DISEASE CHARACTERISTICS: Pathologically confirmed posterior fossa medulloblastoma (CCG diagnosis code 2041) Localized disease required, i.e.: No more than 1.5 square centimeters of residual tumor on postoperative contrast-enhanced CT or MRI (preferably within 72 hours but no more than 14 days after surgery) No evidence of metastatic disease on pre- and postoperative MRI of spine (with dye enhancement) and lumbar cerebrospinal fluid (CSF) cytology within 3 days prior to surgery Cytologic analysis of ventricular CSF allowed only if medical contraindication to lumbar puncture and with approval of study chairperson Brain stem involvement eligible