Thymoma

Definition

The currently most widely used classification is that of the WHO 2004
- 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
- Thymoma with a mixture of lymphocyte-poor and lymphocyte rich areas
Type B1
- Thymoma exhibiting areas reminiscent of both normal thymic cortex and medulla
Type B2
- Thymoma with prominent large epithelial cells with an admixture of numerous lymphocytes
Type B3
- Thymoma composed of sheets of epithelial cells with mild to moderat 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 thymoma exhibiting more than one thymoma type pattern, or
- A combination of thymoma and thymic carcinoma
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

Stage I	Grossly and microscopically encapsulated
Stage IIa	Microscopic transcapsular invasion into fat
Stage IIb	Macroscopic capsular invasion into fat
Stage III	Macroscopic invasion of neighboring organs
Stage IVa	Pleural or pericardial dissemination
Stage IVb	Hematogenous or lymphatic dissemination

Lymphatic metastases are rare
Hematogenous metastases are exceedingly rare
There is no significant difference in disease free or overall survivall between Stage I and II (Gupta et al.. Arch Pathol Lab Med. 2008 Jun;132(6):926-30.)

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

Original posting : February 21, 2008
Updates: July 3, 2008; May 10, 2009; June 20, 2009