Cap Polyposis Presenting with Large Bowel Obstruction Mimicking Advanced Colorectal Cancer: A Case Report.

INTRODUCTION: Cap polyposis is a rare, benign inflammatory colorectal disease that usually presents with diarrhea, mucous stool, or abdominal discomfort. Presentation with large bowel obstruction requiring surgical intervention is extremely uncommon. CASE PRESENTATION: A 42-year-old Japanese man presented with anal pain. Colonoscopy revealed a circumferential stenotic lesion in the sigmoid colon that prevented passage of the endoscope, and advanced colorectal cancer causing bowel obstruction was strongly suspected. A transanal ileus tube was inserted for decompression, followed by elective laparoscopic low anterior resection with a temporary diverting ileostomy. Histopathological examination unexpectedly demonstrated cap polyposis, characterized by elongated and distorted crypts, mucosal erosion, and fibrinopurulent exudates forming cap-like structures. The patient was also positive for Helicobacter pylori infection and underwent successful eradication therapy after surgery and remained recurrence-free during 18 months of follow-up. CONCLUSIONS: Cap polyposis presenting with bowel obstruction is extremely rare and may mimic advanced colorectal cancer. Surgical intervention can be clinically justified in cases where malignancy cannot be excluded preoperatively.