Background and purpose: Beam shaper is a type of applicator used in conjunction with the intraoperative electron radiotherapy. This study aimed at quantitative evaluation of the photon contamination of this applicator using Monte Carlo simulation.

Materials and methods: In this experimental study, at first the head of LIAC accelerator was simulated along with the beam shaper applicator using MCNPX Monte Carlo code. Validity of the simulated model was evaluated by comparing the percentage depth dose curves obtained by Monte Carlo simulation and practical dosimetry. Finally, the photon contamination at different clinical field sizes and electron energies was quantitatively determined.

Results: The results showed that by increase in field size, the photon contamination of the beam shaper applicator was considerably decreased. Furthermore, increment of electron energy could increase the photon contamination.

Conclusion: Increasing the photon contamination at the phantom surface by increment of electron energy and decrement of field size can be attributed to increasing the probability of electron interaction with the steel blades of the beam shaper and production of bremsstrahlung radiation at higher energies. Due to the photon contamination, employing the beam shaper applicator can increase the surface dose.