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.

Keywords: MR-guided radiation therapy; MRI; MRgRT; RT; SABR; SBRT; SMART; ablative radiation therapy; adaptive radiation therapy; image guided radiotherapy; magnetic resonance imaging; mpMRI; multiparametric MRI; plan optimization; radiation therapy; stereotactic ablative radiotherapy; stereotactic body radiotherapy; stereotactic magnetic resonance-guided adaptive radiotherapy; tumor motion management; ultra-hypofractionated radiation therapy.

Figure 1

MRL workflow. CT: Computed tomography; MR: magnetic resonance; MRL: MR linear accelerator; OAR: organ at risk; QA: quality assurance.

Figure 2

Cumulative installations of ViewRay MRIdian and Elekta Unity MRLs over time. ViewRay MRIdian was initially a tri-⁶⁰Co system, with MRL installations beginning in 2017. All existing ViewRay MRIdian systems, except for one, have been upgraded to MRLs. Elekta Unity systems were initially pre-clinical until late 2019. All existing Elekta Unity systems have been upgraded to fully clinical systems. Data used for the creation of Figure 2 were directly provided by Elekta and ViewRay team members.

Figure 3

Cumulative treatments of ViewRay MRIdian and Elekta Unity MRLs per year since 2019. Data used for the creation of Figure 3 were directly provided by ViewRay and from data presented at the 9th annual MR in RT symposium [25].