Coexistence of papillary and medullary carcinoma of the thyroid gland-mixed or collision tumour? Clinicopathological analysis of three cases.

We present three thyroid carcinomas displaying medullary and papillary components. In two cases the papillary component was characterized by typical papillae with a fibrovascular core; in one a follicular variant of papillary carcinoma was found. The papillary component was dominant in two and the medullary in one case. One tumour showed clear-cut borders between the two components, the others displayed an intermingled pattern. Both tumour components were seen in lymph node metastases with immunostaining with antibodies to calcitonin, chromogranin A, carcinoembryonic antigen, other neuroendocrine markers and thyroglobulin. At least two of our cases are true mixed carcinomas probably arising from a common stem cell.