SU-E-T-287: Measured Neutron Levels at the Washington University Proton Therapy Facility

Purpose: To measure neutron dose levels at various locations in the proton radiation therapy treatment room. Methods: Proton beam was delivered to a phantom from the Mevion S250 superconducting synchro-cyclotron that uses passive scattering. The proton beam had a range of 17.7 g/cm2 and was modulated to give uniform dose over the full range. The beam was collimated to 10×10 cm2 using a brass aperture. The beam current/proton dose rate was varied to ensure the neutron detector response was within its linear dose rate measurement range. Dose to the phantom was measured using an IBA FC65-P cylindrical ionization chamber and an IBA Dose 1 electrometer. Neutrons were measured with a Thermo Scientific FHT 762 WENDI-2 neutron dose rate detector including a quality factor of 10. Neutron dose rates were measured at 4 room locations: 1m lateral from isocenter, 4m from isocenter, near the opening to the treatment room maze, and within the maze. Results: Pile-up from prompt neutrons limited the WENDI-2 to a dose rate of ∼2mSv/hr. Beyond that, neutron readings would not scale with proton dose, indicating a reduced efficiency for measuring neutron dose. The highest neutron dose observed was at the location 1m lateral to isocenter. At that location a dose of 0.3mSv per Gray of dose delivered to the phantom was measured. Four meters from isocenter the neutron dose rate reduced to 0.1 mSv per Gray of proton dose. At the opening to the maze we measured 0.07 mSv/Gy. Immediately inside the maze this dose dropped to 0.003 mSv/Gy. Conclusion: The WENDI-2 gave reproducible results as long as the dose rate from prompt neutrons was kept below 2mSv/hr. The Mevion system neutron levels were measured to be well below the 0.1% leakage limit set for x-ray therapy units.