Synchronous Multiple Primary Cancers Involving Rectal Cancer and Diffuse Large B-cell Lymphoma of the Right Colon: A Case Report.

Colorectal cancer is a common malignant tumor of the digestive tract, but cases concurrently complicated with lymphoma are rare. This article reports a unique case of rectal adenocarcinoma combined with diffuse large B-cell lymphoma (DLBCL) in the right colon. An 82-year-old male was admitted to the hospital due to irregular bowel movements accompanied by abdominal pain. Abdominal CT and colonoscopy suggested a malignant tumor in the rectum. During laparoscopic anterior resection of the rectum, a firm mass measuring approximately 6 cm × 5 cm was observed in the mesentery posterior to the descending and horizontal segments of the duodenum, with unclear boundaries from the ileocecal region and ascending colon. Consequently, an open right hemicolectomy was simultaneously performed. Postoperative pathology revealed well-differentiated adenocarcinoma in the rectum, while the right colon tumor and intestinal segment, combined with immunohistochemistry, were consistent with diffuse large B-cell lymphoma (not otherwise specified (NOS)) of non-germinal center origin. This case provides important clinical insights: for patients with colorectal tumors, the possibility of multiple primary cancers should be considered, especially when symptoms and imaging findings are inconsistent. Multisite biopsies and molecular pathological analysis should be employed to reduce misdiagnosis.