Conundrum of a Large Bowel Neoplasm: Collision Tumor.

A 79-year-old Hispanic man was admitted to the intensive care unit with symptomatic iron-deficiency anemia and watery diarrhea. Radiological images revealed diffuse colonic wall thickening, a soft-tissue fullness in the ascending colon, and multiple mesenteric lymphadenopathies. Colonoscopy showed multiple aphthous ulcers throughout the colon and a large deep ulcer with irregular raised borders in the rectosigmoid area. Histological exam of the ulcers showed severe ulcerative colitis, while biopsy of the deep ulcer revealed a well-differentiated adenocarcinoma. Colectomy specimen was consistent with colliding diffuse large B-cell lymphoma and adenocarcinoma.