High-field mri in-room guidance for radiotherapy adaptation

Intra-fractional motion is the main source of uncertainty when targeting treatment sites in the thorax and upper abdomen. Predominately periodic respiratory and cardiac-induced motion, and irregular gastrointestinal activity displace and deform targets and healthy organs compared to their observed position during treatment planning. Adapting treatment to this complex motion pattern requires precise and frequent on-board imaging. Current X-ray based in-room imaging techniques induce additional radiation exposure, suffer from poor soft-tissue contrast in the 2D projection images, and usually require implanted fiducial markers for guidance. In comparison, high-field magnetic resonance imaging (MRI) offers customizable, diagnostic-quality soft-tissue contrast and much greater flexibility in selecting the appropriate image dimension for detecting anatomical motion. In this chapter we will first introduce the clinical rationale for in-room guidance based on MRI. A survey will highlight existing technical implementations while focussing on the Elekta Unity MR-linac. We will then identify suitable MRI techniques for radiotherapy adaptations. This will include an overview of typical sequences used for anatomical imaging. Several possible image dimensions, ranging from real-time 2D cine imaging to retrospectively sorted 4D motion models will be presented in detail. The resulting increase in observer confidence due to the higher image quality is complemented with an array of suitable inter-fractional and intra-fractional treatment adaptations based on on-line MRI. We will emphasize the benefits and challenges of several motion mitigation techniques including beam gating, dynamic tumour tracking, and intra-fractional re-planning based on on-line accumulated dose.