Status and challenges of MRI-only simulation for stereotactic body radiation therapy

The implementation of stereotactic body radiation therapy (SBRT) using magnetic resonance imaging (MRI)-only simulation holds much promise for improving clinical efficiency and reducing treatment burden and toxicity. There have been several clinical implementations of SBRT using MRI-only simulation imaging, including treatments of the prostate using X-ray image-guided SBRT, same-day palliative simulation, and SBRT using online adaptive MRI-simulation imaging. Several challenges exist to the widespread implementation of MRI-only simulation for SBRT. General challenges to the clinical implementation of MRI-only simulation with SBRT include contraindications to MRI such the presence of ferromagnetic implants. SBRT-specific challenges include immobilization and motion management challenges, acquisition and optimization of MRI image sequences for SBRT, and quality assurance of synthetic CT (sCT) and SBRT dose calculations using MRI-only simulation. In this review, we summarize published experiences of SBRT with MRI-only simulation, discuss the challenges to the widespread implementation of this technique and summarize the research that is being conducted to address these challenges.