Development of a Beam Source Modeling Approach to Calculate Head Scatter Factors for a 6 MV Unflattened Photon Beam

Abstract

This study aimed to investigate the accuracy of head scatter factor (S c ) by applying a developed multi-leaf collimator (MLC) scatter source model for an unflattened photon beam.Methods: Sets of S c values were measured for various jaw-defined square and rectangular fields and MLC-defined square fields for developing dual-source model (DSM) and MLC scatter model. A 6 MV unflattened photon beam has been used. Measurements were performed using a 0.125 cm 3 cylindrical ionization chamber and a mini phantom. Then, the parameters of both models have been optimized, and S c has been calculated. The DSM and MLC scatter models have been verified by comparing the calculated values to the three S c set measurement values of the jaw-defined field and the two S c set measurement values of MLC-defined fields used in the existing modeling, respectively.Results: For jaw-defined fields, the calculated S c using the DSM was consistent with the measured S c value. This demonstrates that the DSM was properly optimized and modeled for the measured values. For the MLC-defined fields, the accuracy between the calculated and measured S c values with the addition of the MLC scatter source appeared to be high, but the only use of the DSM resulted in a significantly bigger differences.Conclusions: Both the DSM and MLC models could also be applied to an unflattened beam. When considering scattered radiation from the MLC by adding an MLC scatter source model, it showed a higher degree of agreement with the actual measured S c value than when using only DSM in the same way as in previous studies.