Thymoma
Definition
- 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
- It is important to realize that this classification is not of great clinical value
- 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
- 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
