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-1to 65.5µGy Gy-1, while the gamma dose, which remained consistently lower, ranged between 88.4µGy Gy-1and 6.1µGy Gy-1. The measured dose equivalent varied between 6.3 mSv Gy-1and 82.3µSv Gy-1. 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 PT 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.

ADVACAM Prague Czech Republic

Department of Particle Therapy University Hospital Essen Essen Germany

Department of Physics TU Dortmund University Dortmund Germany

Department of Radiation Oncology UZ Leuven Leuven Belgium

Department of Radiotherapy Universitair Ziekenhuis Brussel Vrije Universiteit Brussel Brussels Belgium

German Cancer Consortium Essen Germany

KU Leuven Department of Oncology Laboratory of Experimental Radiotherapy Leuven Belgium

Radiation Protection Dosimetry and Calibration Expert Group Belgian Nuclear Research Center Mol Belgium

UCLouvain Institut de Recherche Expérimentale et Clinique MIRO Lab Brussels Belgium

West German Cancer Center University Hospital Essen Essen Germany

West German Proton Therapy Centre Essen Essen Germany

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