To derive the effect of these missing contaminating electrons, the treatment head geometry was investigated in order to find some component that was close enough to the beam and that could have an influence. Close to the mirror in the treatment head there is a lot of shielding material made of tungsten and lead and this shielding relatively close to the photon beams. Simulations for the 18 MV photon beam 40 x 40 cm2field were done, where the shielding material surrounding the mirror was included, toinvestigate if the contaminant electron fluence at the phantom surface increased.
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