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Surgical Pathology Criteria

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  • Thymic epithelial cell neoplasm lacking overtly malignant cytologic features

Diagnostic Criteria

  • The currently most widely used classification is that of the WHO 2004
    • It is important to realize that this classification is not of great clinical value
      • Stage is far more important clinically
      • Both aggresive and innocuous behavior may be seen with all types
      • The main value of defining types is to aid in the recognition of lesions with a variety of appearances as thymomas
      • Mixtures are common (Moran 2012) and difficult to classify tumors are not rare
    • A number of other classifications have been used or proposed
  • See individual entries for each type by clicking on the links below
  • Type A
    • Thymoma composed of bland spindle to oval cells with few to no lymphocytes
  • Type AB
    • Spindle cell thymoma with a mixture of lymphocyte-poor and lymphocyte rich areas
  • Type B1
    • Thymoma overwhelmingly composed of immature T cells, exhibiting areas reminiscent of both normal thymic cortex and medulla
  • Type B2
    • Thymoma with prominent large polygonal epithelial cells with a roughly even admixture of numerous lymphocytes
  • Type B3
    • Thymoma composed of sheets of polygonal epithelial cells with mild to moderate atypia and scant lymphocytes
  • Micronodular Thymoma with Lymphoid Stroma
    • Thymoma composed of multiple epithelial nodules surrounded by prominent lymphoid stroma containing mature B and T cells and devoid of epithelial cells
  • Metaplastic Thymoma
    • Thymoma composed of alternating areas of epithelial cells and bland slender spindle cells
  • Combined Thymoma
    • A combination of thymoma and thymic carcinoma
    • A thymoma exhibiting more than one thymoma type pattern may be designated as composite or mixed
  • Malignant Thymoma
    • Generally used to refer to any of the above thymoma types that is behaving aggressively
    • Thymic carcinoma is the preferred term for a cytologically malignant epithelial thymic neoplasm
    • The term "Malignant Thymoma" is confusing and should not be used
      • "Invasive Thymoma" is better
  • Extensive hyalinization and sclerosis may obscure the pattern of the thymoma
  • Microthymoma refers to a small thymoma while Microscopic Thymoma refers to Nodular Hyperplasia of Thymic Epithelium


  • Staging (see menu at left) is much more important than type of thymoma

Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting/last update: 2/21/08,7/8/12, 12/1/13


Thymoma Staging

  • There is no universally accepted staging scheme for thymomas
    • Staging is best discussed in the report comment and not simply reported as a score

TNM for thymomas has been proposed (based on Masaoka scheme):

pT1 Completely encapsulated
pT2 Invades through capsule into fat or normal thymus, but not through pleura or pericardium (may be adherent) or into adjacent organs
pT3 Invades through pleura or pericardium or into adjacent organs (great vessels, lung)
pT4 Pleural or pericardial implants
pN1 Metastasis only to anterior mediastinal nodes
pN2 Metastasis to other intrathoracic nodes
pN3 Metastasis to extrathoracic nodes
pM1 With distant metastases
  • From Tsuchiya et al. Pathol Int 1994; 44:506
  • There is no significant difference in disease free or overall survivall between pT1 and pT2 thymomas (Gupta et al. Arch Pathol Lab Med. 2008 Jun;132(6):926-30.)
    • They suggested collapsing pT1 and pT2 into one but unfortunately did not actually propose a staging system

Moran (2012) has proposed a system that reflects the indolent nature of completely resected pT1 and pT2 thymomas better than does the Masaoka based TNM system above

Stage Features
0 Completely encapsulated
1 Invasive into perithymic fat
2 Direct invasion into
A) Innominate vein, mediastinal pleura, lung
B) Pericardium
C) Great vessels, heart
3 Metastatic disease
A) Intrathoracic structures, diaphragm, lymph nodes
B) Extrathoracic invasion
No data was supplied to justify the substaging of stages 2 and 3
No provision was made for pleural or pericardial dissemination


Thymoma Classification/Lists

Intrinsic Tumors of the Thymus

Non-Thymic Neoplasms that May Arise Primarily in the Thymus

  • Hematopoietic
    • Primary mediastinal large B cell lymphoma
    • Extranodal marginal zone lymphoma
    • Precursor T lymphoblastic lymphoma/leukemia
    • Hodgkin lymphoma
  • Germ cell tumors
    • Seminoma (Germinoma)
    • Embryonal carcinoma
    • Yolk sac tumor
    • Teratoma
    • Choriocarcinoma
    • With associated somatic type malignancy
      • Embryonal rhabdomyosarcoma
      • Angiosarcoma
      • Others
    • With associated hematologic malignancy
      • Acute myelogenous leukemia
      • Acute megakaryoblastic leukemia
      • Others

Miscellaneous Thymic Lesions and Conditions


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