Post-Immunotherapy Robotic-Assisted Resection for Primary Anorectal Melanoma: A Case Report.

Anorectal malignant melanoma (AMM) is a rare and aggressive neoplasm. Here, we report a case of AMM that was treated with robotic-assisted abdominoperineal resection (APR) after immunotherapy. A 68-year-old Japanese woman presented at our hospital because of diagnosis AMM. Computed tomography revealed a mass in the lower anorectal region and enlarged perirectal lymph nodes, but showed no evidence of distant metastases. We determined that radical resection would be difficult; therefore, the patient received immunotherapy with pembrolizumab for nine cycles. The size of pararectal lymph nodes was reduced, and the patient subsequently underwent robot-assisted APR and lymph node dissection. Histopathological examination showed spindle-shaped atypical melanocytes with acidophilic changes indicative of tumor cell necrosis in the rectal mucosa. We found no viable tumor cells in all 48 lymph nodes that were resected, and resection margins were tumor free. The patient was able to complete 15 cycles of adjuvant immunotherapy with pembrolizumab and remained relapse free at the 2-year postoperative follow-up. The present case showed that combination of complete surgical resection and immunotherapy is expected to improve outcomes in AMM patients if immunotherapy is effective.