Twenty adults with medulloblastoma (≥ 22 years old) received postoperative proton CSI. This study reported no acute ≥ grade 3 gastrointestinal or hematologic toxicities attributable to CSI. With a median follow-up of 3.1 years, 4-year actuarial local control, disease-free survival, and overall survival rates were 90%, 90%, and 95%, respectively. These data support the standard use of proton CSI for adult medulloblastoma.