Rhabdoid tumor of the thyroid gland: a variant of anaplastic carcinoma.

Rhabdoid tumor of the thyroid gland is a very rare neoplasm, characterized by significant metastatic potential. All of the 6 cases reported in the recent literature had poor outcomes. We report an additional case involving, to our knowledge, the oldest patient reported so far. A 67-year-old woman had a nodular goiter for all of her adult life and presented with a rapidly growing mass in the right lobe. Histologic examination showed a highly cellular neoplasm with a solid infiltrative growth pattern. Extracapsular invasion was evident. Rhabdoid cells were large, with abundant cytoplasm, eosinophilic inclusions, and eccentric nuclei containing distinct nucleoli. Immunohistochemistry identified vimentin, sarcomeric actin, myoglobin, and cytokeratin expression in the tumor cells; they were negative for desmin, thyroglobulin, and calcitonin. Scattered follicles with nuclear features of papillary thyroid carcinoma were detected; these cells were immunoreactive for thyroglobulin and TTF-1. Reverse transcriptase polymerase chain reaction using specific primers for RET/PTC1 and RET/PTC3 fusion genes identified a RET/PTC3 gene rearrangement in the rhabdoid tumor. Despite radiotherapy, the neoplasm rapidly progressed, with massive local and mediastinal metastasis leading to death 5 months after presentation. The hypothesis that rhabdoid tumor is a variant of anaplastic thyroid carcinoma is supported by the identification of a RET/PTC gene rearrangement, a feature of carcinomas of follicular cell derivation.