Parameterisation of Small Photon Fields in Off-Axis Regions for the SIEMENS 160 MLC

The new SIEMENS MLC160 offers the application of intensity modulated techniques and stereotactic treatements within the whole 40 cm x 40 cm irradiation field. In this work we analyzed dose profiles of small off-axis fields. Larger deviations from symmetrical dose distributions have been found for fields which are generated at the maximum diagonal off-axis positions. Two different methods for an analytical description of the dose distribution generated by these fields are presented, based on the convolution of a rectangular linear- function with a kernel. A sum of two Gaussian distributions as well as a Lorentz-type function are used as such kernels. While the Gaussian method leads to known field parameterization parameters, the Lorentz type kernel offers a larger data reduction and a full analytical description of the profile.