Thymic neoplasia as represented by fine needle aspiration biopsy of anterior mediastinal masses. A practical approach to the differential diagnosis.

OBJECTIVE: To evaluate the cytomorphology of thymoma and to discuss the potential diagnostic pitfalls and a practical approach, which sometimes uses ancillary studies, in distinguishing various lesions of anterior mediastinal masses. STUDY
DESIGN: A review of 25 fine needle aspiration (FNA) specimens of anterior mediastinal masses of thymic origin from 24 patients.
RESULTS: The FNA diagnoses were thymoma in 16 aspirates, thymic/poorly differentiated carcinoma in 2, necrotic fluid/debris in 2, small lymphocytic proliferation in 2, hyperplastic thymic tissue in 1, benign cyst in 1, and scant tissue in 1. In 22 cases that had histologic confirmation, the diagnoses of 19 (86%), including 14 thymomas, concurred with the FNA diagnoses. Sampling error was responsible for the discrepancy in the remaining three cases. The cytologic evaluation of thymoma revealed a characteristic dual population of predominantly small lymphocytes and occasional large, atypical lymphocytes intimately admixed with relatively bland epithelial cells. Differential diagnoses of thymoma in the mediastinal aspirates included a variety of entities, depending on which component predominated.
CONCLUSION: FNA of anterior mediastinal thymic lesions generally yields adequate cellular tissue with distinct cytologic and immunophenotypic features that enable precise classification. Caution in interpretation should be taken when the FNA yields only necrotic fluid or scanty material or when the patient is younger than the typical age range for thymoma. When the FNA diagnosis is coupled with clinical and radiographic findings, a definitive diagnosis can be generally rendered without open biopsy.