Background and Aim: Beam shaper applicator is one of the dedicated applicators for intraoperative electron radiotherapy which is usually employed for large tumors irradiation. Due to the high weight and lack of possibility of direct placement on the patient’s body, a considerable air gap exists between the applicator and patient. Therefore, determination of the effective position of electron source (SSD_eff) is absolutely necessary to correct the applicator output for air gap effect and accurate delivery of the prescribed dose to the patient. The aim of this study was to determine the SSD_eff for all square fields of beam shaper applicator at different electron energies using Monte Carlo (MC) simulation.
Materials and Methods: At first, the head of LIAC accelerator and beam shaper applicator were simulated by MCNPX MC code and then, the validity of the simulated model was evaluated through comparing the calculated percentage depth dose (PDD) curves at different field sizes and energies with the corresponding practiacal values. After verifying the simulated model, the SSD_eff was determined for different field sizes and electron energies using inverse square law (ISL) method.
Results: The results showed that the SSD_eff is a function of electron energy and radiation field size. Dependency of SSD_eff on field size variations was much more than that on the alterations in the electron energy. Generally, increase in the field size or electron energy led to increase in SSD_eff.
Conclusion: The determined SSD_eff values for the electron beam of the beam shaper applicator in our study, can be used to correct the electron beam output for clinical purposes.