MEASUREMENT OF NEUTRON AMBIENT DOSE EQUIVALENT IN PROTON RADIOTHERAPY WITH LINE-SCANNING AND WOBBLING MODE TREATMENT SYSTEM.

The primary objective of this study was to measure secondary neutron dose during proton therapy using a detector that covers the entire neutron energy range produced in proton therapy. We analyzed and compared the neutron dose during proton treatment with passive scattering and line scanning. The neutron ambient dose equivalents were measured with a 190 MeV wobbling and line-scanning proton beam. The center of a plastic water phantom (30 × 30 × 60 cm3) was placed at the isocenter. A Wide-Energy Neutron Detection Instrument (WENDI-2) was located 1m from the isocenter at four different angles (0°, 45°, 90° and 135°). Both wobbling and line-scanning modes of a multipurpose and pencil beam scanning dedicated nozzles were used to obtain a spread-out Bragg peak with 10-cm-width for the measurements. The ambient dose equivalent H*(10) value was normalized by the proton therapeutic dose at the isocenter. For wobbling mode and line-scanning mode, the highest H*(10) values were 1.972 and 0.099 mSv/Gy, respectively. We successfully measured the neutron ambient dose equivalents at six positions generated by a 190 MeV proton beam using wobbling and line-scanning mode with the WENDI-2. These reference data could be used for neutron dose reduction methods and other analysis for advanced proton treatment in the near future.