Poorly differentiated thyroid carcinoma with rhabdoid phenotype: a diagnostic dilemma--report of a rare case.

Rhabdoid tumor is a distinct entity reported in renal and extrarenal sites. Malignant tumors of various types may have rhabdoid phenotype. There are eight case reports of carcinomas of thyroid with rhabdoid phenotype. All of these cases, except two, have been reported in middle-aged women (42-72 yr) and have had an aggressive clinical course with death occurring within few months to 4 yr after diagnosis. We report a case of poorly differentiated carcinoma of the thyroid with rhabdoid phenotype in a 22-yr-old male. The rhabdoid cells were immunopositive for thyroid transcription factor-1, vimentin, epithelial membrane antigen, and focally for cytokeratin. Synaptophysin, chromogranin, thyroglobulin, carcinoembryonic antigen, smooth muscle actin, myogenin, and desmin were all negative. To the best of our knowledge this is the ninth case report of carcinoma of the thyroid with rhabdoid phenotype. This case, unlike the previous reported cases, has certain unusual features including presentation in a young male, the absence of either follicular/papillary differentiation, and immunohistochemical profile of the rhabdoid cells.