Monte Carlo 4D Dose-Rate Distribution for Ultra-High Dose-Rate Radiation Therapy
Abstract
Purpose
Ultra-high dose-rate (>40 Gy/s) or “FLASH” radiation therapy has the potential to improve cancer treatment through reduced radiation side effects. The purpose of this work is to create and validate a particle phase space for an electron FLASH (e-FLASH) treatment beam and investigate whether FLASH dose-rates can be achieved in clinically relevant targets through Monte Carlo (MC) 4D dose-rate distribution calculations.
Methods
10 MeV e-FLASH beams were generated on a Varian TrueBeam linac using electrons for a nominal 10 MV photon beam with the target removed, and scattering foil added in the beam path. Dose was measured with L-alanine in a 20x20x20 cm3 water phantom (100 cm SSD). Beam delivery was controlled via pulse counting, achieving a dose-rate of 59.13 Gy/s (2 cm depth). A 10 MeV electron phase space was obtained from Varian recorded upstream of the linac jaws. Particle transport through the jaws, MLCs and electron applicator was simulated using BEAMnrc. DOSXYZnrc simulated dose delivery to a 20x20x20 cm3 water phantom, for tuning MC calculations, as well as planning treatment volumes (PTVs) of two skin lesions (5 cm and 3.5 cm deep). The prescribed doses (60 Gy and 50 Gy) were delivered in one fraction by a 10x10 cm2 electron field, in 1.01 s and in 0.85 s respectively.
Results
Errors in total MC dose were below 5%. Dose-rates above 40 Gy/s (lower bound for FLASH effect) were achieved only up to a depth of 4 cm, or in 80% of one PTV volume and 95% of the other.
Conclusion
This 10 MeV e-FLASH beam is only able to achieve FLASH dose-rates in PTVs shallower than 4 cm. However, we demonstrated a proof-of-concept method for validating clinical FLASH beams and calculating instantaneous dose-rate distribution for treatment planning. Future work will investigate other clinical sites and beam modalities.