PRIMARY OBJECTIVES: I. To determine whether carboplatin radiosensitization increases long term event-free survival for high risk medulloblastoma/primitive neuroectodermal tumor (PNET) patients. II. To determine whether isotretinoin increases long term event-free survival for high risk medulloblastoma/PNET patients. SECONDARY OBJECTIVES: I. To compare residual disease response to radiation alone versus radiation plus carboplatin. II. To identify molecular prognostic indicators suitable for patient stratification in future trials. III. To evaluate the health-related quality of life (HRQOL) during phases of active treatment specific to treatment modalities. IV. To describe the neuropsychological functioning of the study population and to evaluate the relationship between neuropsychological status and health related quality of life. OUTLINE: Patients are randomized to 1 of 4 treatment arms. ARM I (standard chemoradiotherapy and standard maintenance therapy): CHEMORADIOTHERAPY: Patients undergo radiation therapy once daily (QD) five days a week for 6 weeks. Patients also receive vincristine sulfate intravenously (IV) over 1 minute once weekly for 6 weeks. Six weeks after completion of chemoradiotherapy, patients proceed to maintenance therapy. MAINTENANCE THERAPY: Patients receive cisplatin IV over 6 hours on day 1, vincristine sulfate IV over 1 minute on days 1 and 8, and cyclophosphamide IV over 1 hour on days 2 and 3. Patients also receive filgrastim subcutaneously (SC) or IV beginning on day 4 and continuing until blood counts recover (at least 10 days). Treatment repeats every 28 days for a total of 6 courses in the absence of disease progression or unacceptable toxicity. ARM II (standard chemoradiotherapy plus carboplatin and standard maintenance therapy): CHEMORADIOTHERAPY: Patients receive vincristine sulfate and undergo radiation therapy as in Arm I. Patients also receive carboplatin IV over 15 minutes on each day of radiation therapy. Six weeks after completion of chemoradiotherapy, patients proceed to maintenance therapy. MAINTENANCE THERAPY: Patients receive maintenance therapy as in Arm I. ARM III (standard chemoradiotherapy, standard maintenance therapy plus isotretinoin, and continuation therapy with isotretinoin): CHEMORADIOTHERAPY: Patients undergo chemoradiotherapy as in Arm I. Six weeks after completion of chemoradiotherapy, patients proceed to maintenance therapy. MAINTENANCE THERAPY: Patients receive isotretinoin orally (PO) twice daily (BID) on day 1 and days 16-28 and cisplatin, vincristine sulfate, cyclophosphamide, and filgrastim as in Arm I maintenance therapy. Treatment repeats every 28 days for a total of 6 courses in the absence of disease progression or unacceptable toxicity. Patients then proceed to continuation therapy. CONTINUATION THERAPY: Patients receive isotretinoin PO BID on days 15-28 every 28 days for up to 6 courses in the absence of disease progression or unacceptable toxicity. ARM IV (standard chemoradiotherapy plus carboplatin, standard maintenance therapy plus isotretinoin, and continuation therapy with isotretinoin): CHEMORADIOTHERAPY: Patients undergo chemoradiotherapy as in Arm II. Six weeks after completion of chemoradiotherapy, patients proceed to maintenance therapy. MAINTENANCE THERAPY: Patients receive maintenance therapy as in Arm III. Patients then proceed to continuation therapy. CONTINUATION THERAPY: Patients receive continuation therapy as in Arm III. After completion of study treatment, patients are followed up periodically for up to 10 years.

This randomized phase III trial studies different chemotherapy and radiation therapy regimens to compare how well they work in treating young patients with newly diagnosed, previously untreated, high-risk medulloblastoma. Drugs used in chemotherapy, such as vincristine sulfate, cisplatin, cyclophosphamide, and carboplatin, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. 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.