Thyroid plasmacytoma mimicking medullary carcinoma: a potential pitfall in aspiration cytology.

Extramedullary plasmacytoma (EMP), a malignant neoplasm of the soft tissues composed of plasma cells, may occur either as a solitary plasma-cell tumor (primary EMP) or as a manifestation of multiple myeloma (secondary EMP). We report on the aspiration cytology findings of an intriguing case of EMP of the thyroid that was initially misinterpreted as medullary carcinoma on clinical and cytologic examination. A major contributing cause for the cytologic misinterpretation was the presence of amyloid in the aspirate and the clinical impression of a "thyroid mass." Based on this experience, we suggest that EMP should be considered in the differential diagnosis of a neck mass that yields discohesive cells associated with amyloid/amyloid-like material. Clinical correlation and immunocytochemistry are crucial in avoiding pitfalls in this situation.