Stereotactic Magnetic Resonance-Guided Adaptive and Non-Adaptive Radiotherapy on Combination MR-Linear Accelerators: Current Practice and Future Directions

Abstract

Stereotactic body radiotherapy (SBRT) is an effective radiation therapy technique that has allowed for shorter treatment courses, as compared to conventionally dosed radiation therapy. As its name implies, SBRT relies on daily image guidance to ensure that each fraction targets a tumor, instead of healthy tissue. Magnetic resonance imaging (MRI) offers improved soft-tissue visualization, allowing for better tumor and normal tissue delineation. MR-guided RT (MRgRT) has traditionally been defined by the use of offline MRI to aid in defining the RT volumes during the initial planning stages in order to ensure accurate tumor targeting while sparing critical normal tissues. However, the ViewRay MRIdian and Elekta Unity have improved upon and revolutionized the MRgRT by creating a combined MRI and linear accelerator (MRL), allowing MRgRT to incorporate online MRI in RT. MRL-based MR-guided SBRT (MRgSBRT) represents a novel solution to deliver higher doses to larger volumes of gross disease, regardless of the proximity of at-risk organs due to the (1) superior soft-tissue visualization for patient positioning, (2) real-time continuous intrafraction assessment of internal structures, and (3) daily online adaptive replanning. Stereotactic MR-guided adaptive radiation therapy (SMART) has enabled the safe delivery of ablative doses to tumors adjacent to radiosensitive tissues throughout the body. Although it is still a relatively new RT technique, SMART has demonstrated significant opportunities to improve disease control and reduce toxicity. In this review, we included the current clinical applications and the active prospective trials related to SMART. We highlighted the most impactful clinical studies at various tumor sites. In addition, we explored how MRL-based multiparametric MRI could potentially synergize with SMART to significantly change the current treatment paradigm and to improve personalized cancer care.

Deep learning application for abdominal organs segmentation on 0.35 T MR-Linac images

“…In our institution, MR-guided abdominal SBRT (including gating and adaptive RT) is performed with the MRIdian (Viewray Inc., Oakwood Village, USA) 0.35 T MR-Linac since 2019 ( 11 , 12 ). The success of these treatments and the reduced toxicity highly rely on the exact definition of the different OARs ( 13 , 14 ). Radiation oncologists generally follow the established guidelines to define the volume of interest ( 15 – 17 ).…”

Accurate, repeatable, and geometrically precise diffusion-weighted imaging on a 0.35 T magnetic resonance imaging-guided linear accelerator

“…This facilitates online adaptive radiotherapy incorporating updated anatomy and daily plan reoptimization [5] . Although MRgRT-driven adaptation based on morphological changes has proven successful [6] , MRI’s functional imaging capabilities to interrogate tumor physiology in vivo [7] , [8] , [9] have not been fully exploited [10] .…”

Treatment of Central Nervous System Tumors on Combination MR-Linear Accelerators: Review of Current Practice and Future Directions

“…These advantages work together to effectively expand the therapeutic window for CNS RT by improving dose delivery to the tumor and minimizing the exposure of surrounding healthy tissue [8,9]. The two commercially available MRLs are the ViewRay MRIdian (ViewRay Technologies Inc., Oakwood Village, Ohio) and Elekta Unity (Elekta AB, Stockholm, Sweden) systems [10]. These systems make up the majority of clinical MRL implementations and have been used with great efficacy to treat tumors across the body [10,11].…”

“…The two commercially available MRLs are the ViewRay MRIdian (ViewRay Technologies Inc., Oakwood Village, Ohio) and Elekta Unity (Elekta AB, Stockholm, Sweden) systems [10]. These systems make up the majority of clinical MRL implementations and have been used with great efficacy to treat tumors across the body [10,11]. The MRIdian system integrates a 0.35 T field strength split-bore magnet MRI with a 6 MV flattening filter-free (FFF) linear accelerator [12], while the Elekta Unity combines a 1.5 T MRI with a 7 MV FFF linear accelerator [13].…”