Medullary carcinoma of the thyroid gland: an encapsulated variant resembling the hyalinizing trabecular (paraganglioma-like) adenoma of thyroid.

Two encapsulated medullary carcinomas of the thyroid gland exhibiting an organoid and trabecular pattern with hyalinized stroma and composed of polyhedral to spindled tumor cells with hyaline-appearing cytoplasm are described and compared with three histologically indistinguishable adenomas of follicular cell derivation. Positive immunocytochemical staining for calcitonin and negative staining for thyroglobulin confirms the diagnosis of medullary carcinoma in these tumors, while positive staining for thyroglobulin confirms the follicular cell nature of the adenomas. The distinction between these tumor types is a crucial one, given the prognostic and potential genetic implications of medullary carcinoma.