MR‐linac is the best modality for lung SBRT

摘要

The introduction of MR-linac hybrid modality started a new era in image guidance for radiotherapy. It provides superior soft tissue contrast compared to conventional x-ray imaging, and offers the ability to track and gate treatment delivery, yet challenges and limitations remain in various aspects.1 Our previous Parallel Opposed issues discussed the additional physicist qualification and residency requirements for the use of MRI in radiotherapy settings.2, 3 Herein, we continued our discussion on the clinical use of MR-linac and its potential improvement in treatment efficacy for lung stereotactic body radiotherapy (SBRT). Dr. Andrew Godley believes that “MR-linac is the best modality for lung SBRT,” while Dr. Dandan Zheng explained her doubts and concerns. Dr. Andrew Godley received his Ph.D. in High Energy Physics from the University of Sydney in 2001. Dr Godley continued in particle physics at the University of South Carolina and Fermilab, until 2007 when he trained in medical physics at the Medical College of Wisconsin. Dr. Godley spent 7 years as a staff physicist at the Cleveland Clinic before joining the Miami Cancer Institute in 2018 to help develop their MR-linac and SBRT programs. Dr. Dandan Zheng received her Ph.D. in Applied Science from the University of California Davis in 2007. After conducting a postdoctoral training in Radiation Oncology at Virginia Commonwealth University, she joined the department as an assistant professor in 2009. In 2012 Dr. Zheng moved to University of Nebraska Medical Center where she is currently an associate professor and the medical physics residency program director. Dr. Zheng is an avid peer reviewer/associate editor for many journals, and her research interests include image guidance, radiomics and machine learning, stereotactic RT, dose algorithms, and motion management. One vision of future radiotherapy is for patients to arrive for treatment, lie down on the couch, and the treatment system to automatically locate and irradiate their tumor, regardless of its position or motion, sparing all but the surrounding few millimeters of normal tissue. The hybrid technology of MR-guided linear accelerator is today's embodiment of that vision. It provides three main advantages to patients: monitoring the actual tumor location during treatment delivery, comfort, and accuracy. These advantages would benefit any patient undergoing radiotherapy, but provide specific gains for lung SBRT patients. Clinical trials have shown lung SBRT already achieves 98% tumor control, however there is still a 17% risk of grade 3–4 toxicity4 and it requires oppressive patient immobilization. The fast-growing number of patients eligible for lung SBRT further presses the need for technological improvement. The MR-linac tracking and accuracy reduce the volume of normal lung and chest wall irradiated, decreasing toxicity, and obviating the need for immobilization. MR-linac provides the best targeting of any system available as it directly images the tumor throughout treatment delivery. The reasons are four-fold: First, 2D cine-MRI based tumor tracking enables gating directly on the tumor position relative to the planned position. This cine imaging incurs no additional radiation exposure. Although some conventional systems acquire planar imaging during treatment, the radiation dose over a large area would be excessive to achieve the continuous monitoring of the MR-linac. Second, while using external surrogates facilitates continuous monitoring, these surrogates require at least an additional 1 mm margin to ensure correlation with the tumor.5 Third, the precise gating window of the MR-linac provided by its continuous tumor tracking limits or eradicates multileaf collimator (MLC) interplay issues, which was reported to affect delivery accuracy of other modalities6 and thus reduce target coverage.7 Lastly, the tumor based gating eliminates the need to define an internal target volume (ITV), which is conventionally used t