 Surgery ( by craniotomy , open brain surgery ) may be offered based on the risks of treatment as determined by the <unk> – Martin scale ( grade I @-@ V ) ; this score is higher in larger lesions that are close to important brain structures and have deep venous drainage . High grade lesions ( IV and V ) have an unacceptably high risk and surgery is not typically offered in those cases . <unk> ( using targeted radiation therapy such as by a gamma knife ) may be used if the lesion is small but close to vital structures . Finally , embolization may be used on small lesions that have only a single feeding vessel . 
