread
N1 spread to anterior mediastinal lymph nodes
N2 spread to intrathoracic lymph nodes other than the mediastinal lymph nodes
N3 spread to supraclavicular lymph nodes

M-stage - based on Masaoka et al. (1999):[5]

Stage Features
M0 no hematogeneous spread and extrathoracic lymph nodes with the exception of the supraclavicular nodes
M1 hematogeneous spread and/or extrathoracic lymph nodes

IHC

  • p63 +ve.[6]
  • TdT +ve.
  • Ki-67 variable.[7]
    • ~5-70% for A, AB & B1.
    • ~80-100% for B2 & B3.

A panel:

  • TdT, CD1a, CD3, CD5, CD20, Ki-67, CD117, p63, CK5/6.

Sign out

A. Lymph Node, Station 6, Lymphadenectomy:
- One benign lymph node (0/1).

B. Submitted as "Anterior Mediastinal Tumour (Thymus)", Excision:
- Thymoma, WHO type B2.
- Modified Masaoka stage IIa.
- Three benign lymph nodes (0/3).
- Rim of benign thymus.
- Please see synoptic report.

See also

References

  1. ↑ Mills, Stacey E; Carter, Darryl; Greenson, Joel K; Oberman, Harold A; Reuter, Victor E (2004). Sternberg's Diagnostic Surgical Pathology (4th ed.). Lippincott Williams & Wilkins. pp. 1264. ISBN 978-0781740517.
  2. ↑ Masaoka, A.; Monden, Y.; Nakahara, K.; Tanioka, T. (Dec 1981). "Follow-up study of thymomas with special reference to their clinical stages.". Cancer 48 (11): 2485-92. PMID 7296496.
  3. ↑ Koga, K.; Matsuno, Y.; Noguchi, M.; Mukai, K.; Asamura, H.; Goya, T.; Shimosato, Y. (May 1994). "A review of 79 thymomas: modification of staging system and reappraisal of conventional division into invasive and non-invasive thymoma.". Pathol Int 44 (5): 359-67. PMID 8044305.
  4. ↑ Butnor KJ et al. Thymus. Version 3.1.0.0. 2011. URL: www.cap.org/cancerprotocols. Accessed on: 31 August 2015.
  5. ↑ 5.0 5.1 5.2 Masaoka, A.; Yamakawa, Y.; Fujii, Y. (Mar 1999). "Well-differentiated thymic carcinoma: is it thymic carcinoma or not?". J Thorac Cardiovasc Surg 117 (3): 628-30. PMID 10047676.
  6. ↑ Adam P, Hakroush S, Hofmann I, Reidenbach S, Marx A, Ströbel P (June 2014). "Thymoma with loss of keratin expression (and giant cells): a potential diagnostic pitfall". Virchows Arch.. doi:10.1007/s00428-014-1606-6. PMID 24923897.
  7. ↑ Viti, A.; Bertolaccini, L.; Cavallo, A.; Fortunato, M.; Bianchi, A.; Terzi, A. (Sep 2014). "18-Fluorine fluorodeoxyglucose positron emission tomography in the pretreatment evaluation of thymic epithelial neoplasms: a metabolic biopsy confirmed by Ki-67 expression.". Eur J Cardiothorac Surg 46 (3): 369-74; discussion 374. doi:10.1093/ejcts/ezu030. PMID 24585679.
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