Incidental Colo–Urachal Fistula on FDG PET/CT in Rectosigmoid Cancer: A Rare Finding

A 44-year-old man with a history of inflammatory bowel disease (IBD) presented with bleeding per rectum and pneumaturia. Colonoscopy revealed an ulceroproliferative rectosigmoid growth. Ultrasonography showed normal kidneys and prostate, and laboratory parameters did not indicate urinary infection. Final histopathology revealed a moderately differentiated adenocarcinoma not otherwise specified (NOS).

¹⁸ F fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) performed for staging demonstrated FDG-avid circumferential wall thickening involving the sigmoid colon and upper rectum with luminal narrowing and perilesional fat stranding. Additionally, a linear FDG-avid tract was incidentally noted extending from the sigmoid colon to a persistent urachal remnant at the bladder dome consistent with a colo–urachal fistula explaining the patient’s pneumaturia. No per-rectal contrast was administered during the study and fistulous communication was identified on non-contrast-enhanced CT components.