Experimental evaluation of out-of-field dose for different high-energy electron beams and applicators used in external beam radiotherapy

Out-of-field doses are much smaller than treatment-field doses, but most secondary cancer occurrences occur around the primary treatment area. Studies on out-of-field dose distribution in electron beam radiotherapy are limited in the literature. The current study aims to evaluate out-of-field doses at different depths relative to the beam center axis (CAX) with five high-energy electron beams and four commonly used electron applicators. The dose distribution of out-of-field doses at different depths up to 10 cm from the edge of the 6x6, 10x10, 15x15 and 20x20 cm2 applicators in the water phantom with the PTW Roos 34001 electron ion chamber was investigated according to the beam center axis (CAX). The out-of-field dose of the 6x6 and 10x10 cm2 applicators decreased linearly to 3.5 cm at all energies. Since the penetration of electrons is directly proportional to the energy, it was observed that the out-of-field dose approached the CAX dose as the depth increased, especially at low energies. Our findings show an apparent reduction in out-of-field doses as the distance and depth to the field edge increase. It also shows that the out-of-field dose does not always decrease linearly with distance but depends on the applicator and depth. It is recommended that the radiotherapy team consider out-of-field doses in electron treatments.