MRI-guided radiotherapy: Opening our eyes to the future

When discussing local treatment modalities, physicians often quip that there can be no blind surgeons, suggesting that an inherent advantage to surgery is the ability to precisely visualize the target. Advances in radiation therapy (RT) technology over the years have significantly improved the radiation oncologists’ abilities to precisely sculpt high-dose radiation to a variety of targets, simultaneously improving local control and toxicity profiles. However, even the most sophisticated methods of CT-based image-guided radiotherapy (IGRT) are hampered by limitations that can compromise target localization and motion management. Additionally, realtime adaptive radiotherapy (ART) programs, wherein radiation plans can be molded to fit the often malleable anatomy of tumors and organs-at-risk, have not yet been developed. For these reasons, MRI-guided RT is considered the next frontier of radiation oncology. However, the development of integrated MRI/RT-delivery platforms is complicated by several technical challenges that have precluded development of MRI-guided linear accelerators. By virtue of employing a low-field (0.35 T) MRI and three 60Co sources, a newly available tri-60Co teletherapy system (MRIdian SystemTM, ViewRayTM, Cleveland, OH, USA) has tackled these technical concerns and has thus become the first MRI-guided RT platform with FDA approval for medical use. In this Review, we describe the abilities of this device to accomplish the goals of MRI-guided RT—improved target localization, sophisticated motion management, and on-line ART—by exploring published feasibility, proof-of-principle, and proof-of-practice studies.