RATIONALE: Radiation therapy uses high-energy x-rays to damage tumor cells. Drugs used in chemotherapy, such as vincristine, cisplatin, lomustine, and cyclophosphamide, work in different ways to stop tumor cells from dividing so they stop growing or die. Giving radiation therapy with chemotherapy after surgery may kill any remaining tumor cells. It is not yet known whether standard-dose radiation therapy combined with chemotherapy after surgery is more effective than reduced-dose craniospinal (head and spine) radiation therapy plus either posterior fossa (back of the brain) boost or tumor bed (site of the tumor) boost radiation therapy combined with chemotherapy in treating medulloblastoma.

PURPOSE: This randomized phase III trial is studying standard-dose radiation therapy to see how well it works compared to reduced-dose craniospinal radiation therapy AND posterior fossa boost radiation therapy to see how well it works compared to tumor bed boost radiation therapy when given together with chemotherapy in treating young patients who have undergone surgery for newly diagnosed standard-risk medulloblastoma.