Hi, Thanks for writing in. Arachnoid cysts in brain are benign thin walled cysts arising from the covering in the brain. It has nothing to do with aggressive chemotherapy for malignant mealanoma. It does not cause any serious problem and in most cases is left untreated. Treatment involves complete cyst removal only when there is sufficient problem and symptoms that can be explained purely by the presence of the cysts. Most people have a single cyst and few have multiple cysts. Since he does not have symptoms, the arachnoid cysts are not a concern. However, a close follow up must be done for malignant melanoma and should MRI be done at that time then a record of the increase in size of the arachnoid cyst might be made. These are slow growing and stable lesions. Please do not worry.