Benign Intestinal Epithelization on Serosa Mimicking Stage IV Tumor Post Bowel Perforation in Colonic Adenocarcinoma Following Neoadjuvant Therapy

Differentiation between benign intestinal epithelium and neoplastic epithelium is critical in staging intestinal tumors, especially complicated colorectal cancer. We present a case of treated advanced colon adenocarcinoma in a 72-year- old female who was clinically diagnosed with colo-vaginal-ileal fistula formation and intraperitoneal carcinomatosis following neoadjuvant chemotherapy and vEGFR targeted therapy. She presented acutely with abdominal pain, was found to have bowel perforation on imaging and underwent total colectomy. Pathologic examination revealed rectosigmoid perforation and terminal ileum with fistula. Microscopic examination identified a small amount of residual adenocarcinoma in the rectosigmoid and no tumor in the remaining colon, ileum, fistula and perforated areas; instead, benign reactive intestinal epithelium and related mucosa were present on the serosa adjacent to the perforations, mimicking stage IV carcinoma and carcinomatosis. This case report raises the awareness that benign intestinal mucosa may colonize the serosa, challenging the diagnosis of tumor involvement and significantly impacting tumor staging, treatment and prognosis.