Malignant melanoma arising in melanin-producing medullary thyroid carcinoma

• A case of malignant melanoma arising in medullary thyroid carcinoma is described.
• Malignant melanoma cells expressed Melan-A, HMB-45, and S-100 protein.
• Contrary to expectation, malignant melanoma arising in MTC may have a favorable prognosis.

IntroductionWe report a case of malignant melanoma arising in medullary thyroid carcinoma that has not yet been described.Presentation of caseA 66-year-old woman presented with a mass in her thyroid. The resected mass was black in color, and was composed of a mixture of classic medullary thyroid carcinoma and pleomorphic atypical cells containing melanin pigments. The pleomorphic atypical cells were morphologically consistent with malignant melanoma, and expressed Melan-A, HMB-45, and S-100 protein as determined by immunohistochemistry. Some of these cells were also positive for calcitonin and chromogranin A. Although the malignant melanoma metastasized to the lymph nodes, the patient remained free from local recurrence and distant metastasis and the primary malignant melanoma lesion was not identified for up to 11 years after the thyroidectomy.Discussion11 melanin-producing MTC cases have been reported to date. In the reported cases, the term “malignant melanoma” was not used, likely because the melanin-containing carcinoma cells were not morphologically consistent with malignant melanoma, but with medullary carcinoma.ConclusionMalignant melanoma arising in MTC may have a favorable prognosis.