S3995 A Pain in the Rectum—But Not From the Colon

Introduction: Rectal bleeding is a common gastrointestinal complaint, frequently attributed to benign anorectal disorders or primary colorectal malignancies. However, rare metastatic lesions, such as those from prostate adenocarcinoma, may masquerade as primary gastrointestinal pathology, posing diagnostic challenges. Case Description/Methods: A 65-year-old man with a history of high-grade prostate adenocarcinoma (Gleason 4 + 5, cT2a cNx cMx, diagnosed in 2016) presented with recurrent hematochezia, dyspnea, and fatigue. He had previously undergone intensity-modulated radiation therapy (IMRT) and androgen deprivation therapy (ADT); surgical resectomy was not attempted due to progression of disease. He later developed osseous metastases and was transitioned to enzalutamide after abiraterone intolerance. His history was further complicated by pulmonary embolism and bilateral deep vein thromboses, managed with rivaroxaban. On presentation, the patient was tachycardic and hypoxic with a hemoglobin of 6.3 g/dL. Initial labs revealed an unchanged PSA, which did not reflect the current metastatic burden. Differential diagnoses included radiation proctitis, primary rectal carcinoma, and ischemic colitis. Contrast-enhanced CT showed circumferential thickening of the distal rectum with mucosal irregularity, but no discrete mass. Colonoscopy revealed extensive mucosal ulceration without obvious neoplastic growth. Biopsy demonstrated poorly differentiated adenocarcinoma. Immunohistochemical staining was positive for PSA, NKX3.1, AE1/AE3, and CAM 5.2, confirming a prostatic origin. Given his poor functional status and widespread metastases, further radiotherapy was deferred. Hematochezia resolved spontaneously, and he was managed conservatively with close outpatient follow-up. Discussion: This case underscores the need for gastroenterologists to consider metastatic disease in the differential diagnosis of hematochezia, particularly in man patients with prior pelvic malignancy and radiation. Prostatic metastasis to the rectum is exceedingly rare but can closely mimic primary colorectal cancer both endoscopically and histologically. Endoscopic findings may lack a discrete mass, adding to diagnostic ambiguity. Immunohistochemistry is essential for accurate diagnosis and appropriate management. This case also highlights the value of multidisciplinary collaboration between gastroenterology, oncology, and pathology in complex diagnostic scenarios.