Radiation therapy (RT) is currently one of the mainstays of cancer treatment, with approximately 50% or higher of all cancer patients receiving RT during their treatment [1]. Image-guided radiation therapy (IGRT) has become the standard RT practice to enable accurate and precise delivery, leading to the widening of the therapeutic ratio [2, 3]. Various imaging technologies, such as computed tomog­raphy (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET), are used to guide RT to precisely deliver the dose to targets while avoiding unnecessary dose to nearby critical structures [3]. CT-guided RT, a type of radiation therapy using CT-based technology, is used to guide the delivery of radiation beams to the tumor, ensuring accurate targeting [3]. The latest development of combining PET-CT and radiotherapy delivery involves using biological markers or character­istics of tumors to guide the delivery of radiation treatment, allowing radiation therapy to be tailored more specifically to the individual characteristics of the tumor

for better control of respiratory motion and other motion uncertainty during treatment delivery [12]. It is also worth noting that due to the high strength of the magnetic field, the beam profile is inherently off-central and asymmetric. In addition, the electron return effect (ERE) resulting from the magnetic field causes electrons to change trajectory to ‘return’ to a higher density material at the interface of exiting a higher density material into a lower density material [13].

The ViewRay MRIdian (Oakwood, Cleveland, OH), shown in figure 16.2, is one of the two currently commercially available MRgRT platforms. In contrast to Elekta’s Unity, the ViewRay system makes use of a 0.35 tesla low-field MRI scanner for MRI-guided treatment. This system paved the way for clinical MRgRT, being the first MR-linac device to gain FDA approval in 2012 [14]. The initial system was designed with cobalt-60 sources for irradiation and was later integrated with a linear accelerator, which was cleared by the FDA in 2017 [14]. The MRIdian design consists of a split-bore superconducting magnet with a 70 cm bore diameter perpendicular to a linear accelerator capable of producing 6 MV FFF photon treatment beams [15]. The MRIdian is capable of producing coplanar static IMRT fields and can deliver dose at 650 MU/min with a 0.5 rpm gantry rotation [5]. Currently, the MRIdian uses a balanced steady state free precession pulse sequence for MRI that may be used for treatment planning, set-up verification, and imaging during delivery [5]. Compared to the high-field MR-linac, the 0.35 T MRIdian has the disadvantage of lower image signal–noise ratio (SNR), but the advantages of diminished magnetic susceptibility artifacts, smaller geometric distortion in MR images, and a more minimal electron return effect—resulting in less perturbations to the dose distribution [14]. A significant benefit of the MRIdian system is its ability for real-time imaging during treatment while the radiation beam is on, enabling high-quality monitoring of intrafraction motion during treatment. This feature allows for real-time tracking and automatic gating based on user defined boundaries where the 2D motion is evaluated from a sagittal cine image [5]. Additionally, unlike conventional linacs, the MRIdian has functionality for online adaptive therapy