RATIONALE: Drugs used in chemotherapy work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more tumor cells. Isotretinoin may help chemotherapy work better by making tumor cells more sensitive to the drugs. Radiation therapy uses high-energy x-rays to kill tumor cells. Carboplatin may make tumor cells more sensitive to radiation therapy. It is not yet known which chemotherapy and radiation therapy regimen is more effective in treating brain tumors.

PURPOSE: This randomized phase III trial is studying different chemotherapy and radiation therapy regimens to compare how well they work in treating young patients with newly diagnosed, previously untreated, high-risk medulloblastoma or supratentorial primitive neuroectodermal tumor.