SU‐E‐T‐304: Study of Secondary Neutrons From Uniform Scanning Proton Beams

Abstract

Purpose:Secondary neutrons are unwanted byproducts from proton therapy and exposure from secondary radiation during treatment could increase risk of developing a secondary cancer later in a patient's lifetime. The purpose of this study is to investigate secondary neutrons from uniform scanning proton beams under various beam conditions using both measurements and Monte Carlo simulations.Methods:CR‐39 Plastic Track Nuclear Detectors (PNTD) were used for the measurement. CR‐39 PNTD has tissue like sensitivity to the secondary neutrons but insensitive to the therapeutic protons. In this study, we devised two experimental conditions: a) hollow‐phantom; phantom is bored with a hollow cylinder along the direction of the beam so that the primary proton passes through the phantom without interacting with the phantom material, b) cylindrical‐phantom; a solid cylinder of diameter close to the beam diameter is placed along the beam path. CR‐39 PNTDs were placed laterally inside a 60×20×35 cm3 phantom (hollow‐phantom) and in air (cylindrical‐phantom) at various angles with respect to the primary beam axis. We studied for three different proton energies (78 MeV, 162 MeV and 226 MeV), using a 4 cm modulation width and 5cm diameter brass aperture for the entire experiment and simulation. A comparison of the experiment was performed using the Monte Carlo code FLUKA.Results:The measured secondary neutron dose equivalent per therapeutic primary proton dose (H/D) ranges from 2.1 ± 0.2 to 25.42 ± 2.3 mSv/Gy for the hollow phantom study, and 2.7 ± 0.3 to 46.4 ± 3.4 mSv/Gy for the cylindrical phantom study. Monte Carlo simulations predicated neutron dose equivalent from measurements within a factor of 5.Conclusion:The study suggests that the production of external neutrons is significantly higher than the production of internal neutrons.