A method of monitoring a state of a Linac beam in an MRI-Linac system for real-time in vivo patient dosimetry is provided that includes disposing a Linac beam proximal to a PIN diode of an imaging RF coil to induce RF noise in an MRI image from an MRI imager, and using the MRI imager to monitor the induced RF noise in the MRI image, where a change in a percentage of RF noise in the MRI image is used to determine a state of the Linac beam in the presence of a magnetic field of the MRI imager, where the state of the Linac beam includes an ON state, an Off state, or a relative intensity of the Linac beam.
