Mixed medullary and follicular carcinoma of the thyroid: report of two cases with an immunohistochemical study.

Two cases of mixed medullary and follicular carcinoma of the thyroid (MFC) and two cases of thyroid carcinoma resembling MFC are reported with a description of their histological and immunohistochemical features. Two cases of MFC with lymph node metastasis were histologically distinguishable from each other because one had a follicular structure filled with a thyroglobulin (TG)-positive colloid-like substance and the other did not have it. Although one of the thyroid carcinomas resembling MFC was similar to the case of MFC with a follicular structure in its primary lesion, it showed no lymph node metastasis. The metastatic lesion of the thyroid carcinoma resembling MFC consisted of TG-positive cells and neighboring calcitonin (CT)-positive cells. However the primary lesion exhibited the typical features of papillary carcinoma except for the presence of a small lesion which stained negatively for both TG and CT. The two types of tumor were not intermingled in a single tumor. These cases of thyroid carcinoma resembling MFC have a possibility of being MFC. However they should not be classified as MFC because lymph node metastasis or the coexistence of medullary carcinoma and follicular carcinoma in their primary lesion was not proved.