Rectal diffuse large B-cell lymphoma misdiagnosed as bleeding cancer in an elderly patient.

BACKGROUND: Primary rectal diffuse large B-cell lymphoma (DLBCL) is a rare malignancy that often closely mimics more common rectal pathologies such as adenocarcinoma or inflammatory bowel disease. This resemblance poses considerable diagnostic difficulty, and misdiagnosis may result in delayed treatment and unnecessary procedures, ultimately affecting patient outcomes. CASE PRESENTATION: An 87-year-old female presented with intermittent hematochezia that acutely worsened two hours before admission. Physical examination identified a rectal mass located 5 cm from the anal verge. Pelvic imaging and colonoscopy were suggestive of hemorrhagic adenocarcinoma. Given the patient’s advanced age, significant anemia, and elevated bleeding risk, a multidisciplinary team decided against preoperative biopsy. Instead, transanal minimally invasive surgery was carried out for local disease control. Final histopathological analysis of the resection specimen revealed DLBCL, germinal center B-cell-like subtype. Following surgery, the patient was referred to hematology for guideline-directed chemotherapy and remained free of recurrence during follow-up. CONCLUSION: This case highlights that rectal DLBCL can present with hematochezia as the sole symptom and may be mistaken for rectal adenocarcinoma, particularly in elderly patients. The divergence between the preoperative clinical impression and the postoperative pathological diagnosis emphasizes both the diagnostic challenge and the tailored therapeutic approach required for this rare condition. A high index of suspicion for uncommon malignancies is crucial to ensure appropriate clinical management.