• Able to understand the study procedures and agree to participate in the study by providing written informed consent (by participant or legally authorized representative), and assent when applicable.

• Body weight ≥ 10 kg at time of randomization.

• Histologically diagnosed H3 K27M-mutant diffuse glioma (new diagnosis). Detection of a missense K27M mutation in any histone H3-encoding gene detected by testing of tumor tissue (immunohistochemistry \[IHC\] or next-generation sequencing \[NGS\] in a Clinical Laboratory Improvement Amendments \[CLIA\]-certified or equivalent laboratory). \[Site to provide (as available): ≥ 10 unstained formalin-fixed paraffin-embedded (FFPE) slides from tumor tissue.\]

• At least one, high-quality, contrast-enhanced MRI of the brain obtained prior to starting radiotherapy for submission to sponsor's imaging vendor for central read. For participants who had a surgical resection, this scan must be post-resection; for participants who did not have a resection, this scan may be pre- or post-biopsy.

• At least one, high-quality, contrast-enhanced MRI of the brain obtained 2 to 6 weeks after completion of frontline radiotherapy. If unable to obtain contrast-enhanced imaging due to lack of venous access after multiple attempts, a patient may still be eligible after collection of a nonenhanced MRI of the brain. \[Site to also provide all available MRIs completed prior to initiating treatment with study intervention.\]

• Received frontline radiotherapy

‣ Initiated radiotherapy within 12 weeks from the initial diagnosis of H3 K27M-mutant diffuse glioma.

⁃ Completed radiotherapy within 2 to 6 weeks prior to randomization

⁃ Completed standard fractionated radiotherapy (eg. 54 to 60 Gy in 28 to 33 fractions given over approximately 6 weeks or hypofractionated radiotherapy (eg. 40 Gy in 15 fractions given over approximately 3 weeks).

• Karnofsky Performance Status or Lansky Performance Status ≥ 70 at time of randomization.

• Stable or decreasing dose of corticosteroids and anti-seizure medications for 7 days prior to randomization, if applicable. Stable steroid dose is defined as ≤ 2 mg/day increase (based on dexamethasone dose or equivalent dose of an alternative steroid).