question: Major advances in external beam radiation therapy in the last decade have provided technologies for planning and delivering highly conformal radiation dose distributions made available by refined beam targeting methods, including intensity-modulated radiation therapy using tomotherapy, volumetric modulated arc therapy and many other auspicious tools for 3D dose painting. The emergence of magnetic resonance imaging (MRI) systems integrated with linear accelerators (linacs), commonly known as an MR-linacs, have realized MR-image guided radiation therapy (MRgRT) systems.