A Rare Case of Ileoileal Fistula Concomitant with Adenocarcinoma in A Patient with Crohn's Disease

Introduction: Crohn's disease (CD) is a chronic inflammatory bowel disease that involves any segment of the gastrointestinal tract. Fistulas are clinically challenging complications of CD. Sustained chronic inflammation in the fistulas can lead to cancer. Case Presentation: A 42-year-old man diagnosed with ileocolonic CD at 18 years of age, had been treated with oral mesalazine. At 23 years of age, his computed tomography (CT) results revealed an internal fistula between the ileum and sigmoid colon and ileal-ileal adhesion, with no obvious fistula. No additional treatment was administered because the activity of the patient’s disease was mild. At 29 years of age, he underwent colonoscopy, which revealed narrowing of the terminal ileum due to sustained diarrhea and abdominal pain. Abdominal contrast-enhanced CT revealed ileocecal and ileoileal fistulas in addition to the existing ileosigmoid fistula. Therefore, ileocecal and ileosigmoid fistula resections were performed. Pathological examination revealed adenocarcinoma involving the ileoileal fistula. The pathological stage was pT4aNXM0, based on the Japanese Classification of Colorectal, Appendiceal, and Anal Carcinoma, 9th edition (2018). The patient received postoperative adjuvant chemotherapy, and no recurrence has been reported in the four years since surgery. Conclusion: To prevent complications such as fistulas in patients with CD, it is important to initiate early anti-inflammatory treatment. Furthermore, when managing long-term, poorly controlled internal fistulas, the potential for malignancy must be considered.