<qa>  What is a symptomatic retinal hemangioblastoma that causes- One or more measurable CNS hemangioblastoma, if symptomatic, must be refractory or not amenable to standard therapy, if asymptomatic, must have had at least one prior neurosurgical treatment procedure; andor: at least one untreatable or treatment-refractory retinal hemangioblastoma that is causing impaired visual function 