Isolated metastasis of malignant melanoma into follicular carcinoma of the thyroid gland.

A 38-yr-old woman with a history of malignant melanoma (MM) presented with a thyroidal nodule. Fine needle aspiration biopsy of the thyroid was consistent with metastatic MM. The patient underwent thyroidectomy: microscopic examination revealed a follicular carcinoma nodule harboring a focus of metastatic melanoma. On review of the fine needle aspiration biopsy specimen, the population of cells with more uniform nuclei with focal follicle formation, which initially was interpreted as cells originating from normal thyroid tissue, was seen to actually represent the follicular carcinoma component. Tumor-to-tumor metastasis is an interesting phenomenon and there are only few cases of MM metastasis to other tumors. MM metastasis into a neoplastic thyroid nodule is a very rare combination and may be explained because the nodule in question represents the most highly vascularized component of the thyroid.