Collision tumor consisting of a colorectal adenocarcinoma and dissemination of a gastric adenocarcinoma

Background: Collision tumors, composed of histologically distinct tumor types, are rare entities, especially in the colorectum, and corresponding evidence-based clinical management or treatment strategies are poorly defined. This is the first report of a collision tumor composed of two histologically distinct adenocarcinomas. Case presentation: A 78-year-old male showed severe anemia and a 10% body weight loss over 1 month. Preoperative examination revealed T3N1M0 stage IIIA gastric cancer and T3N0M0 stage IIA rectal cancer. Distal gastrectomy and rectectomy with regional lymph node dissection were performed. Immunohistochemistry revealed two distinct adenocarcinomas with gland duct structures – a colorectal adenocarcinoma and a disseminated gastric adenocarcinoma – that had collided to form an invasive tumor on the serosal surface of the anterior rectum wall. Conclusion: This extremely rare case of a collision tumor supports that precise immunohistochemical identification of all tumor components is needed for guiding decisions affecting overall prognosis, adjuvant treatment and survival