Metastatic Carcinoma of Anal Fistula Caused by Implantation from Rectal Cancer-A Case Report

A 73-year-old man with a history of perianal abscess and anal fistula for approximately 40 years visited our outpatient clinic because of anal pain. An anal tumor was found at the initial visit. Colonoscopy also revealed a circumferential rectosigmoid tumor located 15 cm from the anal verge. The initial diagnosis was double primary cancer: Stage ⅡB anal fistula cancer(T3N0M0)and cStage Ⅱa rectosigmoid cancer(cT3N0M0). He underwent abdominoperineal resection with curative intent. Histopathological examination revealed that both tumors were moderately differentiated adenocarcinoma. In addition, genetic testing revealed that both tumors were KRAS G12V mutated, BRAFV600E wild, and microsatellite stable. The final diagnosis was metastatic carcinoma of anal fistula caused by implantation from rectal cancer.