SU‐E‐T‐369: Evaluating Intensity Modulated Proton Therapy Relative to Passive Scattering Proton Therapy for Increased Vertebral Column Sparing in CSI of Pediatric Patients

Abstract

Purpose:To investigate the trade‐off between vertebral column sparing and thecal‐sac target coverage in craniospinal irradiation (CSI) of pediatric patients treated with passive‐scattering (PS) and intensity modulated (IMPT) proton therapy.Methods:We selected 2 pediatric patients treated with PS CSI for medulloblastoma. Spinal irradiation was re‐planned with IMPT. For all cases, we assumed prescription dose of 23.4 Gy(RBE), with the spinal canal receiving at least 95% of 23.4 Gy(RBE). PS planning was performed using the commercial system XiO. IMPT planning was done using the Astroid planning system. Beam arrangements consisted of (a) PS posterior‐anterior (PA) field, PS‐PA, (b) IMPT PA field, IMPT‐PA, and (c) two posterior oblique IMPT fields, IMPT2 (‐35°, 35°). Dose distributions were re‐calculated using TOPAS Monte Carlo, along with LET distributions, to investigate LET variations within the target and vertebra anatomy. Variable RBE‐weighed dose distributions were also calculated based on a dose and LET‐dependent biophysical model. Dosimetric data were compared among the plans for the target volume, spinal cord and adjacent critical organs (thecal‐sac and cauda equina).Results:IMPT2 resulted in better sparing of the posterior vertebral column (entrance region posterior to thecal‐sac), where planned dose was approximately 6–8Gy(RBE). For IMPT‐PA and PS‐PA the MC‐calculated dose to the posterior vertebral column was, on average, 20Gy and 18Gy respectively. For IMPT2 higher mean‐LET (5keV/µm/(g/cm3)) values were observed in anterior vertebral column (beyond the thecal‐sac) relative to IMPT‐PA and PS‐PA, where mean‐LET was 3.5keV/µm/(g/cm3) and 2.5keV/µm/(g/cm3) respectively. The higher LET region observed for both IMPT plans was in the distal end of treatment fields, where dose delivered was less 5Gy(RBE).Conclusion:The two‐oblique proton beams IMPT2 best spared the spinal column, while reducing the dose to the posterior spinal column from 18–20 to 6–8 Gy(RBE). The best LET distribution was obtained with the PS‐PA fields.