Poster — Wed Eve—28: Monte Carlo Study of the Influence of a Novel Transmission Detector on 6 MV Photon Beam

The Monte Carlo technique is used to investigate the influence of a novel transmission detector (IBA Dosimetry, Germany) on a 6MV photon beam. The transmission detector may be used as an in vivoIMRTquality assurance tool , and therefore will be a source of contaminantelectrons. This could potentially affect prescribed dose to patients. The linear accelerator together with the transmission detector and a plastic block tray for comparison, were modeled using BEAMnrc/EGSnrc. Electron fluence at different SSDs (70, 80, 90, 100 cm), electron energy spectra, electronangular distributions, and surface doses were calculated. Calculated data were validated against measurements using a fixed parallel plate chamber. Calculated surface dose for both open field and non‐open fields (i.e. TRD and block tray) agree within 3% of measurement. The fluence of contaminantelectrons produced in the TRD and block tray fields increases at shorter SSD, but most electrons at shorter SSD are low energy electrons with large angular spread. These electrons contribute less to surface dose at larger SSD because they are either out‐scattered or absorbed in air. Surface doses at different SSDs for the block tray are higher than those of the TRD. Contribution of contaminantelectrons to dose in the buildup region increases with increasing field size. For a 20 × 20 cm 2 field, the contribution of electrons is 9.8 % and 8.6% for block tray and TRD respectively, while for a smaller 3 × 3 cm 2 field is 0.35% and 0.75% for TRD and block tray respectively.