Session Item

Many anatomical and pathological volumetric changes occur on time scales relevant to a radiotherapy treatment course. Examples include tumour growth and regression, organ filling, peristalsis, movement of gas pockets, and respiration. In a traditional CT-based radiotherapy workflow, some of these volumetric changes may be partly counteracted by the use of plan libraries or auxiliary imaging needed for offline re-planning. However, in most cases, large PTV margins are needed to ensure target coverage, although this may lead to excess dose to the OAR. Furthermore, large PRV margins are often needed to ensure a safe and tolerable treatment, which again may lead to insufficient target coverage.

Online adaptive MRgRT has the potential to address many of the issues coursed by volumetric changes. During the MRgRT workflow, target and OAR structures may be propagated from the planning images to the 3D MR images acquired on the MR-linac using deformable image registration. Furthermore, the superior soft-tissue image quality of MR allows for accurate adjustment of the target and OAR delineation while the patient is on the treatment couch. Thereby, the treatment plan may be adapted to the anatomical situation of the day. Additionally, 2D cine MR imaging acquired during treatment delivery may be used to monitor target drift or respiration-induced motion. Some MR-linac systems can account for such movement using couch- or beam-shifts and beam-gating. Therefore, online adaptive MRgRT can lead to reduced PTV and PRV margins since the exact location of targets and OAR is known at the time of treatment.

Although online adaptive MRgRT holds great potential for many disease sites, the workflow is labour-intensive and time-consuming. Dedicated oncologists and medical physicists must often be present during each treatment fraction. Furthermore, 3D MR imaging and manual contour adjustment are relatively slow processes, which may lead to volumetric organ changes during the adaptation process. Such changes cannot always be accounted for using beam-gating and couch- or beam-shifts.

During this talk, a short introduction to the MRgRT workflow will be presented. Examples of volumetric changes occurring during a single treatment fraction as well as the whole treatment course will be given based on recent literature and clinical experience. The potential benefits and challenges of these volumetric changes will be discussed. Strategies for speeding up the workflow or otherwise reducing the effect of intra-fractional volume changes will be presented.