Optimization of photon beam flatness for radiation therapy

In this work, we investigate the relation between lateral fluence/dose distributions and photon beam uniformity, possibly identifying ways to improve these characteristics. The calculations included treatment head scatter properties associated with three common types of linear accelerators in order to study their impact on the results. For 6 and 18 MV photon beams the lateral fluence distributions were optimized with respect to the resulting calculated flatness, as defined by the International Electrotechnical Commission (IEC), at 10 cm depth in six different field sizes. The limits proposed by IEC for maximum dose ratios ('horns') at the depth of dose maximum have also been accounted for in the optimization procedure. The conclusion was that typical head scatter variations among different types of linear accelerators have a very limited effect on the optimized results, which implies that the existing differences in measured off-axis dose distributions are related to non-equivalent optimization objectives. Finally, a comparison between the theoretically optimized lateral dose distributions and corresponding dose measurements for the three investigated accelerator types was performed. Although the measured data generally fall within the IEC requirements the optimized distributions show better results overall for the evaluated uniformity parameters, indicating that there is room for improved flatness performance in clinical photon beams.