Technical Note: On maximizing Cherenkov emissions from medical linear accelerators

PurposeCherenkov light during MV radiotherapy has recently found imaging and therapeutic applications but is challenged by relatively low fluence. Our purpose is to investigate the feasibility of increasing Cherenkov light production during MV radiotherapy by increasing photon energy and applying specialized beam-hardening filtration. MethodsGAMOS 5.0.0, a GEANT4-based framework for Monte Carlo simulations, was used to model standard clinical linear accelerator primary photon beams. The photon source was incident upon a 17.8cm(3) cubic water phantom with a 94cm source to surface distance. Dose and Cherenkov production was determined at depths of 3-9cm. Filtration was simulated 15cm below the photon beam source. Filter materials included aluminum, iron, and copper with thicknesses of 2-20cm. Histories used depended on the level of attenuation from the filter, ranging from 100 million to 2 billion. Comparing average dose per history also allowed for evaluation of dose-rate reduction for different filters. ResultsOverall, increasing photon beam energy is more effective at improving Cherenkov production per unit dose than is filtration, with a standard 18MV beam yielding 3.3-4.0x more photons than 6MV. Introducing an aluminum filter into an unfiltered 2400cGy/min 10MV beam increases the Cherenkov production by 1.6-1.7x, while maintaining a clinical dose rate of 300cGy/min, compared to increases of similar to 1.5x for iron and copper. Aluminum was also more effective than the standard flattening filter, with the increase over the unfiltered beam being 1.4-1.5x (maintaining 600cGy/min dose rate) vs 1.3-1.4x for the standard flattening filter. Applying a 10cm aluminum filter to a standard 18MV, photon beam increased the Cherenkov production per unit dose to 3.9-4.3x beyond that of 6MV (vs 3.3-4.0x for 18MV with no aluminum filter). ConclusionsThrough a combination of increasing photon energy and applying specialized beam-hardening filtration, the amount of Cherenkov photons per unit radiotherapy dose can be increased substantially.