Experimental assessment of peripheral dose in high-energy electron beams used in external radiotherapy

Abstract

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 2300 C/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 5 cm to 65 cm from the field edge for SSD = 100 cm. Thermoluminescent TLD-700 powder dosimeters were used. Measurements were made at both 1 cm and 10 cm depths, using applicator sizes from 6 × 6cm 2 to 20 × 20 cm 2 , the results assuming that the TLD signal is proportional to the dose. Whatever the field size, a peak dose spot appeared at 15 cm from the field edge for Siemens applicators. For Siemens Primus with an applicator size of 10 × 10 cm 2 , 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 × 6 cm 2 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 × 20cm 2 . In contrast for Varian 2300 C/D, the doses at 15 cm from field edge were 0,3%, 0,6% and 1,1% of Dmax for 6,12 and 18 MeV, respectively. No peak dose spot was evidenced for Varian applicator. The doses measured at 10 cm depth were 2–3 times lower than doses at 1 cm depth for out off- field distances from 5 cm to 35 cm, 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 15 cm from field edge ranges from 0,3–2,7% of the Dmax. Measurements made at 10 cm depth show that, depending on beam energy and applicator size and collimator size, the dose at 15 cm 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.