Which proton imaging set-up should we use in a proton therapy facility?

Proton imaging has been proposed to reduce range uncertainties in the treatment planning by circumventing or improving the conversion from Hounsfield Units to relative stopping power (RSP). In the last decades, different types of proton imaging setups have been developed, but not yet integrated into the clinical workflow. In this contribution, we present a comprehensive comparison of four types of proton imaging setups. For this purpose, we develop a mathematical framework to quantify the spatial resolution achievable with each of them. We find that the spatial resolution in setups combining pencil beam scanning with X-ray flat panel detectors is 10% better than in setups with pixel-less detectors such as multilayer ionisation chambers. In both setup types, performance can be significantly improved by reducing the pencil beam size down to 2-3 mm FWHM. In this case, the achievable spatial resolution is only 50% lower than in considerably more complex single proton tracking setups. Our results show that imaging setups combining double scattering with a pixel detector provide sufficient spatial resolution only under very stringent conditions. We further investigate the setups’ integrability in a proton center and analyse their performance in the low dose regime based on characteristic detector sensitivities and image acquisition scenarios. Finally, we develop methods to extract physical properties of the imaged object complementary to the RSP and validate the results against experimental data. Our results indicate that detector hardware already available in proton centers for quality assurance provide well performing proton imaging setups.