CONCURRENT MEDULLARY AND PAPILLARY CARCINOMAS OF THYROID WITH LYMPH-NODE METASTASES - A COLLISION PHENOMENON

The simultaneous occurrence of two distinct neoplasms derived from dif ferent cells of origin is a recognized, albeit rare, entity, In the th yroid, such lesions could consist of medullary carcinoma composed of p arafollicular C cells and well-differentiated carcinoma showing follic ular epithelial cell differentiation. We report a patient whose thyroi d contained calcitonin-immunoreactive medullary carcinoma and thyroglo bulin-positive papillary carcinoma, clearly separated from each other. The tumors metastasized to regional lymph nodes, where they formed fo ci of composite medullary and papillary carcinoma, with each component maintaining a distinct immunophenotypic profile. The composite metast ases are best regarded as collision tumors, as each primary neoplasm e xhibited only one line of differentiation. Given the high incidence of papillary carcinoma, the occurrence of the two tumors may be a coinci dence. Alternatively, a common tumorigenic stimulus triggering neoplas tic transformation of both parafollicular C cells and follicular epith elial cells is a plausible explanation for such a phenomenon.