PRO-GLIO: PROton Versus Photon Therapy in IDH-mutated Diffuse Grade II and III GLIOmas

1. Patients must be 18 to 65 years old on the day of consent. 2. IDH-mutated astrocytoma grade 2 or 3, or oligodendroglioma grade 2 or 3 according to WHO 2021. 3. Indication for radiotherapy. 4. WHO/ECOG performance status 0-2. 5. Ability to undergo MRI. 6. No significant contrast enhancing tumour (more than 1 or 2 punctate contrast enhancing foci) at the time of randomization. In recurrence patients, no contrast enhancement is allowed unless a new biopsy confirms the diagnosis of IDH-mutated astrocytoma grade 2 or 3, or oligodendroglioma grade 2 or 3. 7. Ability and willingness to travel to a proton therapy centre if randomized to the proton therapy arm. 8. Women of child-bearing potential (WOCBP) must agree to use an effective method of contraception during radiotherapy, chemotherapy and 1 year after completion of chemotherapy. Pregnancy is not an ineligibility criterium if radiotherapy is indicated and cannot be postponed 9. Ability to understand the information about the study and included treatment. 10. Signed informed consent. 11. Ability to speak and understand Norwegian or Swedish language. Exclusion Criteria: 1. Prior treatment (except surgery) for diffuse glioma 2. Concomitant or previous malignancies. Exceptions are adequately treated basal cell carcinoma or squamous cell carcinoma of the skin, or in situ carcinoma of the cervix uteri with a follow-up time of at least 3 years, or other previous malignancy with a disease-free interval of at least 5 years 3. Known CDKN2A/B homozygous deletion 4. Presence of any medical, psychological, familial, sociological, or geographical characteristic that might impair patient compliance for study protocol procedures including follow-up 5. Body weight \> 150 kg