The clinical trial will primarily evaluate the safety and immunogenicity of an H3K27M-specific peptide vaccine in combination with the humanized anti-PD-L1 checkpoint inhibitor atezolizumab and standard radiotherapy in a rare disease with poor prognosis. Due to the rarity of the disease, 15 patients will be enrolled in the study to determine safety, defined as regimen limiting toxicity (RLT). Immunogenicity will be assessed based on patient-specific T-cell responses.