Positive Response to Dual BRAF and MEK Inhibition in a Case with BRAF Mutant Anorectal Malignant Melanoma

A 59-year-old male with the chief complaint of bloody stool underwent transanal tumor resection for an anorectal malignant melanoma. Rectal amputation and inguinal lymph node dissection were not performed due to severe anemia with platelet depression and poor general condition. After the operation, treatment with an immune checkpoint inhibitor was started. A gait disturbance was diagnosed as postoperative inflammatory neuropathy based on a nerve biopsy. As steroid treatment was necessary, the immune checkpoint inhibitor was discontinued after a single dose. Tissue diagnosis identified the BRAF gene mutation, and combination therapy with dabrafenib/trametinib was started. Abdominal contrast computed tomography on day 30 and 101 after the start of medication showed decreased right inguinal lymphadenopathy and a partial response (PR) was determined based on the response evaluation criteria in solid tumors (RECIST). Anorectal malignant melanoma is rare. We report a case of anorectal malignant melanoma in which dabrafenib/trametinib combination therapy was effective.