Out-of-field patient doses in proton therapy are dominated by neutrons. Currently, they are not taken into account by treatment planning systems. There is an increasing need to include out-of-field doses in the dose calculation, especially when treating children, pregnant patients, and patients with implants. In response to this demand, this work presents the first steps towards a tool for the prediction of out-of-field neutron doses in pencil beam scanning proton therapy facilities. As a first step, a general Monte Carlo radiation transport model for simulation of out-of-field neutron doses was set up and successfully verified by comparison of simulated and measured ambient neutron dose equivalent and neutron fluence energy spectra around a solid water phantom irradiated with a variation of different treatment plan parameters. Simulations with the verified model enabled a detailed study of the variation of the neutron ambient dose equivalent with field size, range, modulation width, use of a range shifter, and position inside the treatment room. For future work, it is planned to use this verified model to simulate out-of-field neutron doses inside the phantom and to verify the simulation results by comparison with previous in-phantom measurement campaigns. Eventually, these verified simulations will be used to build a library and a corresponding tool to allow assessment of out-of-field neutron doses at pencil beam scanning proton therapy facilities.

Belgian Nuclear Research Center Mol Belgium

Danish Centre for Particle Therapy Aarhus University Hospital Aarhus Denmark

Departament de Física Universitat Autònoma de Barcelona Bellaterra Spain

Department of Medical Radiation Physics and Nuclear Medicine Karolinska University Hospital Stockholm Sweden

Department of Physics Royal Institute of Technology Stockholm Sweden

Department of Radiation Dosimetry Nuclear Physics Institute of the Czech Academy of Sciences Prague Czechia

Faculty of Medical Sciences University of Newcastle upon Tyne Newcastle Upon Tyne United Kingdom

Federal Office for Radiation Protection Neuherberg Germany

Helmholtz Zentrum München Institute of Radiation Medicine Neuherberg Germany

Institut de Radioprotection et de Sûreté Nucléaire PSE Santé Fontenay aux Roses France

Institute of Nuclear Physics Polish Academy of Sciences Krakow Poland

Swedish Radiation Safety Authority Solna Sweden

The Skandion Clinic Uppsala Sweden

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