Fine needle aspiration in the diagnosis of thymic epithelial neoplasms.

Fine-needle aspiration (FNA) biopsy of thymoma is a demanding diagnostic exercise by the cytopathologist because of an overwhelming, often obscuring population of benign lymphocytes in many cases. Diagnosis requires the presence of a dual population of unequivocal epithelial cells and lymphocytes in the correct clinical-radiologic context. Cytologic examination alone is not insufficient to discriminate among the various subtypes of thymoma, nor can capsular invasion or invasion of adjacent structures be determined using FNA. The cytopathology of various thymic carcinomas (including neuroendocrine carcinoma) imitate their appearance in extra-thymic sites, and are generally recognizable using FNA. Separation of moderately differentiated neuroendocrine carcinoma from poorly differentiated small cell neuroendocrine carcinomas is generally not possible.