Carcinomas metastatic to follicular adenomas of the thyroid gland. Report of two cases.

Metastases to the thyroid gland are an uncommon occurrence, and metastasis to a preexisting thyroid neoplasm is even more rare. We report two cases of tumor-to-tumor metastasis where a prostatic and a breast carcinoma metastasized to follicular adenoma of the thyroid gland. The metastatic process in case 1 was initially diagnosed by fine-needle aspiration biopsy and later confirmed with the hemithyroidectomy. Immunostaining for prostate-specific antigen and prostatic acid phosphatase in case 1 and estrogen and progesterone receptors in case 2 demonstrated strong immunoreactivity in the metastatic tumor cells. Flow cytometric DNA analysis of the primary and the metastatic tumors in both cases demonstrated stemline fidelity that supported their association. Our cases exemplify why attention should be given to the possibility of metastasis when distinctly different morphologic features are seen in an otherwise typical tumor and the utility of ancillary tests that may assist in establishing the diagnosis.