A large non-inverted true diverticulum resembling a submucosal tumor of the ascending colon: a report of a rare case.

A 57-year-old woman presented with a chief complaint of right lower abdominal pain. Abdominal radiography and plain abdominal computed tomography revealed a mass with extensive calcification. A colonoscopy demonstrated a 20-mm-sized mass lesion protruding into the intestinal lumen from the ileocecal valve's lower lip. The mass was covered with normal mucosa without erosions. Endoscopic ultrasonography indicated that the tumor originated from or beneath the proper muscle layer. To treat the pain, eliminate the obstruction risk, and obtain a definitive diagnosis, the patient chose surgical resection. Laparoscopic-assisted ileocecal resection was performed under the diagnosis of a submucosal ascending colon tumor. Macroscopically, the tumor showed no gross epithelial abnormalities. However, it was filled with fecal material and had an orifice of approximately 3 mm on the proximal side of the tumor. Histopathologically, the cyst wall consisted of an entire colonic structure, and continuity was noted between the cystic lesion's wall and the ascending colon's wall, leading to a diagnosis of diverticular expansion due to fecal matter. No malignancies were detected. A diverticulum can collect feces and protrude into the colonic lumen, resembling colonic submucosal tumors with calcification. Although rare, this condition should be included in the differential diagnosis of colon tumors.