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Purpose MRI is a promising alternative imaging tool in radiation therapy for its ability to image soft tissue in the human body and metabolic processes therein,and relative safety of the no-ionization nature of the magnetic fields.This article reviews the implementation of MRI technique into radiotherapy.MethodsInitial approach started with standalone accelerator and MRI units where patients received MRI scan on a flat table top with immobilizations in place before transporting them to the next door for radiation treatment on the same table.Then new technology called MRI-linear accelerator is introduced to implement real-time adaptive radiotherapy.Compared to perpendicular configuration of MRI and accelerator,inline approach reduces the electron return effect of magnetic field on the exit skin dose and dosimetric hotspots.ResultsA rotating Biplanar Linac-MRI system built at the Cross Cancer Institute in Edmonton,Alberta is equipped with a superconducting open bore MRI with 0.6T field strength and a 6MV linac.The Linac-MRI system tested at the University of Sydney,Australia is equipped with a custom designed open bore MRI with 1T field strength and a 6MV linac.Viewray,OH,USA introduced a system which involves a double donut superconducting magnet with 0.35T open bore field strength and a ring of three robotic Cobalt-60 units that are 120 degrees apart simultaneously delivering a dose rate of 550 cGy/min.One other highly equipped prototype is a combination of modified 6MV Elekta accelerator and 1.5T Philips Achieva MRI system built at UMC Utrecht in Netherlands.ConclusionThe development of MRI guided radiotherapy has set a pathway for an advanced treatment planning and delivery methods.Investigation of geometric accuracy and electron density accuracy further enhances the role of MRI as an ultimate tool for MRI guided radiotherapy.