Comparing experimental assessment of the peripheral dose outside the applicator in electron beams of ELEKTA with Treatment planning system

Introduction: The use of electrons in the electron therapy to destroy tumoral tissue is dedicated significant contribution of different methods of radiation therapy. Scattered radiation due to exited electrons of the applicator affect the dose out of the field in the patient's normal tissue. The aim of this study is to determine the peripheral dose outside the applicator in the electron beams of Elekta accelerator. Materials and Methods: In this study the peripheral dose outside the applicator in electron beams was measured using linear accelerator Elekta Synergy Plateform and it was compared with the obtained results from the treatment planning system (TPS) Isogray model. The peripheral dose profiles were measured and compared Using the solid water phantom set and the film dosimetry system of EBT3 at energy levels 6, 10 and 18 MeV in the applicators with the different dimensions and at different angles of 0, 10 and 20 at the depth 0 and 0.5 and 1cm and for each energy level at depth of maximum dose (Dmax). The peripheral dose Profiles has been normalized to the edge of the field. Results: The highest Peak dose was observed in 18 MeV beam outside the applicator. Peak dose was reduced with increasing the electron beam energy. Using the applicator 20 × 20 cm2 for energy 18 MeV, a dose peak of 1.6% was observed on the surface at a distance of 2 cm from the outer edge of applicator and also for energy 6 MeV, 1.15% at the same distance from the edge of the applicator. It was found that the dose peak decreases with increasing depth and increases with increasing field size. Also the dose peak increased with increasing gantry angle from zero to 20 degrees. The result of measured peripheral dose outside the applicator by using TPS of Clarkson algorithm showed that the dose Calculation can be evaluated only up to distance of 3cm from the edge of the field. Conclusion: Totally, anticipation of dose distribution of the electron beams in collision with matter is different and difficult in various accelerators because of the complexity of the behavior of electrons. However, according to the results of this study in Elekta accelerator, noticeably dose peak was observed in use of high-energy electron beams because of more incidence of bremsstrahlung radiation outside the treatment field. The radiotherapy team can have prevented from damage to organs at risk outside the applicator by awareness of the value and location of peripheral dose outside the treatment field with applying the principles of radiation protection like shields. The evaluated TPS is not a good way to measure the peripheral dose outside the field.