Cribriform-morular variant of papillary thyroid carcinoma displaying poorly differentiated features.

Cribriform-morular variant of papillary thyroid carcinoma (CMVPTC) usually occurs in the setting of familial adenomatous polyposis (FAP) although it can rarely arise sporadically. Poorly differentiated thyroid carcinoma (PDTC) is a follicular cell-derived neoplasm with more aggressive behavior than well-differentiated carcinomas such as CMVPTC. We report the case of a 35-year-old woman without FAP history who presented a left neck mass and complained of back pain. Imagiological examinations revealed a nodule in the left lobe of thyroid and multiple nodular lesions in the bone and lungs suggestive of metastases. The patient was submitted to total thyroidectomy and radioactive iodine. The tumor was composed of CMVPTC and PDTC components that shared the same somatic APC gene mutation (p.Cys520Tyr_fsX534). Besides this mutation, no CTNNB1, BRAF, N-RAS, and H-RAS gene mutations were detected in any of the 2 components. To the best of our knowledge, this is the first report of a sporadic CMVPTC with transformation into PDTC. Although the majority of CMVPTCs carry an indolent clinical outcome, the coexistence of poorly differentiated areas may justify the aggressiveness of the CMVPTC reported here.