Mucoepidermoid carcinoma of the thyroid gland showing marked ciliation suggestive of its pathogenesis.

Mucoepidermoid carcinoma of the thyroid gland is a rare tumor first described by Rhatigan et al. in 1977. Its pathogenesis is still controversial. With regard to its most likely origin, some authors have suggested that it arises directly from follicular epithelium whereas others have proposed that it arises from ultimobranchial body (diverticulum from the fourth pharyngeal pouch) remnants, also known as solid cell nests (SCN). Herein is reported a unique case of thyroid mucoepidermoid carcinoma. The patient, a 67-year-old man, presented with a non-tender thyroid mass and vocal cord fixation. The tumor was poorly defined, necessitating subtotal thyroidectomy with composite resection of the adjacent structures. Pathologically, the tumor cells had characteristics of mucoepidermoid carcinoma, along with layers of columnar cells showing marked ciliation resembling respiratory-type epithelium, suggesting that this rare tumor had originated from SCN. p63 immunopositivity in the tumor provided additional evidence for the pathogenesis.