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

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 cm² applicator 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.