We report a case of advanced colon cancer which was supposed to have arisen from a hyperplastic polyp in a 68-year-old man. Colonoscopy revealed a depressed reddish area with a surrounding elevated lesion that was of a faded color compared with the normal mucosa. After the mucosal surface had been sprayed with crystal violet dye, magnifying colonoscopy showed an amorphous area in the central depression and the surrounding, slightly elevated lesion had an asteroid pattern. The depressed area was therefore considered to be a colonic cancer surrounded by a hyperplastic polyp. Endoscopic ultrasonography showed that the lesion was infiltrating further than the deep submucosal layer and it was therefore decided to treat the patient by laparoscopically assisted right hemicolectomy. The depressed lesion was found to be a well-differentiated adenocarcinoma invading the muscularis propria (diagnosed as IIc + IIa-like advanced adenocarcinoma). The surrounding flat elevated lesion was found to be hyperplastic mucosa. No adenomatous lesions were found. There have been few reported cases in which a preoperative diagnosis of carcinoma in a hyperplastic polyp has been made, but the possibility of carcinogenesis from hyperplastic polyps has come under consideration recently. This case was considered to be important because it raises the possibility that nonpolypoid cancer can develop from a hyperplastic polyp.
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