Progress toward a proton computed tomography apparatus

There is today a growing up interest in proton therapy for tumors treatment, because these particles permit to tightly shape the dose to the target. Anyway, the spatial accuracy of proton therapy is limited by the uncertainty in stopping power distribution, which is calculated from the photon attenuation coefficients measured by X-ray tomography. A proton computed tomography apparatus (pCT) could be used to directly measure stopping power and reduce this uncertainty. Single proton tracking is a promising way to face difficulties due to multiple Coulomb scattering. The design of a proton radiography (pCR) prototype and its practical realization are described in this paper. The prototype is based on a silicon microstrip tracker to characterize particle trajectory and on a segmented YAG:Ce calorimeter to measure their residual energy. The aim is to detect protons with initial kinetic energy K = 250-270 MeV and with a particle rate of ~1MHz. This prototype is the first step toward the design of a complete pCT system to measure tissue electron density with an accuracy better than 1% and with a spatial resolution better than 1 mm.