S4106 Missed by Non-Invasive Screening: A Case of Obstructing Rectal Adenocarcinoma Highlighting the Critical Role of Colonoscopy in Early Detection

Introduction: Rectal cancer is a significant cause of gastrointestinal morbidity, often presenting with nonspecific symptoms that can delay diagnosis. This case report highlights the diagnostic challenges and management of a 77-year-old man with alternating diarrhea and constipation, whose malignancy was not detected by prior non-invasive screening methods, ultimately leading to the discovery of an obstructing rectal adenocarcinoma during a prolonged hospital stay. Case Description/Methods: A 77-year-old man with a history of duodenal ulcer presented with a 6-month history of alternating diarrhea and constipation associated with rectal discomfort and pain. He had regular non-invasive colon cancer screening with fecal immunochemical test but reported no prior colonoscopies. Initial computed tomography imaging revealed moderate to large stool burden in the colon and rectum but no significant abnormalities. After admission, the patient was managed with a bowel regimen, which initially improved his symptoms. However, rectal pain persisted 5 days post-admission, which was prolonged due to a comorbidity with Afib, and diarrhea recurred despite conservative management. Gastroenterology consultation suggested possible irritable bowel syndrome or overflow diarrhea. Despite symptomatic management, his condition failed to improve. A colonoscopy revealed a large, fungating, ulcerated, circumferential rectal mass obstructing the lumen at mid to distal rectum. Biopsies confirmed invasive rectal adenocarcinoma. Imaging with computed tomography and magnetic resonance imaging was initiated for staging, and a diverting loop colostomy was planned. Following multidisciplinary discussions, the patient initially deferred surgical intervention in favor of colonic stenting. However, advanced endoscopy deemed stenting inappropriate due to expected persistent pain. The patient subsequently underwent a successful diverting loop colostomy and was discharged 3 days postoperatively. He was referred for outpatient oncology follow-up for further management, including palliative and oncologic care. Discussion: Rectal adenocarcinoma can present with vague gastrointestinal symptoms that mimic benign conditions, delaying diagnosis and treatment. This case underscores the limitations of non-invasive CRC screening tools, including the fecal immunochemical test, which may fail to detect certain malignancies, especially those in advanced stages. Despite the increasing availability of alternative screening methods, colonoscopy remains the gold standard for detecting and diagnosing colorectal malignancies.