Real-time intrafraction motion monitoring in external beam radiotherapy

Abstract

Radiotherapy (RT) aims to deliver a spatially conformal dose of radiation to tumours while maximizing the dose sparing to healthy tissues. However, the internal patient anatomy is constantly moving due to respiratory, cardiac, gastrointestinal and urinary activity. The long term goal of the RT community to ‘see what we treat, as we treat’ and to act on this information instantaneously has resulted in rapid technological innovation. Specialized treatment machines, such as robotic or gimbal-steered linear accelerators (linac) with in-room imaging suites, have been developed specifically for real-time treatment adaptation. Additional equipment, such as stereoscopic kilovoltage (kV) imaging, ultrasound transducers and electromagnetic transponders, has been developed for intrafraction motion monitoring on conventional linacs. Magnetic resonance imaging (MRI) has been integrated with cobalt treatment units and more recently with linacs. In addition to hardware innovation, software development has played a substantial role in the development of motion monitoring methods based on respiratory motion surrogates and planar kV or Megavoltage (MV) imaging that is available on standard equipped linacs. In this paper, we review and compare the different intrafraction motion monitoring methods proposed in the literature and demonstrated in real-time on clinical data as well as their possible future developments. We then discuss general considerations on validation and quality assurance for clinical implementation. Besides photon RT, particle therapy is increasingly used to treat moving targets. However, transferring motion monitoring technologies from linacs to particle beam lines presents substantial challenges. Lessons learned from the implementation of real-time intrafraction monitoring for photon RT will be used as a basis to discuss the implementation of these methods for particle RT.

Medical physics challenges in clinical MR-guided radiotherapy

“…These include partial Fourier acquisition, different k-space read-out strategies and the use of deep learning to shorten reconstruction times [24,26]. Parallel imaging capabilities of today's clinical MR-linacs are also still constrained by the available hardware [25,41,44]. The development of faster sequences could enable motion monitoring of the heart, which could be either used to achieve heart dose reduction [45] or to guide cardiac ablation interventions with MR-linacs in the future [46,47].…”

“…In clinical practice as well as in research studies, the sagittal slice orientation has been most often used for cine MRI [24,32], but as MR imaging allows for arbitrary slice orientations, the impact of different orientations on the tumor tracking accuracy has been evaluated [25,43]. No consensus has been found yet [44], but the ideal slice orientation and position might be entity-and patient-specific [43] and depend on the beam angle [48].…”

First prospective clinical evaluation of feasibility and patient acceptance of magnetic resonance-guided radiotherapy in Germany

“…These new hybrid systems for MRgRT do not only offer three-dimensional MRI for soft tissue target and organ at risk visualization, but also allow for continuous cine-MRI before as well as during treatment [12,13]. Cine-MRI during treatment enables advanced motion management based on real-time soft tissue anatomic feedback such as treatment beam gating [11,16,17]. This eliminates the need for invasive implantation of fiducial markers as well as the application of internal target volumes (ITV) in order to account for intrafractional motion, and thereby offers the potential for margin reduction and hence a lower risk of ensuing toxicity [18].…”

Percutaneous insertion of hepatic fiducial true-spherical markers for real-time adaptive radiotherapy

“…There are high expectations regarding RAR without fiducial markers, such as on-line MRI in the treatment room, for the treatment of liver tumors. However, only 2dimensional MRI movies have been used thus far [26]. In addition, automatic real-time contouring of liver tumors with MRI are not always sufficiently accurate for automatic 22 gating of irradiation [26].…”

“…However, only 2dimensional MRI movies have been used thus far [26]. In addition, automatic real-time contouring of liver tumors with MRI are not always sufficiently accurate for automatic 22 gating of irradiation [26]. Therefore, it remains important to implant fiducial markers into the liver in real-world RAR.…”