Characterisation of the Secondary Neutron Radiation at the University Proton Therapy Dresden

Radiation therapy is one of the most frequently used treatment modalities for cancer. While most patients receive photon therapy, a growing number of patients are treated with particles, mainly protons. Protons offer a more localized dose deposition compared to photon therapy. This allows a reduction of the dose that is applied by the primary beam to the healthy tissue outside the target volume. At the same time, the use of protons leads to a change in the composition of the secondary radiation field, when compared to photons. In most locations, the out-of-field dose is dominated by secondary neutrons. The out-of-field dose varies significantly in shape and magnitude. Currently, there is no established procedure to monitor the secondary neutron dose to the patients. This paper describes the simulation and measurement of the secondary neutron radiation field at the University Proton Therapy Dresden. The simulation uses a detailed model of the beam delivery system, an IBA universal nozzle. The simulations have been validated by a comprehensive set of extended range Bonner sphere spectrometer measurements, with the NEMUS spectrometer operated by the Physikalisch-Technische Bundesanstalt. The set of measurements covers all possible machine configurations for the double scattering mode. An excellent agreement between unfolded measurements and simulation predictions is achieved. The data show that the neutron field is varying strongly on the scale of a human body. This indicates that the use of fluence to dose conversion tables is not justified for neutron dose calculation in patients. Therefore, the presented data is of high importance for future studies of the organ doses in different treatment scenarios.