A rare malignant thyroid carcinosarcoma with aggressive behavior and DICER1 gene mutation: a case report with literature review.

Background: Malignant biphasic tumor also known as carcinosarcoma is an uncommon neoplasm that is composed of both malignant epithelial and mesenchymal components. Most reported cases of carcinosarcoma affect the female genital tract; however, other sites including head and neck, lung, and breast have been described. Carcinosarcoma of the thyroid is an extremely rare and aggressive malignancy with an ominous clinical course similar to anaplastic carcinoma.

Case presentation: We report a case of a 45-year-old female who was found to have a biphasic thyroid carcinosarcoma. Her clinical course declined significantly shortly after she underwent a total thyroidectomy and she developed distant metastases to the lungs. Histopathological features of the primary and metastatic tumor were identical. The tumor is composed of an intimately intermixed epithelial component of poorly differentiated follicular thyroid carcinoma and a spindle cell sarcoma with rhabdomyosarcoma differentiation. Molecular analysis using a next-generation sequencing based assay revealed a DICER1 (E1705K) point mutation in neoplastic cells.

Conclusion: To our knowledge, the E1705K point mutation within the DICER1 gene is the first reported mutation in carcinosarcoma of the thyroid. A comprehensive review of the relevant literature is also included for discussion.