SU-E-T-108: Development of a Novel Clinical Neutron Dose Monitor for Proton Therapy Based On Twin TLD500 Chips in a Small PE Moderator

Abstract

Purpose: In proton therapy, it could be desirable to measure out-of-field fast neutron doses at critical locations near and outside the patient body. Methods: The working principle of a novel clinical neutron dose monitor is verified by MCNPX simulation. The device is based on a small PE moderator of just 5.5cm side length for easy handling covered with a thermal neutron suppression layer. In the simulation, a polystyrene phantom is bombarded with a standard proton beam. The secondary thermal neutron flux produced inside the moderator by the impinging fast neutrons from the treatment volume is estimated by pairs of α-Al2O3:C (TLD500) chips which are evaluated offline after the treatment either by TL or OSL methods. The first chip is wrapped with 0.5mm natural Gadolinium foil converting the thermal neutrons to gammas via (n,γ) reaction. The second chip is wrapped with a dummy material. The chip centers have a distance of 2cm from each other. Results: The simulation shows that the difference of gamma doses in the TLD500 chips is correlated to the mean fast neutron dose delivered to the moderator material. Different outer shielding materials have been studied. 0.5mm Cadmium shielding is preferred for cost reasons and convenience. Replacement of PE moderator material by other materials like lead or iron at any place is unfavorable. The spatial orientation of the moderator cube is uncritical. Using variance reduction techniques like splitting/Russian roulette, the TLD500 gamma dose simulation give positive differences up to distances of 0.5m from the treatment volume. Conclusion: Applicability and basic layout of a novel clinical neutron dose monitor are demonstrated. The monitor measures PE neutron doses at locations outside the patient body up to distances of 0.5m from the treatment volume. Tissue neutron doses may be calculated using neutron kerma factors.