A convergent electron beam approach for small-field irradiation with reduced bremsstrahlung.

Abstract

Objective. To investigate whether converging electron beams using quadrupole electromagnets can mitigate fluence loss and bremsstrahlung contamination in small irradiation fields.Approach. A three-stage quadrupole electromagnet system was installed beneath a linear accelerator head to converge 6, 9, 12, and 15 MeV electron beams. Dose distributions, dose rate, and bremsstrahlung contamination were evaluated for 2 cm and 1 cm fields. The 2 cm convergent field was compared with a 2 cm tubular applicator, whereas the 1 cm convergent field was evaluated at 5.1 and 10 cm air gaps. Monte Carlo simulations at 6 and 9 MeV assessed detector volume-averaging effects and the origins of bremsstrahlung photons.Main results. The dose rate of the 2 cm convergent field was 1.05-2.53 times higher than that of the 2 cm tubular applicator, while bremsstrahlung contamination was reduced by 0.40-1.79 percentage points. The 1 cm convergent field achieved a therapeutic field width ⩽1 cm and penumbra ⩽0.85 cm at all energies. At a 5.1 cm air gap, the dose rate was comparable to that of a 3 cm tubular applicator at 9 MeV and exceeded that of a 10 × 10 cm²applicator at 12 and 15 MeV. Bremsstrahlung contamination was 1.08%, 0.87%, 1.04%, and 1.32% at 6, 9, 12, and 15 MeV, respectively, and remained lower than that of the 2 cm tubular applicator at all energies. At a 10 cm air gap, output decreased and dose profiles broadened. Bremsstrahlung contamination increased to 2.56%, 1.70%, 1.72%, and 2.11%, respectively, but remained lower than that of the 2 cm tubular applicator at 12 MeV and above.Significance. Converging electron beams can mitigate fluence loss while reducing bremsstrahlung contamination in small fields. This externally mounted system provides proof of concept for high-spatial-resolution electron beam delivery and future electromagnetic spot-scanning applications.