A case of metastatic carcinoma of anal fistula caused by implantation from rectal cancer

This case involved an 80-year-old man who was seen for melena. Further testing revealed a tubular adenocarcinoma 50 mm in size in the rectum. In addition, an anal fistula was noted behind the anus along with induration. A biopsy of tissue from the external (secondary) opening of the fistula also revealed adenocarcinoma. Nodules suspected of being metastases were noted in both lung fields. The patient was diagnosed with rectal cancer, a cancer arising from an anal fistula, and a metastatic pulmonary tumor, and neoadjuvant chemotherapy was begun. A laparoscopic abdominoperineal resection was performed 34 days after 6 cycles of mFOLFOX-6 therapy. Based on pathology, the rectal cancer was diagnosed as moderately differentiated adenocarcinoma, and this adenocarcinoma had lymph node metastasis (yp T3N2aM1b). There was no communication between the rectal lesion and the anal fistula, and a moderately differentiated tubular adenocarcinoma resembling the rectal lesion was noted in the anal fistula. Immunohistochemical staining indicated that both the rectal lesion and anal fistula were cytokeratin 7 (CK7) (−) and cytokeratin 20 (CK20) (+), and the patient’s condition was diagnosed as implantation of rectal cancer in an anal fistula. In instances where an anal fistula develops in colon cancer, cancer implantation in that fistula must also be taken into account, and further testing should be performed prior to surgery.