Development of simple proton CT system with novel correction methods of proton scattering

For safe and effective proton therapy, the proton range in a patient's body is characterized by the water equivalent length (WEL), and must be accurately determined. Current treatment planning is based on X-ray computed tomography images, which might cause uncertainty because of the different energy loss processes between protons and X-rays. We develop a simple, novel, and real-time proton CT system. The system uses a CCD camera and scintillator, which is thin enough for protons to penetrate. Since protons lose energy when they pass through a phantom, different emissions corresponding to the proton energy loss are acquired in the scintillator. Images of the scintillator were gathered by the CCD camera with 70 MeV and 200 MeV proton beams. Since blurring due to proton reactions such as multiple Coulomb scattering and nuclear reactions significantly degrades the obtained images in both beams, we developed two kinds of effective correction methods. One method is applied to broad beam systems, while the other is applied to narrow beam systems. We successfully obtain clear images with minor proton reaction effects by applying these correction methods. Moreover, we confirm that the WEL values estimated from the acquired CT images agree well with the theoretical values for materials such as polymethyl methacrylate (PMMA) and isopropyl alcohol, within 1-σ uncertainty. Through simulations, we found that nuclear reactions significantly contribute to the uncertainty of WEL values.