• Hadron-therapy is a leading-edge technique which exploits the particular energy deposition prole (Bragg peak) protons or heavy-ions exhibit to target and destroy tumors within the human body. The beneficial aspects of this technique over other, more established, procedures, such as X-ray therapy, have been extensively reported. The effectiveness of a hadron-therapy treatment is strictly related to the accuracy of the knowledge of the tissues density or, equivalently, stopping power (SP) distribution: an accurate 3D map of the body SP makes it possible to precisely determine the position of the Bragg peak as a function of the beam energy. However, the effectiveness of the hadron-therapy procedure is currently limited by the necessity to rely on body density maps produced with X-ray Computed Tomography (X-ray CT), which cannot deliver maps accurate enough to fully exploit the intrinsic accuracy of the technique. This is mainly due to the different behavior inside matter of X-ray and hadrons. A Computed Tomography performed with protons (pCT), instead of X-ray, would therefore improve the accuracy of the SP maps and lead to an enhancement of the treatment effectiveness, as the particles used for both the imaging process and the treatment present the same energy-loss behaviour. Recent studies confirmed that pCT can potentially be a factor 2:5 better, with respect to X-ray CT, in terms of SP values accuracy, with, at the same time, at least a factor 50 lower deposited dose. However the spatial resolution is expected to be worse for pCT than X-ray CT, mostly due to protons multiple Coulomb scattering (MCS).
• The iMPACT Project, innovative Medical Proton Achromatic Calorimeter and Tracker, aims to design and develop a pCT scanner, with the ultimate goal of demonstrating the viability of the pCT technique in a realistic clinical environment. A pCT scanner is composed by a tracker and a calorimeter. The tracker must provide the particles position and angle before and after they pass through the target (the human body), while the calorimeter has to measure the residual energy of the passing particles (Figure 1). By combining the track and energy information of about one billion protons passing through the target from different directions, it is possible reconstruct a 3D image of the target itself.
• The goal of the iMPACT project is therefore to build a proton scanner fast enough to take a full 3D image of the targeted body part (by recording about 1 billion protons passing through it) in few seconds, so that the patient would not move, or even breath, during the irradiation. This would demonstrate the feasibility of a clinically viable pCT system. A successful outcome, in the long term, would mean improving the quality of cancer heavy-ions treatment, as well has creating a 3D body imaging tool less harmfull than current CT scanner, due to the lower dose released to record an image.
