Determination of Effective Electron Source Position for Beam Shaper Applicator in Intraoperative Radiotherapy

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 impossibility of direct placement on the patient’s body, a significant air gap exists between the applicator and patient. Therefore, knowing the effective position of electron source (SSDeff) is quiet essential to correct the applicator output for this air gap effect and deliver an accurate prescribed dose to the patient. The current research aims to determine the SSDeff for all produced square fields by beam shaper applicator at different electron energies using Monte Carlo (MC) simulation.

At first, the head of LIAC accelerator in conjunction with beam shaper applicator was 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 measured ones. After verifying the simulated model, the SSDeff was determined for different field sizes and electron energies using inverse square law (ISL) method.

The obtained results showed that the SSDeff is a function of electron energy and field size. Dependency of SSDeff  on field size variations was much more than variations of electron energy. Generally, the SSDeff increments with either increasing the field size or electron energy.

The determined SSDeff values for the electron beam of the beam shaper applicator at the current study, can be applied to correct the electron beam output in clinical practice.