Compton Imaging System for Dose Monitoring in Carbon-ion Therapy

To obtain accurate radiotherapy for carbon therapy, it is critical to accurately monitor the three-dimensional (3D) dose distribution of patients. This paper designs a double-layer Compton imaging system and a simple back-projection algorithm. Using the Geant4 toolkit, the structure of the Compton imaging system is optimized, the detection efficiency and imaging performance of the Compton imaging system are evaluated, and the 3D dose distribution of 200 MeV/μ carbon ion beam bombarding PMMA target is analyzed. In addition, four pairs of LYSO crystals with 1.5 mm×1.5 mm×10.0 mm and 12×12 pixels were used to perform Compton imaging experiments on a ²²Na point source with a diameter of about 3 mm. Due to the deviation of the reconstructed dose distribution, factors affecting the precision of 3D dose monitoring are also analysed. For the 0.847 MeV γ point source, the reconstructed image has an FWHM increased to 2.38 mm on the cross-section and 7.02 mm on the coronal plane. The compton imaging system detects the 4.439 MeV prompt γ-rays generated by carbon ion beam bombardment of PMMA target, and the deviation of the reconstructed 3D dose distribution from the real 3D dose distribution is 9.3%. Using the ²²Na point source imaging experiment of the LYSO Compton imaging system prototype, a reconstructed image with a FWHM of 4.05 mm was obtained, which verified the practicability of the compton imaging method.