Purpose: The first clinical biology-guided radiation therapy (BgRT) system – RefleXion TM X1 - was installed and commissioned for clinical use at our institution. This study aims at evaluating X1 treatment planning feasibility of complex craniospinal targets for pediatric medulloblastoma patients and comparing plan quality to multi-isocenter linac-based VMAT plans. Methods: Five pediatric patients treated with multi-isocenter craniospinal irradiation (CSI), planned using Eclipse VMAT and delivered using Varian Trilogy C-arm linac, were selected for this retrospective study. All PTV targets had a craniocaudal length<50cm (current X1 TPS limit) and received 36 Gy in 20 fractions. The target volumes and organs-at-risk (OARs) used for VMAT plans were used to generate plans using RefleXion X1. The near maximum dose to PTV (PTV D2%), OARs Dmean and Dmax, and treatment times were collected for analysis. A paired-sample t-test was performed to detect significance at p <0.05. Results: All five RefleXion X1 CSI plans were successfully generated and were clinically acceptable for treatment. PTV D2% was higher for X1 compared to VMAT plans at 41.3 Gy and 39.2 Gy, respectively, (p = 0.08). For the X1 plans, the average Dmean to the bowel, cochleas, heart, kidneys, lungs, and oral cavity were 10.4 Gy, 38.5 Gy, 12.5 Gy, 18.2 Gy, 13.7 Gy, and 16.0 Gy, respectively. The difference was not found to be statistically significant (p>0.05) compared to VMAT plans which showed Dmean to the bowel, heart, kidneys, lungs, and oral cavity at 11.7 Gy, 38.4 Gy, 11.6 Gy, 15.6 Gy, 13.3 Gy, and 15.0 Gy, respectively. The average treatment beam-on time for X1 plans was 16.7 min versus 3.6 min for VMAT plans (p<0.01). However, RefleXion X1 platform enabling one-isocenter treatment and 90 cm long kVCT scan has a potential to decrease the setup/imaging time, and thus the total treatment time compared to multi-isocenter linac-based VMAT where the total treatment time up to 43.5 min was observed. Conclusion: Apart from a higher maximum dose to PTV, X1 plans showed comparable dosimetry to multi-isocenter VMAT plans. Although the average beam-on time with X1 was longer, there is a potential for a more streamlined setup and IGRT using a single-isocenter plans.

Purpose/Objectives: To evaluate dosimetric differences between auto-planned Volumetric Modulated Arc Therapy (VMAT) Total Body Irradiation (TBI) technique and 2D AP/PA TBI technique. Methods: Ten pediatric patients treated with VMAT-TBI on Varian c-arm linac were included in this study. VMAT-TBI plans were generated using our in-house developed and publicly shared auto-planning scripts. For each VMAT-TBI plan, a 2D AP/PA plan was created replicating the institution’s clinical setup with the patient positioned at extended SSD with a compensator to account for differences in patient thickness, 50%-transmission daily lung blocks and electron chest-wall boosts prescribed to 50% of the photon prescription. Clinically relevant metrics were analyzed and compared between the VMAT and 2D plans. Results: All VMAT-TBI plans achieved PTV D90%≥100% of prescription. VMAT-TBI PTV D90% significantly increased (6.2%±2.4%, p<0.001) compared to the 2D technique, whereas no differences were observed in global Dmax (p<0.2) and PTV V110% (p<0.4). Compared to the 2D plans, significant decreases in the Dmean to the lungs (-25.6%±11.5%, p<0.001) and lungs-1cm (-34.1%±10.1%, p<0.001) were observed with the VMAT plans. The VMAT technique also provided additional sparing to other organs: for 12Gy prescription, kidneys Dmean of 64.7%±3.3%; for 2Gy prescription, testes/ovaries (Dmean of 31.6%±10.7%), brain (Dmean of 74.8%±1.6%) and thyroid (Dmean of 72.5±3.5%). Conclusions: Superior lung sparing with improved target coverage and similar global Dmax were observed with the VMAT plans as compared to 2D plans. In addition, VMAT-TBI plans provided greater dose reductions in gonads, kidneys, brain, and thyroid.