This is an overview of research advancements in motion-imaging and on research and clinical implementation of 4D particle therapy.

The main aspects regarding 4D proton therapy treatment strategies that are being implemented were:

a) bias on patient selection: a broadening in the spectrum of clinical indications treated by PBS-PT has been observed in recent years, shifting from the more traditional lung, liver, and breast cancer to include several other indications. It was proposed that a closer look into a model-based approach to define which patient would receive the greatest benefits might be beneficial in selecting the right patients. The importance of 4D imaging for motion qualification and the decision on a specific 4D treatment strategy was also highlighted.

b) selection of motion, monitoring and mitigation approach: Different techniques were described for motion management, as well as for mitigation to minimize the negative dosimetric effect of motion. For instance, if the motion amplitude does not exceed a certain threshold, motion is passively managed either through rescanning or by the addition of margins added to the target volume. On the other hand, if the motion amplitude does exceed a threshold, an active mitigation approach or even additional devices can be used to reduce the extent of movement. However, mapping the internal tumor motion can be a limitation. To this extent, fluoroscopy, a real-time imaging approach has been established at Hokkaido University for prostate, liver, pancreas, and lung cases.

c) treatment planning: Several aspects of treatment planning were discussed. 4D imaging and contouring seem to take the central stage with various solutions proposed (from 4DCT, internal target volume margin definition approaches, density override). In addition, the selection of beam angles was also discussed, with the consensus being that selection of motion robust beam angles has to be on top of the list

d) implementation of robust 3D and 4D optimization protocols. On the research front, an overview of the latest 4D solutions was discussed, with a focus on the modeling approaches based on different imaging modalities. The installation of CTs in the treatment room (installed either in robotic arms or on rails) was also discussed. The application of artificial intelligence to improve treatment precision at reduced costs and operation time was also discussed.

Finally, the prospects of FLASH and Arc proton therapy were also discussed. The advantages of FLASH in mitigating the impact of motion were also considered.