Characterization of Stray Radiation Produced by a Proton Beam in an Anthropomorphic Phantom with Dental Implants Using a Pixel Detector

Abstract

<h3>Purpose/Objective(s)</h3> We investigated the scattered radiation produced by a collimated proton beam in the case of head and neck cancer using an anthropomorphic head phantom with human-like anatomy, with two dental implants inserted. Using high-resolution semiconductor pixel detectors, we measured and analyzed the backscatter radiation with particle identification resolving power. This research is focused on giving quantitative information about the radiation field components. <h3>Materials/Methods</h3> A treatment plan was designed on a standard X-ray CT which contained the implants at the center of the tumor. Two irradiations of an anthropomorphic head phantom were performed with clinical proton beams: one without metallic implants (plastic inserts) and one with dental implants. Two titanium cylindrical inserts of 1 cm length and 0.33 cm diameter were placed in the phantom's upper jaw like the molar's replacements. The planning target volume (PTV) is shaped so that the dental inserts are centered in the target volume. A scattered fixed proton beam with a nominal energy of 170 MeV was delivered to the phantom using collimators and boluses designed to shape and stop the beam according to the treatment plan. Scattered primary and secondary radiation was detected with hybrid semiconductor pixel detectors, placed at 4 cm behind the PTV. Detailed description of the mixed-radiation field was performed with spectral (energy loss) and tracking (direction) sensitivity. <h3>Results</h3> Measurements included a low intensity proton beam to assess the particle tracking response and identification resolving power in both situations (with and without implants). In this manner, the components of the radiation field were observed and analyzed. Based on their characteristic directional-sensitive spectral-track signal in the detector, both primary and secondary protons can be distinguished from light charged particles. This was done by analyzing single particle pixelated clusters morphology, spectral and directional sensitive parameters. Directional fluxes and linear energy transfer (LET) spectra of charged particle groups provide information about the scattered radiation field. <h3>Conclusion</h3> We present novel information about dental metallic implants' influences in a proton-based treatment. Preliminary results show differences in the LET distributions of the scattered radiation when the implants are in place, respectively, when they are missing. Further analyses will be done to provide a detailed dissemination of contribution for every type of particle.