Assessment of positional reproducibility in the head and neck on a 1.5-T MR simulator for an offline MR-guided radiotherapy solution

Magnetic resonance imaging (MRI) has attracted greater interest in radiotherapy (RT) applications in recent years (1-8). MRI introduces a number of benefits that may confer an advantage over the conventional use of X-ray based imaging methods in RT, such as non-ionizing radiation, improved soft tissue contrast and multi-planar imaging capability. In recent years, modified clinical wide-bore MRI scanners, named MR-simulator (MR-sim), have been introduced for RT treatment planning purpose. MR-sims enable the use of RT immobilization devices during MR scan to ensure the identical patient positioning of MR scan to that of RT treatment so as to greatly reduce the image registration error to planning CT and thus improve delineation accuracy. In addition, the recent development of the hybrid MR-guided radiotherapy (MRgRT) modalities, such as MR-LINAC (9-11) and MR-Cobalt 60 RT machine (12), further extend the role of MRI in RT from treatment planning to on-board patient positional verification and real-time treatment delivery guidance and monitoring