Imaging for ion beam therapy: current trends and future perspectives

Since the pioneering use of planar X-ray imaging in early experimental sites of proton and light ion cancer therapy, imaging has always been a cornerstone of ion beam therapy (IBT). This contribution highlights current trends and future perspectives of imaging in modern IBT. Several flavours of image guidance are under investigation to enhance IBT. A first class of in-room imaging techniques aims at providing insights on updated patient anatomy prior to or ideally during treatment. Owing to the unique characteristics of IBT, these methods do not only target a correct localization of the tumour and critical structures as in photon therapy, but also aim at extracting the tissue stopping properties for accurate (re)planning. A second class of techniques, predominantly performed during beam delivery, aims at capturing different secondary emissions induced by the irradiation to identify the beam stopping position and ideally reconstruct the dose delivery for inter- or intra-fractional treatment adaptation. Finally, a third class of imaging techniques is being explored to provide novel insights on the underlying biological mechanisms to open new opportunities for more effective and better tolerated treatments. 70 years after the worldwide first proton treatment, image guidance of IBT continues to be an evolving area which combines advanced instrumentation with progress in computational areas, including artificial intelligence, and beam delivery schemes. Especially on-site imaging opens new opportunities to innovate the IBT chain with daily treatment adaptation, real-time verification of in-vivo range and dose delivery along with biological guidance for treatment personalization.