calculation of excess dose to the eye phantom due to a distanced shielding for electron therapy in head and neck cancers

for superficial lesions, the electrons may be used for radiation therapy. the high energy photons and electrons are produced by a linear accelerator (linac).   many of electron fields need the shielding of normal or critical organs. the electron shields are usually lead slabs with few millimeter thickness which should be placed near the skin, less than 1 centimeter away from skin. in the inspection of   patients setting in a clinic by a physicist, it was noted that, in some cases the technician places the shields far away from skin in the way that the shadow of the field still matches the shielded area. this is due to a conceptual mistake in which one assumes that electrons travel in a straight line and matching the shadow of lead slab is enough for the shielding. this project is about monte carlo simulation of this case and dosimetry in which the excess dose to the tissue under the shield is calculated.   in this study beamnrc and  dosxyznrc is used for simulation of the linac and the electron shields . the water phantom as well as the linac head (nepton linac) is simulated in the electron mode. the simulation is performed in 3 various cases in which the lead shield is placed in distances of 1, 20, 40 centimeters from the surface of the phantom. in all cases, the edge of the shield is matched with the light field, so the shields get smaller as they move from the surface because of the divergence of the light field.  the simulations were done in two energies, 6 and 13 mev.  the experiments also were done with edr2 film dosimetry and the simulation results were validated using the experimental results. in all cases the dose under the shield were normalized to the dose in the center. the dose of the normal organ under the shield was around 10 %, 30%, 35 % with respect to the center for shield distances of  1, 20, 40 cm respectively. so there is a considerable increasing of the dose due to the distanced shielding. in this work exact amount of the dose from this mistake is calculated and simulated.