Implantation metastasis from a colorectal cancer into a perianal fistula is very rare. Such lesions are commonly mistaken as benign perianal abscesses or fistulas and diagnosed only after pathological analysis of surgically excised fistulas. Once diagnosed, the management of this condition remains controversial. We report a case of perianal fistula that was unexpectedly found to harbor adenocarcinoma on biopsy. Further investigation by colonoscopy and computed tomography scan revealed a sigmoid adenocarcinoma. Abdominoperineal resection was performed. Histology and immunohistochemical staining was identical in both primary and metastatic tumors. We herein review the literature on the metastasis of colorectal cancer to a benign perianal fistula presumably acquired through implantation of viable malignant cells shed from the primary tumor and discuss the approach to this rare scenario in colorectal cancer surgery.
Keywords: Rectosigmoid cancer; abdominoperineal resection; anal fistula; implantation metastasis.