Collision tumor of a primary colonic adenocarcinoma and a pancreatic ductal adenocarcinoma.

Collison tumors are rare entities characterized by the juxtaposition of two histological distinct neoplastic populations in the same organ without intermixture of the different malignant components. Due to their infrequency, especially in the digestive tract, limited prior cases have been reported and the pathogenesis, biological behavior and treatment remain controversial. We present the case of a 79-year-old woman with a solid pancreatic mass and a rectosigmoid wall thickening, accompanied by multiple hepatic lesions. These findings suggested a pancreatic neoplasm with liver metastasis and a probable synchronous colonic tumor. Surprisingly, the histological study of surgical rectosigmoid resection showed a collision tumor comprised of two independent components: a primary colonic adenocarcinoma and a pancreatic ductal adenocarcinoma with one lymph node metastasis of the intestinal component. Immunohistochemical and molecular studies, confirmed the histomorphological findings. This case outlines the effectiveness of an exhaustive pathological sampling and the importance of the histological, immunohistochemical and molecular studies to increase diagnostic accuracy. To the best of our knowledge, this is the first reported case of collision between these two types of adenocarcinomas in the rectosigmoid region. We highlight the importance of reporting this entity, in order to contribute to the available literature.