Medulloblastoma is one of the most common pediatric cancers. Standard protocol for treatment includes surgical resection of the tumor followed by radiation therapy and adjuvant chemotherapy. Pediatric patients are more sensitive to radiation therapy than adult patients and measures are taken to limit the amount of dose that they receive to their organs at risk. One method of reducing unnecessary radiation exposure is by decreasing the prescribed radiation dose or decreasing the irradiated volume. A second method could be using proton radiation therapy, which has less scattered radiation compared to traditional photon radiation therapy. A Children’s Oncology Group study was performed that endeavored to evaluate whether reductions in dose and volume could be performed while still maintaining the high disease control rates from the standard treatments. The initial outcomes of this randomized study – COG ACNS 0331 has determined that a reduction of craniospinal dose from 23.4 Gy to 18.0 Gy resulted in an inferior treatment with higher tumor recurrence; however, the reduction for the boost volume from the entire posterior fossa to just the involved field consisting of the tumor bed with a small margin did not result in higher tumor recurrence. If a reduction of 5.4 Gy caused significant recurrence, is there an increased probability of recurrence in the posterior fossa when involved fields using proton radiation therapy are used instead of using photon radiation therapy? This thesis is to determine the dose in the posterior fossa using involved fields with proton radiation therapy and comparing them with photon radiation therapy.