SU-E-T-21: Modeling a MLC Scatter Source for In-Air Output Factors.

PURPOSE Scattered radiation from multi-leaf collimators (MLCs) is no longer negligible for calculating in-air output ratio, Sc for small and irregular fields often used in intensity-modulated radiation therapy (IMRT). An extra-focal source model for scattered radiation from MLCs, namely MLC scatter source, has been developed to improve the accuracy of the Sc calculation. METHODS A conventional dual-source model was made by using Sc data that were measured for collimator-defined fields of Varian Clinac IX linear accelerator. Then, an MLC scatter source at the center of the MLC position of the linear accelerator was assumed in the model. The MLC scatter source model consisted of two Gaussian functions of which parameters were iteratively optimized against the Sc data measured for different MLC fields with fixed collimator sizes. To evaluate the effectiveness of the developed source model, measurements were made for various MLC-defined irregular or square fields. The calculated Sc data by using (1) the developed source model and (2) the conventional dual source model were compared with the measured data. RESULTS The mean discrepancy between the measured Sc and calculated Sc from the developed source model was 0.08+-0.28%, while one from the conventional source model was 0.44+-0.39%. CONCLUSIONS The developed MLC scatter source model in conjunction with the dual source model could improve the accuracy of the Sc calculation in IMRT fields.