Rectal Cancer Metastasis to the Anal Verge: An Unusual Case Presentation and Review of the Literature

BACKGROUND: Anal metastasis of colorectal adenocarcinoma is very rare, represented by only a handful of case reports in the literature. Previously, reports of metastasis to this region had occurred following a history of anorectal disease, such as anal fistulae. Antecedent trauma to the area from hemorrhoidectomy, fissures, or perineal retractor injury have also been implicated. CASE PRESENTATION: Herein we report the case of 69-year-old man without any history of anal disease presenting with a metachronous metastasis of a colorectal-type adenocarcinoma to the anal verge. He was previously treated for T1N0 rectal adenocarcinoma at the rectosigmoid junction with a low anterior resection 5 years prior, then had a T3N0 local recurrence at the colorectal anastomosis treated with neoadjuvant chemoradiation, and eventually a Hartmann’s procedure 4 years later. Subsequently, on surveillance flexible sigmoidoscopy, a new tumor was identified on the perianal skin extending from the anal verge. Histopathology demonstrated colorectal-type adenocarcinoma. Flexible endoscopy identified no other residual or recurrent disease in the colon or rectal stump. The patient was treated with wide local excision and advancement flap reconstruction. CONCLUSION: Isolated metastasis to the anus is an extremely rare occurrence for colorectal adenocarcinoma. There exists little evidence to inform management. One option is to treat like a locally recurrent rectal cancer with aggressive tri-modality management consisting of chemoradiation, abdominal perineal resection, and adjuvant chemotherapy. In the absence of metastatic disease, local resection and close surveillance remain an option. As always, patient factors should guide management.