Exhibit Hall | Forum 5
Purpose: To investigate practical aspects of a novel Bragg peak FLASH-RT technique, including spot dose-rate evolution, dose and dose-rate calculation accuracy, and plan robustness, for clinical application in lung cancer treatment.
Methods: The inverse treatment optimization synergy with proton range pull-back and compensation can achieve tumor distal tracking using single-energy Bragg peak while still preserving FLASH dose-rate delivery. The multiple Coulomb scattering (MCS) between protons and universal range shifter (URS) and range compensator (RC) can enlarge beam divergence causing the spot-peak-dose-rate to decrease significantly. A fast Monte Carlo (MC) tool, MCsquare, was also commissioned to compare the proton convolution superposition(PCS) dose and dose-rate calculation. A clinical robustness evaluation method (±5mm setup errors and ± 3.5% range uncertainties) was performed to assess the dosimetric impact of setup errors and CT calibration uncertainties for Bragg peak planning.
Results: Spot dose-rate decreased when the air gap increased, and the central axis dose-rate at the Bragg peak was reduced by a factor of ~2 between 5cm and 25cm air gaps. The largest dose-rate difference between PCS and MC occurred at the distal part of the proton beam traveling through air-soft tissue interfaces and the field edge area or penumbra region. The dose-rate-volume-histogram(DRVH) decreased using the MC method but still maintained acceptable V40Gy/s coverage for all OARs. The D95 second worst-case scenario for iCTV decreased from 95% to 92.7%(±2.7%), and median D95 was 96.3%(±1.9%) when range and setup uncertainties were present.
Conclusion: Minimizing the air gap is essential in maintaining proton fluence intensity and a smaller penumbra, which is crucial for OAR sparing. MC is recommended for FLASH dose and dose-rate calculation in lung tumor treatment planning. The robustness analysis demonstrated that novel Bragg peak planning can maintain excellent coverage when incorporating clinical setup and range uncertainties.