A Study of Lower Radiotherapy Dose to Treat Children With CNS Germinoma

Inclusion Criteria: * Patients must be ≥ 3 years and \< 30 years at the time of study enrollment * Patients must be newly-diagnosed primary localized germinoma of the suprasellar and/or pineal region by pathology and/or serum and/or CSF hCGbeta 5-50 mIU/mL AND institutional normal AFP (or ≤ 10 ng/mL if no institutional normal exists), including tumors with contiguous ventricular or unifocal parenchymal extension. No histologic confirmation required * Patients with EITHER (A) bifocal (pineal + suprasellar) involvement OR (B) pineal lesion with diabetes insipidus (DI) AND hCGbeta ≤ 100 mIU/mL in serum and/or CSF AND institutional normal AFP (or ≤ 10 ng/mL if no institutional normal exists) in both serum and CSF. No histologic confirmation required * Patients with hCGbeta 51-100 mIU/mL in serum and/or CSF and institutional normal AFP (or ≤ 10 ng/mL if no institutional normal exists) in both serum and CSF. Histologic confirmation of germinoma IS required * Patients with germinoma of the basal ganglia and or/thalamic primary sites are eligible * Patients with metastatic germinoma including non-contiguous disease or distant disease in the brain, ventricles, or spine are eligible * Patients with germinoma admixed with mature teratoma are eligible * Patients must have a performance status corresponding to Eastern Cooperative Oncology Group (ECOG) scores of 0, 1 or 2. Use Karnofsky for patients \> 16 years of age and Lansky for patients ≤ 16 years of age * Patients must have eligibility confirmed by Rapid Central Imaging Review performed on APEC14B1-CNS * Imaging studies must be obtained within 31 days prior to study enrollment and start of protocol therapy. (Note: for patients that have had surgery and post-operative imaging performed, it is the post-operative MRI that must be obtained within 31 days prior to enrollment.) * Patients must have a cranial magnetic resonance imaging (MRI) with and without gadolinium at diagnosis/prior to enrollment. If surgical resection is performed, patients must have pre-operative and post-operative brain MRI with and without gadolinium. The post-operative brain MRI should be obtained within 72 hours of surgery. If patient has a biopsy only, post-operative brain MRI is recommended but not required * Patients must have a spine MRI with gadolinium obtained at diagnosis/prior to enrollment * Patients must be enrolled, and protocol therapy must begin, no later than 31 days after definitive surgery or clinical diagnosis, whichever is later * Patients must have eligibility confirmed by Rapid Central Tumor Marker Review performed on APEC14B1-CNS * Lumbar CSF must be obtained prior to study enrollment unless medically contraindicated. If a patient undergoes surgery and lumbar CSF cytology cannot be obtained at the time of surgery, then it should be performed at least 10 days following surgery and prior to study enrollment. False positive cytology can occur within 10 days of surgery. Of note, lumbar CSF should not be performed prior to obtaining spine MRI, as this can make interpretation of the spine MRI less clear * Patients must have CSF tumor markers obtained prior to study enrollment unless medically contraindicated. Ventricular CSF obtained at the time of CSF diversion procedure (if performed) is acceptable for tumor markers but lumbar CSF is preferred. In case CSF diversion and biopsy/surgery are combined, CSF tumor markers should be collected first. Ideally serum and CSF tumor markers should be collected at the same time and processed without delay * For patients with solid tumors: Peripheral absolute neutrophil count (ANC) \>= 1000/uL (Must be performed within 7 days prior to enrollment unless otherwise indicated) * For patients with solid tumors: Platelet count \>= 100,000/uL (transfusion independent) (Must be performed within 7 days prior to enrollment unless otherwise indicated) * For patients with solid tumors: Hemoglobin \>= 8.0 g/dL (may receive red blood cell \[RBC\] transfusions) (Must be performed within 7 days prior to enrollment unless otherwise indicated) * For pediatric patients (age 3-17 years): A serum creatinine based on age/sex as follows (Must be performed within 7 days prior to enrollment unless otherwise indicated): * Age: 3 to \< 6 years; maximum serum creatinine (mg/dL): 0.8 (male); 0.8 (female) * Age: 6 to \< 10 years; maximum serum creatinine (mg/dL): 1 (male); 1 (female) * Age: 10 to \< 13 years; maximum serum creatinine (mg/dL): 1.2 (male); 1.2 (female) * Age: 13 to \< 16 years; maximum serum creatinine (mg/dL): 1.5 (male); 1.4 (female) * Age: ≥ 17 years; maximum serum creatinine (mg/dL): 1.7 (male); 1.4 (female) OR a 24-hour urine creatinine clearance ≥ 70 mL/min/1.73 m\^2 OR a glomerular filtration rate (GFR) ≥ 50 mL/min/1.73 m\^2. GFR must be performed using direct measurement with a nuclear blood sampling method OR direct small molecule clearance method (iothalamate or other molecule per institutional standard). * Note: Estimated GFR (eGFR) from serum or plasma creatinine, cystatin C or other estimates are not acceptable for determining eligibility. * For adult patients (age 18 years or older) (Must be performed within 7 days prior to enrollment unless otherwise indicated): * Creatinine clearance ≥ 70 mL/min, as estimated by the Cockcroft and Gault formula or a 24-hour urine collection. The creatinine value used in the calculation must have been obtained within 28 days prior to registration. Estimated creatinine clearance is based on actual body weight * Total bilirubin ≤ 1.5 x upper limit of normal (ULN) for age (Must be performed within 7 days prior to enrollment unless otherwise indicated) * Serum glutamic-pyruvic transaminase (SGPT) (alanine transaminase \[ALT\]) ≤ 135 U/L (Must be performed within 7 days prior to enrollment unless otherwise indicated) * Note: For the purpose of this study, the ULN for SGPT (ALT) has been set to the value of 45 U/L * No evidence of dyspnea at rest, no exercise intolerance,