Session Item

After investigating real-time motion management for moving targets in particle therapy for more than a decade, the status of its clinical implementation and its clinical impact needs to be critically assessed. While geometrical target concepts, e.g internal target volumes (ITV) considered during treatment planning, were smoothly transferred from the photon community, robust optimisation based on time-resolved computed tomography still lacks behind, for both, photon- and particle beam therapy. Image quality and the assessment of respiratory motion can still be significantly improved even though novel developments based on surface guidance got widely available and implemented in many particle therapy centres. Motion management strategies during beam delivery, e.g gating or tracking, suffer from the poor implementation of anatomical real-time imaging during treatment and advanced motion prediction models. With improved hardware and software, the necessity for real-time data transfer and processing became crucial posing a new challenge to industry and research institutions.

The complexity of particle therapy and its reliance on accurate patient positioning increases the demand for motion management strategies in comparison to photon therapy. Especially considering particles heavier than protons, like carbon ions but also helium, come along with additional challenges due to steeper dose gradients and a variable RBE. In combination with longer delivery times caused by the pulsed beam characteristics of a synchrotron, additional challenges need to be considered when envisaging the clinical implementation of real-time motion management in such a setting.

All treatment facility-specific settings and technologies as well as the treatment site-specific characteristics require individual commissioning and workflow development. Thus, generalised guidelines for applying motion management strategies for moving targets, e.g. lung and liver, in particle therapy barely exist. Patient selection criteria might differ among the different particles and types of accelerators due to varying motion sensitivity and the availability of resources. The longer in-room times, especially in gated treatments, might hinder the application of advanced techniques to targets with large motion amplitude. In that aspect, the novel developments in the field of motion amplitude reduction and control like ventilated breath hold or even hyperventilation might reduce the necessity for consideration of the motion during beam delivery.

Real-time adaptive particle therapy does not only address intrafractional movements but also interfractional changes, which gained importance during the last years. National and international research projects on real-time adaptive particle therapy foster the ongoing success of clinical implementation of adaptive treatment workflows. Essential insights and improvements concerning anatomical and delivery accuracy by daily plan adaptations and robust treatment planning strategies could already be reached and pave the way towards a wide application. Those developments will have an impact on the treatments of pathologies suffering from inter- and intra- fractional anatomical changes, e.g. head and neck or gynaecological tumours, as well as on moving targets, like lung, liver and pancreas. This poses the question if we do need to extend our understanding of the 4th dimension to a longitudinal time scale and develop adaptive strategies in a broader sense aiming for the best clinical outcome.

Still, considering real-time motion management, the particle community does not lack much behind the photon world, supported by the implementation of in-room imaging devices and fast and reliable accelerator technology. Even though photon therapy took a big step towards real-time adaptation with the MR-linac technology, automation of contouring, treatment planning and QA needs to be brought into clinical routine before real-time motion management will become reality.