RONC-27. PROTON THERAPY REDUCES DOSE TO CRITICAL CENTRAL NERVOUS SYSTEM STRUCTURES IN MEDULLOBLASTOMA: A DOSIMETRIC ANALYSIS OF CHILDREN’S ONCOLOGY GROUP (COG) ACNS0331

BACKGROUND: Recently published data demonstrated proton therapy (PRT) significantly reduced cognitive decline relative to photons for pediatric medulloblastoma. These findings imply that reductions in dose to critical CNS structures during the boost phase may account for better outcomes over time. Here, we examine differences in dosimetric data for medulloblastoma patients treated on ACNS0331 with photon (Intensity Modulated Radiation Therapy, 3D-Conformal Radiation Therapy) vs PRT to identify potential structures responsible for cognitive benefit. METHODS: COG ACNS0331 was a randomized trial examining the impact of reduced craniospinal irradiation (CSI) dose (standard vs low dose, in patients aged 3–7) and volume (whole posterior fossa vs involved field) in pediatric medulloblastoma patients. We identified 136 patients (IMRT=95, 3DCRT=28, Proton=13) enrolled on ACNS0331 with complete radiation and imaging data and re-contoured 10 critical brain structures to calculate dose. RESULTS: Proton therapy significantly reduced the dose to critical structures. For example, temporal lobe mean dose and V30 were 30Gy/38% (PRT), 40Gy/89% (IMRT), 41Gy/84% (3DCRT)), hippocampi mean dose were 51 Gy (IMRT), 52 Gy (3DCRT), and 44Gy (PRT) and cochlear mean dose were 43 Gy (IMRT), 49 Gy (3DCRT), and 31Gy (PRT). Dose to several other critical structures were also significantly reduced including the whole brain, supratentorium, cerebellum, and pituitary. CONCLUSIONS: Proton therapy greatly reduces dose to critical CNS structures when compared to IMRT or 3DCRT. Further studies are needed to correlate dose reductions in these structures with improved cognitive outcomes.