Answer: Thymoma, WHO type B3

Histology: none provided

Discussion: The attached non-neoplastic thymus exhibits follicular hyperplasia and is consistent with the patient’s clinical history of myasthenia gravis. The majority of the tumor exhibits an even proportion of epithelial cells and lymphocytes (consistent with WHO type B2 / cortical thymoma; not shown), however, multiple foci of solid sheets of epithelial cells with no or mild atypia (WHO type B3 / epithelial thymoma / well-differentiated thymic carcinoma) are also present.

Immunohistochemistry demonstrates that the tumor cells are cytokeratin (AE1/AE3) and p63 positive, but CD5 and c-Kit (CD117) negative. The admixed small lymphocytes are CD99 positive. The tumor penetrates the capsule, infiltrates into thymic soft tissue and exhibits multiple pleural and pericardial implants.

This tumor demonstrates hybrid histology with both type B2 and B3 areas present. Immnohistochemistry can aid in the diagnosis of thymomas by highlighting the epithelial component (cytokeratin and p63) and demonstrating the presence of CD99 positive lymphocytes. The latter can be helpful in differentiating between thymomas and thymic carcinomas which generally lack CD99 positive cells in the background. Another stain that can be helpful in differentiating between thymic carcinoma and thymoma is c-kit (CD117), with the majority of carcinomas expressing c-kit in contrast to thymomas that generally are negative. Since the majority of thymic carcinomas express CD5 this marker can be used to confirm thymic origin in cases where the origin of the primary tumor is not known. The majority of patients with myasthenia gravis (65%) have thymic hyperplasia, a minority (10%) will develop thymoma, whereas 25% will have no microscopic abnormality.

In summary, the tumor shows both type B2 and B3 histology, lacks significant cytologic atypia and has an immunohistochemical phenotype that is consistent with a thymoma.