PURPOSE: To measure the neutron dose equivalent per therapeutic proton dose (H/D) in a passive scattering proton therapy system and study its dependence on the proton energy, aperture-to-isocenter distance, spread-out Bragg peak (SOBP) width, and field size. METHODS AND MATERIALS: We performed four experiments of varying proton energies, aperture-to-isocenter distances, SOBP widths, and field sizes. Etched track detectors were used to measure the neutron dose equivalent at both an in-field (isocenter, beyond the protons' range) and out-of-field (30 cm lateral to the isocenter) location in air. RESULTS: For a nonmodulated beam with all the protons stopping in the aperture and an aperture-to-isocenter distance of 30 cm, the H/D values measured at the isocenter were approximately 0.3 mSv/Gy for all snouts with a 100-MeV beam. The H/D values increased to 10.7, 14.5, and 15.1 mSv/Gy, respectively, for small, medium, and large snouts when the beam energy increased to 250 MeV. At the out-of-field location, H/D values increased from 0.1 to 2.7, 3.0, and 3.2 mSv/Gy, respectively, for small, medium, and large snouts. When the aperture-to-isocenter distance was changed from 10 to 40 cm, the H/D value at the isocenter dropped 70%. The H/D value doubled for the modulated beam relative to the nonmodulated beam. Open apertures reduced the neutrons produced in the nozzle, but increased those produced in the phantom. CONCLUSIONS: Our data showed that changes in the four factors studied affect the H/D value in predictable ways which permits an estimate of a patient's neutron exposure.
PURPOSE: To measure the neutron dose equivalent per therapeutic proton dose (H/D) in a passive scattering proton therapy system and study its dependence on the proton energy, aperture-to-isocenter distance, spread-out Bragg peak (SOBP) width, and field size. METHODS AND MATERIALS: We performed four experiments of varying proton energies, aperture-to-isocenter distances, SOBP widths, and field sizes. Etched track detectors were used to measure the neutron dose equivalent at both an in-field (isocenter, beyond the protons' range) and out-of-field (30 cm lateral to the isocenter) location in air. RESULTS: For a nonmodulated beam with all the protons stopping in the aperture and an aperture-to-isocenter distance of 30 cm, the H/D values measured at the isocenter were approximately 0.3 mSv/Gy for all snouts with a 100-MeV beam. The H/D values increased to 10.7, 14.5, and 15.1 mSv/Gy, respectively, for small, medium, and large snouts when the beam energy increased to 250 MeV. At the out-of-field location, H/D values increased from 0.1 to 2.7, 3.0, and 3.2 mSv/Gy, respectively, for small, medium, and large snouts. When the aperture-to-isocenter distance was changed from 10 to 40 cm, the H/D value at the isocenter dropped 70%. The H/D value doubled for the modulated beam relative to the nonmodulated beam. Open apertures reduced the neutrons produced in the nozzle, but increased those produced in the phantom. CONCLUSIONS: Our data showed that changes in the four factors studied affect the H/D value in predictable ways which permits an estimate of a patient's neutron exposure.
Authors: Daniel R Gomez; Falk Poenisch; Chelsea C Pinnix; Tommy Sheu; Joe Y Chang; Nada Memon; Radhe Mohan; Marc A Rozner; Anne H Dougherty Journal: Int J Radiat Oncol Biol Phys Date: 2013-11-01 Impact factor: 7.038
Authors: Xin Wang; Falk Poenisch; Narayan Sahoo; Ronald X Zhu; MingFwu Lii; Michael T Gillin; Jing Li; David Grosshans Journal: J Appl Clin Med Phys Date: 2016-09-08 Impact factor: 2.102