Bladder-Adherent Primary Appendiceal Carcinoma Masquerading as a Carpeting Rectal Lesion Detected by a Fecal Immunochemical Test: A Case Report

Introduction: Primary appendiceal carcinoma is rare and comprises up to 1% of all colorectal malignancies. Its invasion into adjacent organs, such as the bladder and rectum, especially as a presenting characteristic, is even less common. Case Presentation: A 75-year-old asymptomatic male tested positive on a screening fecal immunochemical test (FIT). Colonoscopy showed a rectosigmoid tumor and normal appendiceal orifice. Staging MRI surprisingly showed that the cancer was, in fact, of appendiceal origin, coursed posteriorly to invade the rectosigmoid and form adhesions with the urinary bladder. Staging CT did not show metastatic disease. Low anterior resection, en bloc appendectomy, and right hemicolectomy were performed along with cystectomy and ileal conduit. Hematoxylin and eosin stains showed appendiceal adenocarcinoma invading through the appendiceal wall into the rectal muscularis and submucosa. Features of neuroendocrine carcinoma were not identified on immunohistochemistry. This was a colonic type of adenocarcinoma of the appendix. Conclusion: This is a rare case of appendiceal carcinoma invading the rectum and presenting as a positive screening fecal immunochemical test in an asymptomatic individual. We effectively demonstrate the use of preoperative MRI to identify the appendiceal origin of the tumor, as well as to demonstrate the extent of tumor spread, which assisted with operative management and treatment planning.