Chemotherapy and Radiation Therapy in Treating Young Patients With Newly Diagnosed, Previously Untreated, High-Risk Medulloblastoma/PNET

Inclusion Criteria: * Age greater than or equal to 3 and less than 22 years at the time of diagnosis * Newly diagnosed, previously untreated: (1) M0 medulloblastoma with \> 1.5 cm\^2 residual; (2) M+ medulloblastoma; patients with diffusely anaplastic medulloblastoma are eligible regardless of M-stage or residual tumor * As of amendment # 2, enrollment of patients with supratentorial PNET has been discontinued * All patients with M4 disease are not eligible * A pre-operative magnetic resonance imaging (MRI) scan of the brain with and without contrast is required; NOTE: computed tomography (CT) scans are NOT sufficient for study eligibility since radiation therapy planning and response will be based on MRI scans only * Post-operative head MRI scan with and without contrast (preferably within 72 hours post-surgery); for patients who undergo stereotactic biopsy only, either a pre or post-operative MRI is sufficient; for patients with M2 and M3 disease, a post-op MRI is strongly encouraged, but not mandatory * Spinal MRI imaging with and without gadolinium is required within 10 days of surgery if done pre-operatively or within 28 days of surgery if done post-operatively; for posterior fossa tumors, pre-operative MRI scans are preferred because surgically-induced inflammation/blood can be difficult to distinguish from tumor * Lumbar cerebrospinal fluid (CSF) cytology examination must be obtained pre-operatively or within 31 days following surgery; the optimal time for o…