Magnetic resonance linear accelerator technology and adaptive radiation therapy: An overview for clinicians

Abstract

Radiation therapy (RT) continues to play an important role in the treatment of cancer. Adaptive RT (ART) is a novel method through which RT treatments are evolving. With the ART approach, computed tomography or magnetic resonance (MR) images are obtained as part of the treatment delivery process. This enables the adaptation of the irradiated volume to account for changes in organ and/or tumor position, movement, size, or shape that may occur over the course of treatment. The advantages and challenges of ART maybe somewhat abstract to oncologists and clinicians outside of the specialty of radiation oncology. ART is positioned to affect many different types of cancer. There is a wide spectrum of hypothesized benefits, from small toxicity improvements to meaningful gains in overall survival. The use and application of this novel technology should be understood by the oncologic community at large, such that it can be appropriately contextualized within the landscape of cancer therapies. Likewise, the need to test these advances is pressing. MR‐guided ART (MRgART) is an emerging, extended modality of ART that expands upon and further advances the capabilities of ART. MRgART presents unique opportunities to iteratively improve adaptive image guidance. However, although the MRgART adaptive process advances ART to previously unattained levels, it can be more expensive, time‐consuming, and complex. In this review, the authors present an overview for clinicians describing the process of ART and specifically MRgART.

History of Technological Advancements towards MR-Linac: The Future of Image-Guided Radiotherapy

“…Adaptive planning can be further classified into online and offline adaptation [ 43 , 44 ]. The former refers to changes in the treatment plan that are made while the patient remains on the treatment table in the treatment position, while the latter refers to changes made to the treatment plan that occur between fractions.…”

Towards Accurate and Precise Image-Guided Radiotherapy: Clinical Applications of the MR-Linac

“…Using the onboard MR imaging capabilities of MR-Linacs, changes in treatment setup or anatomy can be accounted for in adaptive planning. During this process, daily fractional scans are used by the treating physician and physicist to adjust the radiotherapy plan to not only optimize the target coverage, but also avoid critical adjacent OARs [ 5 , 14 , 15 ]. This can be observed in Figure 1 , where the initial volumes for treatment are modified based on daily MRI to avoid the spillage of the 30 Gy isodose line, in yellow, into the stomach, in brown.…”

“…Daily localization in the current paradigm utilizes kilovoltage X-ray imaging, CBCT, megavoltage CT, or CT on rails, all of which ultimately rely on differences in electron density for discrimination between nearby organs. The result is very poor soft tissue contrast, making verification and adaptation of treatment difficult [ 15 , 40 , 41 ]. Figure 3 shows the wide range of imaging available clinically and the resulting ability to depict different soft tissues.…”

“…While this effect is worrisome when treating patients Although these benefits are theoretically sound, there are multiple concerns that arise when treating pediatric patients via adaptation on a MR-Linac. This technique often requires extensive time for volume delineation, replanning, and assessment of new dosimetry, translating to prolonged time on the treatment couch, which may not be feasible in a young population [15]. Secondly, pediatric patients often have difficulty tolerating long procedures and will require use of anesthesia [59,60].…”

Development and implementation of optimized endogenous contrast sequences for delineation in adaptive radiotherapy on a 1.5T MR-Linear-accelerator (MR-Linac): A prospective R-IDEAL Stage 0-2a quantitative/qualitative evaluation of in vivo site-specific quality-assurance using a 3D T2 fat-suppressed platform for head and neck cancer

“…Hybrid MRI-linear accelerator devices can acquire a variety of imaging data during each fraction of radiotherapy and incorporate them into MR-compatible treatment planning systems [19], [20]. Moreover, these daily images can be used in on-line or off-line adaptive planning workflows when major changes in anatomy or tumor function are detected [21], [22]. Thus, a major area of research is in sequence development for these devices to better visualize relevant structures and discover and acquire useful imaging biomarkers for treatment response and resistance [23], [24].…”