Determination of depth and field size dependence of MLC transmission

Abstract

Treatment of large and complex targets may result in IMRT plans with relatively large contribution from MLC transmission. In such cases, comprehensive characterization of direct and scatter MLC transmission is important. We designed a set of tests (open beam, closed static multi-leaf collimator or MLC, and dynamic MLC gap) in order to determine dosimetric MLC properties as a function of field size and depth at the central axis. We developed a generalized model of MLC transmission to account for direct MLC transmission, MLC scatter, beam hardening, and leaf-end transmission (dosimetric gap). The model is consistent with the beam model used in IMRT optimization. We tested the model for extreme asymmetric fields relevant for large targets and split IMRT fields. We applied our MLC scatter estimation formula to clinically relevant cases and showed that MLC scatter is contributing a relatively large undesired background dose, especially in low-dose regions. This, in turn, may increase toxicity of normal organs, due to volume effect. For complex IMRT of large-volume targets, direct MLC transmission dose is found to be as high as 30% and MLC scatter up to 10% within the target wolume for the selected cases. We identified that the dose discrepancies between IMRT planning system (Eclipse^TM (Varian)) and ionization chamber measurements (inside and outside of the field) are due to an inadequate model of MLC transmission in the planning system (constant value model) and can be decreased if a more comprehensive model of MLC transmission is applied. In this study, we measured MLC transmission properties for Varian 6EX (6MV) and Varian 21EXs (6,10MV); however, the experimental method and theoretical model are general.