Assessment of out-of-field radiation dose in proton therapy by combining in-phantom measurements with different passive detectors.
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INTRODUCTION: Proton therapy is an advanced radiotherapy technique that reduces radiation exposure to healthy tissues while maintaining tumor control. However, out-of-field radiation dose remains a concern due to their potential contribution to secondary cancer risks, particularly in pediatric patients and in the fetus of pregnant patients. Accurate assessment of out-of-field dose is challenging due to the mixed radiation field composed of neutrons, protons, electrons, photons, and alphas with a wide energy range. METHODS: This study investigates how in-phantom measurements with lithium fluoride (LiF) MTS (LiF:Mg,Ti) and MCP (LiF:Mg,Cu,P) thermoluminescent detectors (TLDs) with different concentrations of 6Li and 7Li and bubble detectors (BD-PNDs) can be combined to assess total out-of-field dose in proton therapy. A previous measurement campaign, during which a five-year-old phantom was treated for a brain tumor, was used as a representative case. The out-of-field dose and detector response for this case were modelled by combining Monte Carlo radiation transport simulations, fluence to dose conversion factors and detector response functions. RESULTS AND CONCLUSIONS: The modelled and experimental measurements were in good agreement, considering the limitations of the simulations. The results of this study demonstrated that MTS-7 and MCP-7 detectors with 99.9% 7Li enrichment can provide a good estimate of the absorbed dose to water and dose equivalent from all particles excluding neutrons, while BD-PNDs can provide a reasonable estimate of the neutron dose equivalent. Combining MTS-7 or MCP-7 TLDs with BD-PNDs thus allows to get a complete picture of the out-of-field dose in proton therapy by in-phantom measurements.