Comparison of Neurocognitive Outcome in Two Standard Regimen for Treatment of Low-risk Medulloblastoma

Inclusion Criteria for screening: * Age at diagnosis \< 5 years * Patients with institutional suspicion or diagnosis of SHH-activated MB * Patient and family in social circumstances that will allow neuropsychological follow-up * Ability of parents/legal representatives to understand the patient information and to personally sign and date the informed consent to participate in screening procedures * Patient and the parents/legal representative are able and willing to participate in the entire study (if patient is eligible) Exclusion Criteria for overall study: * Patients previously treated for any other brain tumor or any type of malignant disease * Patients, in whom compliance with toxicity management guidelines and study procedures cannot be assured * History of hypersensitivity to an investigational medicinal product or to any drug with similar chemical structure or to any excipient present in the pharmaceutical form of an investigational medicinal product. * Patients/parents who do not wish to abstain from treatment with live vaccines during study participation * Patients with a language barrier too extensive to complete neuropsychological tests based on the investigator's judgement * Patients with severe premorbid developmental delay (based on the investigator's judgement), which will not allow WPPSI-IV assessment after 2.5 years * Patients cannot undergo MRI Inclusion Criteria for Bridging chemotherapy (carboplatin/etopiside) in interventional arms: * Patients with SHH-activated MB, TP53-wt demonstrated by IHC for Gab1 or p75-NGFR, Yap1, beta-catenin, and TP53 (lack of strong and widespread nuclear p53 positivity) on central review according to WHO classification 2021. * No clinical evidence of extra-CNS metastases * Negative CSF cytology * No prior therapy for MB other than surgery * No other medical contraindications to chemotherapy: * No uncontrolled invasive fungal infection or other severe systemic infection requiring system/parental therapy * No other severe organ dysfunctions, which cannot be clinically controlled * No concomitant use with yellow fever vaccine and with live virus and bacterial vaccines * No demyelinating form of Charcot-Marie-Tooth syndrome * Assessment of hearing function completed * No evidence of cancer predisposition syndromes other than Gorlin syndrome or ELP1, GPR161 germline alterations. * Provided written informed consent by parent(s)/parent representative(s) by bridging chemotherapy * Patient should be enrolled within 28 days after diagnosis. Bridging chemotherapy can start as early as criteria for enrollment are met, and must start no later than 33 days after diagnosis Exclusion Criteria for bridging chemotherapy: * One or more of the inclusion criteria for bridging chemotherapy are lacking * Other histology than SHH MB Inclusion Criteria for randomization: * Patient has received bridging chemotherapy as described in this protocol * Patients with centrally reviewed SHH-activated MB, TP53-wt, according to WHO classification 2021 * Absence of metastatic disease on central radiology review * Exclusion of TP53-mutation by DNA sequencing of the TP53-gene from tumor tissue by central review. Results from local institution will be accepted if raw data of this analysis is forwarded to the national central review institution * Confirmation of SHH activation by DNA methylation-based classification on central review. Results from local institution will be accepted if raw data of this analysis is forwarded to the national central review institution * No amplification of MYC (amplification of MYCN allowed). Array-based technologies (850k array, molecular inversion probe assay (MIP)), array-based comparative genomic hybridization (array-CGH) or next generation sequencing (NGS) DNA sequencing coverage MYC locus will be used. If these alternative assays give any indication of possible amplification, FISH will be performed on central review. * No other medical contraindications to chemotherapy: * No uncontrolled invasive fungal infection or other severe systemic infection requiring system/parental therapy * No other severe organ dysfunctions, which cannot be clinically controlled * No concomitant use with yellow fever vaccine and with live virus and bacterial vaccines * No demyelinating form of Charcot-Marie-Tooth syndrome * Retrospective assessment of pre-operative health-related QoL (HR-QoL) measured by Pediatric Quality of Life Inventory * Provided written informed consent by parent(s)/parent representative(s) for randomization Exclusion criteria for randomization: -Patients are excluded from the interventional study if any of the following criteria are met: 1. One or more of the inclusion criteria for randomization are lacking 2. Patients with metastatic disease 3. TP53-mutated SHH MB 4. MYC amplified MB 5. Pre-existing condition incompatible with scheduled therapy (e.g. Fanconi anemia) 6. Patients with non-communicating hydrocephalus, e.g. due to perinatal intracranial haemorrhage, adueductal stenosis, or meningitis 7. Contraindication for any components of the randomized therapies, including HDCT and intraventricular chemotherapy. Note: postperative hydrocephalus is not a contraindication for i.ventr. MTX.