Radiological assessment of Cerrobend and tungsten carbide as aperture materials in proton therapy

The use of patient-specific apertures in pencil beam scanning (PBS) proton therapy improves target conformity but increases secondary neutron production and induces aperture activation. While brass is commonly used for apertures, Cerrobend and tungsten carbide may offer cost-effective alternatives.

This study evaluates the viability of Cerrobend and tungsten carbide for PBS apertures by examining secondary neutron production and activation using Tool for Particle Simulation (TOPAS). Comparisons were made with brass and also nickel and tungsten alloy, which are materials proposed for dynamic collimation systems (DCS).

TOPAS was used to simulate the irradiation of 230 MeV proton beams on fully closed apertures, with neutron fluence and dose scored in water phantom. Custom scorers recorded isotopes formed in apertures, with residual activity calculated using phase space files and Python scripts.

Nickel yielded the lowest secondary neutron dose among all tested materials. The ambient dose equivalent from neutrons produced by tungsten carbide and Cerrobend were 10 % lower and 28 % higher compared to brass, respectively. Over a 6-week treatment course with 5-fractions per week, Cerrobend and tungsten carbide showed similar and 1.4 times higher activation than brass, respectively. The residual activity of Cerrobend and tungsten carbide fell below brass 30 and 130 days post-treatment, respectively. Nickel's initial activity was 25 % of brass but residual activity remained three times higher after one year.

Brass remains the optimal aperture material, but tungsten carbide and Cerrobend could be viable alternatives for PBS proton therapy. Nickel's high residual activity could pose challenges for DCS.