Collision tumor: a colonic adenocarcinoma and a gastric adenocarcinoma.

Background: Collision tumors are defined as the juxtaposition of two histologically distinct tumors, which form a single mass, without histological mixing of the two tissues. They are extremely rare, especially in the digestive tract.

Clinical Presentation: An 81-year-old patient was admitted for anemia, diarrhea and palpable abdominal mass. The colonoscopy showed a substenosing tumor of the transverse colon. On abdominal CT, the colonic mass invaded the stomach. After surgical treatment with transverse colectomy and associated partial longitudinal gastrectomy, pathological results showed that it was a collision between a primary gastric adenocarcinoma and a primary colonic adenocarcinoma.

Conclusion: We report a first case of collision tumor between a primary gastric adenocarcinoma and a primary colonic adenocarcinoma. Current knowledge on the subject is extremely limited.