(Sunday, 7/29/2018) 1:00 PM - 1:55 PM
Room: Karl Dean Ballroom A2
Purpose: In the pencil beam scanning (PBS) proton therapy, a collimator is used for reducing the lateral penumbra at shallow depth. To date, while the dose increase caused by the collimator-scattered protons has been estimated, its biological effect has not been explored. The scattered-protons may have a bigger impact on biological dose because of their low energy. The aim of this study was to investigate the physical and biological dose impact caused by using collimators in the PBS system.
Methods: The PBS system with an applicator mounting a 2 or 4-cm brass collimator and a 4-cm energy absorber was simulated with Geant4 Monte Carlo code (ver. 10.1.p03). Penetration range (5-20 cm), width of the spread-out Bragg peak (SOBP, 5-10 cm), and field size (2×2-16×16 cm²) were systematically changed. Comparisons were made between the physical and biological doses and dose-averaged LET (LETd) for protons not having hit the collimators and those for all protons. Their difference was evaluated at the surface and the center of the target along the beam central axis.
Results: The dosimetric impact of collimator-scattered protons increased with range and decreased weakly with SOBP width. Since at large field size the angle of the beam with respect to the collimator inner-wall is increased, the collimator scattering increases as well. At the surface, the maximum increase of physical dose and LETd was 3.8-30.3% and 0.01-0.44 keV/μm, respectively, and the maximum biological dose increase was 6.8-30.7%. At the center of the target, no significant changes were observed.
Conclusion: While the increase in physical dose is significant for small and large deep-seated targets, the increase in LETd was found to be negligible in all tested cases. Therefore, the increase in biological dose caused by edge-scattering fully attributes to the increase in physical dose.
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