Dose perturbations at high-Z interfaces in kilovoltage photon beams: Comparison with Monte Carlo simulations and measurements

Dose perturbations, backscatter dose perturbation factor (BSDF), and forward dose perturbation factor (FDPF) near high-Z material interfaces have been reported to be intense and significant in kilovoltage photon beams. Validity of estimation of dose perturbations is critical in the evaluation of radiation effects associated with high-Z interfaces. The magnitude of dose perturbations has been debated due to limitations in the measuring devices (mainly window thickness and chamber perturbations). Monte Carlo (MC) simulations have been proposed for the interface effects but poor statistics in small spatial bins (1μm) near the interface makes MC data questionable even with a well designed code. A moving-window least-square polynomial fit (MLPF) method is proposed to smooth MC simulated data. This method is shown to be useful in achieving reasonable accuracy from statistically poor MC data obtained within a reasonable computation time. The EGS4 and PENELOPE MC simulated data at the interfaces between high-Z materials and a water phantom have been used to calculate the BSDF and FDPF which differ significantly from the measurements suggesting the necessity of further refinements in MC approaches and better measuring devices for interface effects.