An unusual case of colonic mixed adenoendocrine carcinoma: collision versus composite tumor. A case report and review of the literature.

We report a case of collision tumor of the cecum in a 50-year-old woman and discuss the morphological criteria used for classifying mixed glandular-endocrine neoplasms of the digestive tract in a large series of reported cases. The cecal tumor showed clear geographic polarization, with no admixture or transition of the 2 neoplastic components. A lymph nodal metastatic deposit contained both tumors. Immunohistochemical stainings were consistent with the carcinomatous and carcinoidal nature of the 2 neoplasms, except for a minute area of carcinoembryonic antigen positivity in the carcinoid. A review of the literature shows that many mixed glandular-endocrine tumors of the digestive tract escape precise classification using established morphological criteria. Collision and composite tumors can share common histological and immunohistochemical features, including polyclonal metastases and carcinoembryonic antigen expression in the carcinoidal area. Mixed tumors of the cecum showing a collision-like morphology appear to behave more aggressively than other colonic adenoendocrine carcinomas.