Peripheral dose from bremsstrahlung and scattered electrons in high-energy electron beams in external radiotherapy remains poorly studied. The purpose of this work is to assess peripheral dose outside applicators at the patient level.

Commissioning was performed for 6, 9, 12 and 18 MeV electron beams on three linear accelerators Varian 2300C/D (Varian applicator), Siemens Primus KD2 (Deva applicator) and Siemens Oncor (applicator EA3). The peripheral dose was measured in a water phantom as a function of off-axis distance from 5cm to 65cm from the field edge for SSD=100cm. Thermoluminescent TLD-700 powder dosimeters were used. Measurements were made at both 1cm and 10cm depths, using applicator sizes from 6×6cm² to 20×20cm², the results assuming that the TLD signal is proportional to the dose.

Whatever the field size, a peak dose spot appeared at 15cm from the field edge for Siemens applicators. For Siemens Primus with an applicator size of 10×10cm², this peak reached 2,1%, 1%, 0,9% and 1,3% of Dmax for 6, 9, 12 and 18 MeV electron beams, respectively, doubled for 6×6cm² field size. For Siemens Oncor, with the above applicator size, this peak dose reached 0,8%, 1,2% and 1,3% of Dmax for 6, 9 and 12MeV, respectively, doubled for 20×20cm². In contrast for Varian 2300C/D, the doses at 15cm from field edge were 0,3%, 0,6% and 1,1% of Dmax for 6,12 and 18MeV, respectively. No peak dose spot was evidenced for Varian applicator. The doses measured at 10cm depth were 2–3 times lower than doses at 1cm depth for out off- field distances from 5cm to 35cm, and were 4–7 times lower for larger distances.

This work analyzes peripheral doses for different electron beams energies and three different applicator types. It shows that depending on beam energy, applicator size and type, the peak dose at 15cm from field edge ranges from 0,3–2,7% of the Dmax. Measurements made at 10cm depth show that, depending on beam energy and applicator size and collimator size, the dose at 15cm from field edge is 0,07–0,2% of Dmax. Although the debate on signal to dose conversion coefficients for mixed fields is still ongoing, our results should be considered because, to date, peripheral dose with electron beams are not fully accounted for by TPS.