Modeling and Maximizing Cherenkov Emissions from Medical Linear Accelerators: A Monte Carlo Study

Purpose: Cherenkov light is a natural byproduct of MV radiotherapy; recent results demonstrate that it can activate the drug psoralen sufficiently to induce cytotoxicity and increase MHC1 signal in vitro. Here, we investigate Cherenkov radiation from common radiotherapy beams using Monte Carlo, as well as methods to maximize Cherenkov production per unit dose, using filters placed in the beam path.

Methods: GAMOS, a GEANT4-based framework for Monte Carlo simulations, was used to model primary photon beams using spectra from a Varian linear accelerator and mono-energetic electron beams. Cherenkov photon spectra and track length along with dose were scored when irradiating a sphere of water with radius 50cm and SSD=50cm. Further measurements were taken with photon beams irradiating a 17.8cm3 cubic water phantom at 1mm3 detectors with depths of 8 to 9cm; SSD was set to 94cm. Finally, measurements were taken with filters of varying material and thickness placed 15cm below a 10MV FFF beam source.

Results: Simulated Cherenkov spectra were found to have strong overlap with the psoralen absorbance spectrum; dose and Cherenkov photon track length measurements established that higher beam energies had greater Cherenkov production per unit dose, with 18MV providing greater Cherenkov/dose than 6MV by a factor of 4. Simulations with filters suggest that copper and iron filters increase Cherenkov per dose more than aluminum for a given filter thickness, but that aluminum yields a greater boost for a given dose rate.

Conclusion: Initial work has been completed to show that the Cherenkov spectrum produced by radiotherapy beams is well suited for activation of psoralen, and that higher energy photon beams will result in more psoralen activation due to greater Cherenkov radiation per unit dose. We have also demonstrated that significant boosts in Cherenkov/dose can be achieved with the use of filters without overly compromising dose rate. Future work should expand analysis to include optical properties of tissues as well as additional filter materials.