Oral Malignant Melanoma Achieving Long-Term Survival With Nivolumab Monotherapy Following Surgical Treatment: A Report of a Rare Case.

Oral malignant melanoma (OMM) is a rare and aggressive tumor with a poor prognosis, often presenting with early cervical lymph nodes or distant metastases. We report a case of a 69-year-old woman with primary OMM of the left maxillary gingiva, initially accompanied by indeterminate pulmonary nodules. Following partial maxillectomy, she received nivolumab monotherapy for an enlarged pulmonary nodule and para-aortic lymph nodes. After 11 cycles, a partial response (PR) was achieved in the lung metastasis according to the Response Evaluation Criteria in Solid Tumors (RECIST, version 1.1). However, new metastases later developed in the gallbladder, brain, right frontal scalp, adrenal glands, tongue, and colon. Whole-brain radiation therapy was administered as best supportive care for brain metastases. At the patient's request, nivolumab monotherapy was continued during the radiation treatment. Notably, regression of the brain metastases was observed after whole-brain radiotherapy while continuing nivolumab, suggesting a potential local synergistic effect. Nivolumab was administered for a total of 21 cycles, and the patient achieved 20 months of survival, which exceeded the previously reported median survival for OMM. This case highlights the aggressive clinical behavior of OMM, its capacity to metastasize to multiple organs, and the potential benefits of immune checkpoint inhibitors (ICIs) and radiation therapy. Further accumulation of cases is needed to define optimal treatment strategies for this rare malignancy.