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The use of proton beams in radiation therapy allows for the deposition of high doses at the tumor position while minimizing the dose to the surrounding healthy tissue. However, in addition to proton radiation, the patient is also exposed to secondary radiation, which produces an unwanted out-of-field dose. As neutron radiation can provide the largest contribution to this out-of-field dose for proton therapy, it is important to characterize the stray neutron field in the therapy room. As part of a collaboration with HZDR, PTB has carried out measurements with the Bonner sphere spectrometer NEMUS at the OncoRay Proton Therapy Facility in Dresden. The analysis of Bonner sphere measurements is typically done using unfolding codes. However, it is very difficult to implement reliable uncertainty propagation in standard unfolding codes. An alternative approach, which does provide reliable estimates of uncertainties of neutron spectra which lead to rigorous estimates of uncertainties of the dose, is to analyze the Bonner sphere data using Bayesian parameter estimation [1][2]. In this work, we extend previous approaches and apply this method to secondary neutrons from radiation therapy proton beams. This requires introducing a parameterized model which can describe the main features of the neutron spectra. We choose the parameterization based on information that is available from measurements and detailed Monte Carlo simulations. To demonstrate the validity of this approach, we consider the results of an experiment carried out at the experimental hall of the OncoRay facility. Measurements were done with the following experimental set-up: a brass target was placed in the proton beam and the proton beam was pulsed with 10 Hz to mimic the operation of a range modulator wheel. Bonner spheres were placed at different angles with respect to the incoming proton beam. We selected a set of 7 polyethylene spheres and 3 extended spheres with lead or copper inserts. The results of the analysis are the spectra of secondary neutrons with their corresponding doses with uncertainties. The approach that we describe here provides a basic method to assess neutron spectra and their uncertainties and will be extended in future applications to include additional parameters; e.g., those describing the settings of the proton beam. References: [1] M. Reginatto, Radiat. Prot. Dosim 120, 64-69 (2006). [2] M. Reginatto, Rad. Meas. 44, 692-699 (2009). |
