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Case 1 - Type B3 thymoma is composed of lobules of squamoid epithelial cells.

A sheetlike growth of epithelial cells with a relative paucity of lymphocytes are the defining features of type B3.

Mild to moderate atypia of the epithelial cells is a feature of Type B3.

Case 2 - In this case, the epithelial cells are again the predominant component, but these are darker and more spindled nuclei compared to the previous case.

Nuclear grooves are easy to see in the epithelial cells. Lymphocytes are a minor component and sprinkled throughout.


Type B thymomas under the WHO classification are subdivided into three subtypes designated B1, B2, and B3, on the basis of the proportional increase of the epithelial component and the emergence of

atypia of the neoplastic cells. Essentially, Type B3 thymomas are composed predominantly of epithelial cells admixed with a minor component of immature lymphocytes. Lobules are often separate by thick fibrous septae. The epithelial cells exhibit a range of morphologies.

In the majority of cases, the epithelial cells are squamoid, but do not contain intercellular bridges (Fletcher). In other cases, the cells may have raisinoid nuclei resembling koilocytes or exhibit large vesicular nuclei and prominent nucleoli (type B2 morphology). Spindled morphology similar to type A thymomas can also be present, however, mild atypia is present and this is not seen in type A. Lymphocytes are a small component of type B3 (Fletcher).

Type B3 thymoma has been formerly referred to as the well-differentiated thymic carcinoma in older classifications.


Resection and for higher stage cases, adjunctive chemo/radiotherapy may be used as well.


Intermediate malignancy between Type C and B1 largely related to intermediate level of invasiveness. It may be unresectable at the time of presentation and metastasizes are seen in up to 20%. Reported 10 year survival varies from 50-70% (AFIP fascicle).


Thymus : Thymoma, B2 type

Thymus : Thymoma, B1 type


Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 1326.

Last updated: 2011-09-18
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