Medullary thyroid carcinoma with a paraganglioma-like pattern and melanin production: a case report with ultrastructural and immunohistochemical studies.

We report a case of medullary thyroid carcinoma with a paraganglioma-like pattern and melanin production. Macroscopically, a rectangular black area within a gray-white background was seen on the cut surface of the tumor. Histologically, the tumor was found to have a trabecular or nestlike architecture with many pigmented dendritic cells, resembling the "sustentacular cells" of paraganglioma, scattered among nonpigmented polygonal tumor cells. Fontana-Masson staining and bleaching with potassium permanganate and oxalic acid revealed that this pigment was melanin, which was also confirmed by electron microscopy. Immunohistochemically, pigmented dendritic cells were positive for calcitonin, as were the surrounding typical medullary thyroid carcinoma cells. These cells were also positive for S100 protein but not for HMB-45, compatible with sustentacular cells. These findings suggest that this tumor could be considered to bridge the gap between two variants of medullary thyroid carcinoma, the melanin-producing (pigmented) type and the paraganglioma-like type.