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A Multi-Stage Neutron Shielding System for Passively Scattered Proton Therapy

A Wroe

A Wroe1*, G McAuley2 , J Slater1 , (1) Loma Linda University Medical Center, Loma Linda, CA, (2) Loma Linda University, Loma Linda, CA


SU-K-205-17 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 205

Purpose: To design, evaluate and clinically implement a shielding system that will reduce out-of-field dose in passively delivered proton therapy.

Methods: A multi-stage neutron shielding system (MNSS) was designed and retrofitted to the Gantry 1 treatment nozzle at Loma Linda University Medical Center. The system uses multiple borated polyethylene plates situated after the primary beam modifying devices to locate increasing levels of shielding between the sources of secondary particles in the nozzle and the patient. Additionally, the shielding system was designed to allow for ease of retrofit to an existing proton nozzle system without impacting design, access for maintenance or treatment beam characteristics. The effectiveness of the MNSS was evaluated through measurements with Landauer Luxel+ dosimeters while Geant4 Monte Carlo simulations with a replicated setup were also used to evaluate microdosimetric spectra at various locations inside the phantom and treatment room.

Results: The MNSS did reduce out-of-field dose equivalent at the level of the patient by up to 46% for 149MeV protons and up to 41% for 250MeV protons. The average reduction for all measurement positions was 24.5% and 19% for the 149MeV and 250MeV beam energies respectively. The MNSS had less of an impact in reducing out-of-field dose equivalent at depth within the phantom and it is hypothesized that this is because the internal dose equivalent is increasingly made up of neutrons created within the patient that the MNSS is unable to shield. The installation of the MNSS also reduced dose equivalent within the treatment room by up to 80% which has benefit for electronic and computer support systems.

Conclusion: The MNSS proved to be an effective and inexpensive method to significantly reduce out-of-field dose to the patient and electronic systems in passively delivered proton therapy. Following this evaluation the system was deployed permanently at our facility.

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