Metastatic small cell carcinoma to the thyroid gland: a pathologic and molecular study demonstrating the origin in the urinary bladder.

Small cell carcinomas may occur in the thyroid gland. Infrequently, they are primary tumors, and have been interpreted as variants of medullary thyroid carcinoma. However, the vast majority of small cell carcinomas involving the thyroid gland are metastatic tumors. In some cases, demonstration of the primary tumor is not easy. An example of a small cell carcinoma metastatic to the thyroid is presented in this report. The primary tumor was a small cell carcinoma that occurred as a minor component in a transitional carcinoma of the urinary bladder. The microscopical and immunohistochemical features of both tumors, in the thyroid and the bladder, were identical. Moreover, both tumors exhibited an identical mutation in p53, as well as similar loss of heterozygosity at 10q23 and RASSF1A promoter hypermethylation, clearly indicating that the bladder tumor was the site for the primary tumor of the patient.