Monte Carlo Simulation of Thallium-Bromide Semiconductor Detector for Range Verification of a Carbon Ion Radiotherapy Beam through Prompt Gamma-Ray Detection

Thallium-Bromide is a semiconductor material which is well suited for photon detection. Composed of high atomic number elements and a high mass density of 7.56 g/cm3, TlBr has superb photon stopping power. External beam radiation therapy is an ever-changing environment with technology rapidly improving. With proton therapy facilities becoming more widely available, and carbon ion facilities showcasing the benefits of carbon ion radiation therapy, there is an industry need toprovide accurate dosimetric treatment plans to verify patients are receiving proper care. Current clinical practices apply safety margins to clinical target volumes to safe treatment. Through research to improve detection techniques, safety margins may decrease leading to more accurate dose deposition and improved treatments. In this study, Monte Carlo simulations were completed to evaluate TlBr performance in the detection of prompt gamma-rays generated from the irradiation of a Polymethyl methacrylate (PMMA) phantom with carbon ions. TlBr was able to detect the prompt gamma ray profiles of three different materials (cortical bone, adipose tissue, PMMA) with correlation to actual simulated Bragg peaks. The spectra of secondary particles were captured using different collimator configurations. The spectral data confirmed that most prompt gamma rays detected are of 4.44MeV, originating from the de-excitation of 16O. Using opposed detectors, TlBr was able to detect range shifts of 2, 4, 6, 8, and 10mm when a phantom was set in motion along the beam axis. Furthermore, using the detection of 511keV coincident photons from positron annihilation, the location of deposition in the transverse plane was possible. This study demonstrated the power and usefulness of Monte Carlo simulations for many applications including energy, dose deposition, motion, spectra, and time-of-flight. The results of modeling showed the capability of TlBr to accurately determine the prompt gamma ray profiles produced from carbon ion irradiation