Histologic features, growth patterns and classification of atypical thymomas.

Atypical thymomas are a rare form of primary thymic epithelial neoplasm that are characterized by conservation of most of the organotypical features of thymic differentiation, but show atypical cytological features. The spectrum of histologic features and growth patterns these tumors can exhibit have not been extensively documented or illustrated in the literature. The basic histologic growth patterns seen in atypical thymomas include the epithelioid or "Squamoid" subtype and the spindle cell subtype. The histologic picture may often be confused with thymic carcinomas or metastatic disease, in particular squamous cell carcinoma due to the overlap in histologic features. In addition, these subtypes may be seen in combination with each other or in combination with conventional types of thymoma including type A, type AB, type B1 and type B2. Cases of thymic carcinoma arising from atypical thymoma have also been documented in the literature. The biologic behavior of atypical thymomas is intermediate between conventional thymoma and thymic carcinoma. The tumors tend to present with aggressive behavior and an increased rate of metastasis and thus achieving the correct diagnosis is of utmost importance. We present a review of the various morphologic appearances of these tumors to emphasize the wide spectrum of histologic features that they can display, with a discussion of the current nomenclature and approach to these neoplasms.