Andrea Matamoros

Objective: One major advantage of proton therapy (PT) over conventional photon radiotherapy is reduced dose delivered to normal tissue. However, the complexity of the secondary radiation field composed of a mixture of particles with a wide energy range makes its characterization a challenging task. Approach: Measurements with a miniaturized Timepix detector were carried out in three positions out-of-field (7.4 cm, 14.1 cm, and 18.5 cm from the isocenter), inside a phantom resembling a 5 year old undergoing proton pencil beam scanning treatment for a brain tumor. Total and particle-specific deposited energy, absorbed dose, and dose equivalent in water were calculated. Results were compared with thermoluminescent detectors (TLDs) measurements and Monte Carlo (MC) simulations modelling the experimental setup. Main results: The proton absorbed dose in water normalized to the target dose, ranged from 4.8 mGy/Gy to 65.5 µGy/Gy, while the gamma dose, which remained consistently lower, ranged between 88.4 µGy/Gy and 6.1 µGy/Gy. The measured dose equivalent varied between 6.3 mSv/Gy and 82.3 µSv/Gy. Good agreement was observed for the two farthest-locations when comparing the absorbed dose in water estimated by the MiniPIX Timepix detector with TLD measurements and MC simulations. However, the closest position showed an overestimation for both the absorbed dose and the dose equivalent, while the farthest position exhibited an underestimation for the dose equivalent. Significance: Out-of-field dosimetry in proton therapy is challenging due to the complexity of the secondary mixed radiation field. Multiple detectors are typically required, but many are too large for use in anthropomorphic phantoms. This study demonstrates that the MiniPIX Timepix detector can accurately determine absorbed dose, dose equivalent and particle-specific contributions (electrons/gammas, protons, and ions). Unlike passive detectors such as TLDs, it enables active measurements with high time resolution, allowing dose rates analysis. The results, validated through experimental data and MC simulations, support the detector’s potential for reliable out-of-field dose assessment and improved patient safety.