Room: AAPM ePoster Library
Purpose: To determine the ability to deliver a clinically acceptable pediatric whole brain radiotherapy plan at FLASH dose rates via two opposing 40 MeV electron beams produced by a practically realizable linear accelerator system.
Methods: The EGSnrc Monte Carlo modules, BEAMnrc and DOSXYZnrc, were used to simulate whole brain irradiation of a pediatric patient using two lateral 40 MeV electron beams. Electron beam phase space files were generated to model a diverging beam with a diameter of 10 cm at 50 cm SAD (defined at brain midline). The electron beams were collimated using a 10 cm thick block composed of an initial 5 cm of aluminum oxide and then 5 cm of tungsten. For comparison, 6 MV photon plans were calculated with the Varian AAA algorithm. Electron beam parameters were based on novel linear accelerators designed for the PHASER system and powered by a commercial 6 MW klystron.
Results: Calculations of the linear accelerator’s performance indicate an average beam current of at least 6.25 µA which would provide a dose rate of 115 Gy/s at the center of the brain (depth of 7 cm), high enough for the cognitive-sparing FLASH effect. The electron plan was less homogenous with a homogeneity index of 0.108 in comparison to the photon plan’s index of 0.087. The homogeneity of both 6 MV photon and 40 MeV electron plans would increase with intensity modulation which was beyond the scope of this initial investigation. Overall, the dosimetric characteristics of the 40 MeV electron plan were suitable for treatment.
Conclusion: Monte Carlo simulations indicate that two lateral 40 MeV electron beams can be used for pediatric whole brain irradiation at FLASH dose rates and serve as motivation for a simple clinical FLASH radiotherapy system that can be implemented in the near term.
Funding Support, Disclosures, and Conflict of Interest: KKB, PGM, and BWL have received research support from Varian Medical Systems. SGT, PGM, and BWL are co-inventors on Stanford patents pertaining to PHASER related technologies. SGT, PGM, and BWL are co-founders of TibaRay. SGT and BWL are board members of TibaRay.