Cytopathology of thymic epithelial neoplasms

A cytologic diagnosis of thymoma is extremely challenging. In part, this is because the tumor is uncommon and aspirates are infrequently encountered, a technically proficient interventional radiologist is needed, epithelial cells may be difficult to recognize in lymphoid rich aspirate smears, and there is inherent sampling error in a tumor that frequently displays heterogeneous histopathology. Critical to the cytologic diagnosis of most WHO Type B thymomas is the recognition of a distinct population of epithelial cells mixed with lymphocytes. This is more easily accomplished using Papanicolaou or H&E stains, and often requires a cytokeratin stain for verification (in the correct clinical-radiologic context) because these cells are cytologically bland and have a varying amount of cohesiveness. WHO Type A thymoma may contain only epithelial cells and thus mimic a spindle cell neoplasm, or mesothelial cell clusters. Limitations of the cytologic method include an unproven ability to definitively separate thymoma into specific WHO subtypes using cytology alone, and to determine capsular invasion. Non-neuroendocrine thymic carcinomas mimic their extra-thymic counterparts in cytologic aspirates, and their malignant nature is usually readily recognizable. Thymic neuroendocrine carcinomas (NEC) are also cytologically identical to their more common pulmonary sites of origin, but identification of moderately-differentiated NEC is generally not possible.