Purpose: A dosimetric comparison of pediatric cranio-spinal irradiation (CSI) plans between a multi-isocenter linac-based VMAT and a novel ring-based gantry delivery system, RefleXion X1.
Methods: Five pediatric CSI patients previously treated using a multi-isocenter Eclipse-planned linac-treated VMAT were selected for this retrospective study. All PTV targets had a cranio-caudal-length<50cm (current X1 TPS limit) and received 36Gy in 20fx. The target volumes and organs-at-risk (OARs) used for VMAT plans were used to generate plans on the RefleXion X1 using 20mm jaw. The near maximum (D2%) dose to PTV, OARs Dmean, and treatment times were collected for analysis. A paired-sample t-test was performed to detect significance at p <0.05.
Results: With all plans achieving PTV D95%=100% of prescription, PTV D2% was higher for X1 compared to VMAT plans at 41.2Gy and 39.2Gy, respectively, (p = 0.04). For the X1 plans, the average Dmean to the bowel, heart, kidneys, lungs, and oral cavity were 10.4Gy, 12.3Gy, 18.9Gy, 14.0Gy, and 15.3Gy, respectively. The difference was not found to be statistically significant (p>0.05) for VMAT plans which showed Dmean to the bowel, heart, kidneys, lungs, and oral cavity at 11.7Gy, 12.4Gy, 18.9Gy, 13.3Gy and 14.4Gy respectively. The average treatment beam-on time for X1 plans was 16.7min. In comparison, the average treatment beam-on time for VMAT plans was almost half as much, 8.7min. However, the potential benefit of X1 might lie in ability to use a single isocenter and scan kVCT 90cm cranio-caudally to shorten IGRT time. Whereas for VMAT plans the average total treatment time including all pretreatment imaging was 27.8min.
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.