Poster — Thurs Eve‐26: Influence of MLC leaf edge and tongue and groove effect on IMRT dose distributions

An important consideration when using multileaf collimators (MLCs) for IMRT delivery is the correct account of leaf edge effects. To study these effects, two maps were constructed from a clinical beam's step and shoot delivery sequence: a time-weighted leaf edge position (LEP) map where the pixel intensity was proportional to the length of time that a leaf edge defined the edge of any segment within the field, and a (TG) map where pixel intensity was proportional to the length of time that adjacent segments matched along a leaf edge. We investigated the correlation between LEP or TG maps with dose error maps (obtained by subtracting calculated from either measured (CM) or from re-calculated (CC) data). Re-calculated data were obtained by modifying selected MLC photon modeling parameters from their commissioned values. We calculated the correlation coefficient between corresponding regions of CM and CC maps with TG and LEP maps. A NAT analysis of the CM maps indicated that the NAT index was minimized for tongue and groove width at the commissioned value of 0.1cm. A higher correlation coefficient was seen between CM and LEP maps (0.62±0.11) than between CM or CC and TG maps across all MLC modeling parameters used. The low correlation between both LEP or TG maps and CC maps suggests that the higher correlation observed between both LEP or TG maps and CM maps cannot be attributed to the choice of MLC modeling parameters alone. Further work is needed to pinpoint the cause of this correlation.