Incidentally Found Primary Cerebral Malignant Melanoma Associated with Ota Nevus—Wide Dissemination after an Initial Phase of Slow Growth

Primary cerebral malignant melanoma accounts for 1% of all melanomas and for 0.7% of all primary tumours of the central nervous system (CNS). We report an incidentally found primary malignant melanoma in the right temporal lobe of a 76-year-old woman with an Ota nevus on her right eyelid and sclera. The lesion was initially characterised by slow growth, followed by tumoural bleeding and wide leptomeningeal dissemination. Magnetic resonance imaging (MRI) during brain checkup demonstrated an 8-mm mass in the right uncus with high intensity on T1-weighted images. The mass grew slowly over the next two years, but asymptomatic tumoural haemorrhage eventually developed. The patient underwent tumour removal via right frontotemporal craniotomy. Extensive subarachnoid dissemination from the right temporal tumour was found; the pathologic diagnosis was malignant melanoma. Despite adjuvant therapy comprising whole-brain radiation and nivolumab, the patient died of severe leptomeningeal dissemination at 4.5 months after the operation (33 months after the initial MRI study). Our literature review found 46 cases of primary CNS malignant melanoma, predominantly in middle-aged to elderly individuals. The most frequent symptom was headache, followed by visual disturbance, nausea/vomiting, and hemiparesis. Only one other case of primary cerebral malignant melanoma had been found incidentally; 11 previously reported patients manifested congenital nevi (Ota nevus, n = 5; other nevi, n = 6). Six patients suffered tumoural haemorrhage, 4 experienced leptomeningeal dissemination, and 5 developed extracranial metastases. The median survival time of the patients was 31 months.