Atypical granular cell tumor of the thyroid: cytomorphologic features on fine needle aspiration.

Granular cell tumors are uncommon soft tissue neoplasms of nerve sheath origin, which are predominately benign and are characterized by abundant granular cytoplasm, uniform nuclei, and indistinct cell borders. Features of malignancy include spindle cell morphology, necrosis, prominent nucleoli, increased nuclear to cytoplasmic ratio, nuclear pleomorphism, and increased mitotic rate. Granular cell tumors are most common in the soft tissues of the head and neck, but have only been rarely described in the thyroid gland. Here we report a case of an atypical granular cell tumor of the thyroid seen on fine needle aspiration, which displayed focal atypical cells with spindle cell morphology, increased nuclear to cytoplasmic ratio, and prominent nucleoli. The differential diagnosis of such findings is also presented.