Invasive Moderately-Differentiated Mucinous Adenocarcinoma Incidentally Identified in Perforated Acute Diverticulitis With Abscess Formation

Colonic adenocarcinoma rarely arises within a diverticulum. As illustrated in this case report, such an occurrence presents difficulties in diagnosis, posed by the overlap of radiological findings in diverticulitis and adenocarcinoma and the potential absence of luminal lesion. Here we present a 77-year-old male with acute left lower abdominal pain, fever and leukocytosis. Computarized tomography (CT) imaging revealed extensive colonic diverticulosis, with sigmoid colon wall thickening, extraluminal air and a pericolic abscess. No enlarged lymph nodes were detected. The clinical diagnosis was acute sigmoid diverticulitis with micro-perforation and abscess formation. Partial sigmoidectomy was performed and diverticulitis confirmed; no tumor was identified in the resected colon. However, fragments of mucinous adenocarcinoma were incidentally recognized in the pericolic abscess contents. Immunohistochemical (IHC) studies supported a primary colonic adenocarcinoma, warranting further clinical workup. A subsequent distal sigmoidectomy revealed moderately-differentiated mucinous adenocarcinoma invading through the visceral peritoneum, and arising in a low-grade mucinous neoplasm within a diverticulum. The case illustrates that in a patient with diverticulitis and no evidence of a luminal neoplasm, the unexpected finding of adenocarcinoma in the resected bowel may be attributable to adenocarcinoma arising within a diverticulum.