Cholecystocolic fistula of Crohn disease mimics colon adenocarcinoma invasion of gallbladder on F-18 FDG PET/CT.

About one-third of patients with Crohn disease develop intra-abdominal fistulas, and of these, cholecystocolic fistulas are among the rarest. F-18 FDG PET/CT can detect a vast variety of fistulas, although most are discovered incidentally. We present a very unusual case of a 79-year-old woman referred for an F-18 FDG PET/CT for evaluation of a solitary pulmonary nodule. A colon mass was detected incidentally that appeared to directly invade the gallbladder; however, histologic evaluation after surgery revealed a colon adenocarcinoma, with an adjacent cholecystocolic fistula arising from a transmural fissure ulcer in a nonneoplastic colon segment consistent with clinically silent Crohn disease.