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New Analysis Method of Proton Beam Profile Broadenings in Spot-Scanning Proton Nozzle Using Response Function of Angular Distribution

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H Ueda

H Ueda*, T Matsuura , S Hirayama , M Furusaka , K Umegaki , Hokkaido University, Sapporo, Hokkaido


SU-I-GPD-T-201 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In spot scanning proton therapy, highly precise beam control is required in the treatment nozzle so that the proton beam should not spread out during transportation by restraining the divergence of the beam angle and spot size simultaneously. In order to evaluate the beam broadening behavior passing through the various nozzle components, we propose a new method to calculate angular divergence profile of proton beam in the nozzle and demonstrate how useful it is to understand the beam broadening mechanism.

Methods: Angular divergence of the proton beam for each nozzle component is calculated by the Monte Carlo simulation code; Geant4, assuming that the initial beam has no divergence. Then the angular divergence profiles generated in the various nozzle components are fitted by the analytic function formula with triple Gaussian distributions. The fitted profiles can be treated like analytic response functions and the angular divergence profile in the nozzle can be easily and systematically calculated by using Fourier transformation and its convolution theorem. The beam broadening behavior during transportation in the nozzle is carefully evaluated.

Results: The beam profiles are well characterized by the proposed angular divergence analysis, i.e., by triple Gaussian profile analysis. Primary Gaussian part of the beam profile is mainly generated by air and dose monitors with various window components. The secondary and tertiary Gaussian parts are so called wide-angle scattering and generated mainly by spot position and profile monitors with metal window and wire components.

Conclusion: We conclude that the scattering of the nozzle component can be analyzed using the proposed response function method of the angular distribution. Multiple convoluted angular scattering can be determined from the response function of the individual nozzle components. Then, the angular distribution from small to large angle region can be quantitatively evaluated by the proposed method.

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