Abstract
We report a very rare case of rectum and anus collision cancer. A 48-year-old man seen for anal pain was found in colonoscopy to have a 3 cm polypoid mass in the lower rectum, part of which was suspected to have invaded the anal epithelium. Biopsy yielded a diagnosis of moderately to highly differentiated adenocarcinoma. Imaging studies, including 18F-fluorodeoxy-glucose positron emission tomography (FDG-PET)/CT, did not show any clear distant metastasis. Under a diagnosis of rectal cancer with anal canal invasion, we conducted abdominoperineal resection and lymph node dissection (D3). Resected specimens showed a type 1 mass above the dentate line. Cross-sections contained a tumorous lesion in the wall continuous with the mass. The histological diagnosis of the type 1 mass was moderately to highly differentiated adenocarcinoma originating in the rectum. The histological diagnosis of the tumorous lesion was adenocarcinoma mainly with poor differentiation and originating in the anal canal. The patient, thus had collision cancer of the rectum and anal canal. Some collision cancer cases have been seen in the lungs and stomach, but, to the best of our knowledge, only 16 large intestine cases, including ours, have been reported in Japan.
