A case of tumor-to-tumor metastasis of cutaneous malignant melanoma.

We report a case of tumor-to-tumor metastasis of a cutaneous malignant melanoma to a synchronous thyroid Hurthle cell carcinoma. A 42-year-old male underwent a biopsy of right inguinal lymphadenopathy which showed metastatic melanoma. The primary lesion was identified on his right posterior leg, and staging workup discovered a synchronous left thyroid lobe nodule concerning for a follicular neoplasm. He underwent excision of the primary melanoma, right inguinal lymphadenectomy, and total thyroidectomy. The resected thyroid contained a 6.6-cm, well-encapsulated left-sided nodule, red-brown in color and homogenous in consistency, with areas of focal hemorrhage and no grossly identifiable calcification. Microscopically, large tumor cells with distinct cell borders were present, with deeply eosinophilic and granular cytoplasm, large nuclei with prominent nucleoli, and loss of polarity consistent with oncocytes. A microscopic single focus of vascular invasion was identified, and a diagnosis of angioinvasive Hurthle cell carcinoma was made. Within the Hurthle cell carcinoma, multiple deposits of metastatic melanoma were seen. These findings were indicative of tumor-to-tumor metastasis of the cutaneous melanoma to the angioinvasive Hurthle cell carcinoma. Our findings show the ability of melanoma to metastasize to a pre-existing neoplasm.