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Measurements of the secondary neutron field at the OncoRay Proton Therapy Facility in Dresden


Bonnerkugel-Messung mit einem antropomorphen Phantom an der OncoRay Protonentherapieeinrichtung in Dresden (Universitätsklinikum Carl Gustav Carus Dresden).

Modern radiation therapy techniques aim for an optimized dose deposition within the tumor region while minimizing the integral dose to healthy tissue and risk organs. Technical advances in beam production and delivery have established proton therapy as a growing cancer treatment technique. Due to their unique physical properties, proton beams allow for the deposition of high doses at the position of the tumor while keeping the dose to the surrounding tissue relatively low. However, in addition to proton radiation, the patient is also exposed to secondary radiation in the form of neutrons, photons and charged particles (produced when protons interact with atomic nuclei in the material of the device, therapy room, and in the patient) which produces an out-of-field dose that affects healthy tissue. The determination of the additional dose from secondary radiation is of importance in order to assess the risk of secondary cancers. Neutron radiation can provide the largest contribution to this out-of-field dose, and is of concern in the case of pediatric treatments and the treatment of pregnant patients.

Estimates of the out-of-field dose require accurate information about the quality and extent of the neutron radiation component for specific facilities, as a function of beam delivery and for particular patient configurations. This information is currently very limited and subject to large uncertainties. Therefore, it is important to characterize the stray neutron field in the therapy room.

As dose conversion factors for neutrons are strongly energy dependent, the determination of the neutron dose requires spectrometry, which must be carried out over a wide range of neutron energies, from thermal energies of about 10-8 MeV to energies that are comparable to the maximum proton energy; i.e., a few hundred MeV. While such spectra can be simulated using Monte Carlo particle transport codes, it is important to benchmark these calculations using spectrometric measurements carried out under conditions that are similar to the ones assumed for the simulations.

As part of a collaboration with HZDR , PTB has carried measurements of neutron spectra at the OncoRay Proton Therapy Facility in Dresden (University Hospital Carl Gustav Carus in Dresden). The main aim of these measurements was the validation of computer calculations that provide the basis for evaluating the neutron dose in patients. To achieve this goal, we have carried out simulations (HZDR) and measurements (PTB) of secondary neutrons for different proton irradiation conditions in the proton therapy room, as well as additional simulations and measurements in an experimental hall using a simpler set-up than the one of the therapy room. The approach that we followed involved generating detailed simulations of the beam generation process using GEANT4 to cover all possible configurations and carrying out measurements for selected configurations using an extended range Bonner sphere spectrometer (ERBSS).

The measurements of secondary neutrons were done using the PTB Bonner sphere spectrometer NEMUS (NEutron MUltisphere Spectrometer). NEMUS is the secondary standard of PTB for the dissemination of the unit for the ambient dose equivalent for neutron radiation in unknown radiation fields, e.g., in workplaces and in the environment. NEMUS consists of a set of moderator spheres that are equipped with a detector in the centre that measures the thermalized neutrons. The response of NEMUS to neutrons is validated and traced back to primary PTB standards of neutron sources and reference monoenergetic neutron fields. NEMUS is not sensitive to other radiation qualities, like, e.g., gamma radiation.

The experimental campaign at the OncoRay Proton Therapy Facility in Dresden took place in December 2015. The benchmarking of the detailed Monte Carlo simulations is currently under way.