The peripheral dose outside the applicator in electron beams of an Elekta linear accelerator.

Peripheral doses out of field could have short and long terms biological effects on patients treated with electron beams. In this study, peripheral dose outside the applicator was measured using the 6, 10 and 18 MeV beams of an Elekta synergy linac. For these beams dose profiles were measured using EBT3 film at various depths within a solid water phantom. Measurements were performed using 6 × 6, 10 × 10, 14 × 14 and 20 × 20 cm2 applicators at gantryangles of 0°, 10° and 20° and depths of 0, 0.5, 1 cm and depth of Dmax (maximum dose) for each energy. The peripheral dose profiles were normalized to the distance of 2 cm from the edge of each field. The largest peak of the peripheral dose was observed for 18 MeV 3 cm from the outer edge of the applicator. Peak dose increased with increasing energy. Peak dose at 18 MeV electron beam was 1.6% at the surface of phantom and at the distance of 2 cm from the outer edge of the applicator when the applicator of 20 × 20 cm2 was used. Peak dose at 6 MeV electron beam was 1.15% at the same distance in the same applicator size. It was found that the peak dose decreased with increasing depth and increased with increase in field size. Also, the peak dose moved towards CAX with increase in gantry angle.In general dose to tissue out of field could be reduced using appropriate shielding for each applicator and beam energy.