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 alinear accelerator (linac). many of electron fields need the shielding of normal or critical organs. the electron shields are usually leadslabs with few millimeter thicknesses which should be placed near the skin, less than 1 cm away from skin. in the inspection of patientssetting 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 theshadow of the field still matches the shielded area. this is due to a conceptual mistake in which one assumes that electrons travel in astraight line and matching the shadow of lead slab is enough for the shielding. this project is about monte carlo simulation of this caseand dosimetry in which the excess dose to the tissue under the shield is calculated. in this study, beamnrc and dosxyznrc are usedfor simulation of the linac and the electron shields. the water phantom as well as the linac head (nepton linac) is simulated in theelectron mode. the simulation is performed in three various cases in which the lead shield is placed in distances of 1, 20, 40 cm fromthe 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 movefrom the surface because of the divergence of the light field. the simulations were done in two energies, 6 and 13 mev. the experimentsalso were done with edr2 film dosimetry and the simulation results were validated using the experimental results. in all cases, the doseunder the shield was normalized to the dose in the center. the dose of the normal organ under the shield was 2, 38, 43% with respectto the center for shield distances of 1, 20, 40 cm, respectively. so there is a considerable increasing of the dose due to the distancedshielding. in this work exact amount of the dose from this mistake (distanced shielding) is calculated and simulated.