Ray Tracing Algorithm for Virtual Source Modeling based on Evaluation of Rounded Leaf End Effect of Multileaf Collimator

Abstract

Rounded leaf end effect of multileaf collimator plays a role in penumbra characteristics of dose profile. It strongly influences dose target volume conformity and organ at risk sparing for radiation therapy. Previous studies have shown that parameters in virtual source model could be fitted using dose distribution results obtained from Monte Carlo simulation or dosimetric measurement. However, there is a lack of research in literature reporting on virtual source modeling with the rounded leaf end effect incorporated. In this study, we present an analytical method for virtual source modeling by evaluation of the rounded leaf end effect of multileaf collimator on dose profile. We confine our study to single source modeling, with source energy distribution of Gaussian shaped. Ray Tracing algorithm is introduced and parameter estimation is conducted to revise the source size. Model verification is performed with numerical simulation, which is conducted based on the Monte Carlo codes of EGSnrc/BEAMnrc. It is shown that Ray Tracing algorithm employing the focal spot size of photon source would result in reduced penumbra width, compared with the simulation counterpart. This observation is probably related to the aperture effect of multileaf collimator. In contrast, results of penumbra width and radiation field edge offset using revised source size obtained with parameter estimation agree well with Monte Carlo simulation. In summary, the method we propose could provide insight into virtual source modeling using Ray Tracing algorithm.