In radiation oncology, continuous physiological changes cause the patients anatomy during treatment to deviate from the one captured on pretreatment imaging. Traditionally, the impact of inter-fraction motion (day-to-day changes) dominates relative to the impact of intra-fraction motion. However, with the introduction of online adaptive radiotherapy a new treatment plan can be generated based on the anatomy of the day. Consequently, the impact of inter-fraction anatomical variation can be effectively minimized. Intrafraction motion, on the other hand, may still impact the delivered dose, the in-room image quality, and consequently requires safety margins. Intra-fraction motion has thus become one of the dominant sources of geometric uncertainty for MR-guided radiotherapy (MRgRT).

MRgRT, integrating an MRI scanner with a linear accelerator, offers nonionizing high-quality images. MRgRT therefore provides a powerful tool for intrafraction motion management. The versatility of MRI also provides a variety of imaging techniques that balance soft-tissue contrast, spatial resolution, temporal resolution, and acquisition time. Imaging techniques can be tailored to the intrafraction motion frequency for different body sites and combined with corresponding correction methods.

In this paper we aim to provide a comprehensive overview of MRgRT intrafraction motion management. We will give an overview of the various sources of intra-fraction motion for different disease sites and then describe the MR imaging techniques to visualize and quantify intra-fraction motion. We will explore motion management strategies in the context of MRgRT and describe the current technical capabilities of the commercially-available MRgRT systems. Moreover, we will describe how these motion management capabilities are currently being used in clinical studies and protocols. We will finalize the paper with a discussion and future outlook.