A First Reported Case of Metastatic Anorectal Amelanotic Melanoma with a Marked Response to Anti-PD-1 Antibody Nivolumab: A Case Report

Introduction: Anorectal amelanotic melanoma (AAMM) is a rare disease with poor prognosis. A standard treatment strategy for AAMM has not been established.

Presentation Of Case: We report a case of successful treatment of AAMM with nivolumab. A 67-year-old man was referred for colonoscopy which revealed type I tumor in the rectum. AAMM was diagnosed with immunostaining histopathological biopsy findings. Enhanced computed tomography (ECT) revealed the rectal tumor without distant organ metastasis. We performed laparoscopy-assisted abdominoperineal resection. ECT at three months after surgery revealed liver metastases and right ischial bone metastasis. Although we had started dacarbazine monotherapy, black spots that were suspicious of skin metastases had appeared on systemic skin. Therefore, we started nivolumab therapy. ECT at 3 months after initiation of nivolumab showed shrinkage of liver metastasis. We have continued strict follow-up every 2 months and checked no oncologic progression at 17 months after initiation of nivolumab.

Discussion: The anti-PD-1 antibody have improved prognosis of malignant melanoma. However, there are no reports of nivolumab for treatment of AAMM.

Conclusions: Our patient is the first reported case of AAMM treated with nivolumab. We consider that nivolumab will be effective for non-cutaneous malignant melanoma.