Assessment of out-of-field radiation dose in proton therapy by combining in-phantom measurements with different...

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.

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.

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.