Used to prevent peritoneal or vascular seeding in tumors that may metastasize through CSF (e.g. medulloblastoma, 1) PNETs, ependymoma); may eventually become occluded by tumor cells and need replacement; may be able to radiate tumor filter to “sterilize” it. Not often used despite the fact that the risk of “shunt mets” appears to be low 2).