Fine needle aspiration cytology of mixed medullary-follicular thyroid carcinoma - A case report

Background Mixed medullary-follicular thyroid carcinoma (MMFTC) is a rare tumor that ha's been regarded as a clinicopathologic variant of medullary thyroid carcinoma. MMFTC represents a diagnostic challenge by fine needle aspiration cytology (FNAC). Case A 77-year-old woman bad a palpable mass on the left side of the neck. It was diagnosed as follicular neoplasm by FNAC; she underwent total thyroidectomy. Pathology revealed follicular carcinoma. Radioactive iodine was administered. An enlarging mass was present in the left mandible later. FNAC showed suspicious follicular neoplasm with predominance of oncocytic cells. Pathology revealed follicular carcinoma with parafollicular cell differentiation. Immunobistochemical analysis demonstrated positive status for thyroglobulin and calcitonin. Simultaneous expression of thyroglobulin and calcitonin within the same neoplastic cell was considered. She underwent several courses of radioactive iodine therapy without significant effect. Interestingly, her serum calcitonin level was not elevated. Conclusion Coexpression of thyroglobulin and calcitonin in the same cell is very rare. The component of medullary carcinoma should be considered when encountering an atypical thyroid carcinoma with predominance of cells showing oncocytic changes on FNAC and with clinically poor response to conventional treatment. Immunohistochemistry and pathologic analyses are helpful to confirm the diagnosis, especially in the absence of elevated serum calcitonin level.