There is provided a radiotherapy system comprising a linear accelerator, beam control circuitry for the linear accelerator, an electronic control apparatus for the control circuitry arranged to adjust properties thereof, and a monitor for detecting properties of the radiation beam produced by the linear accelerator, wherein the control apparatus is adapted to retain a set of beam properties and periodically activate the accelerator, measure the current beam properties via the monitor, compare the measured beam properties to the retained beam properties, and potentially adjust the control circuitry properties to align the beam properties towards the retained beam properties. The beam properties that are measured may include beam flatness and beam width. The retained beam properties can be the properties of the beam produced by the linear accelerator when new, or the properties of a standard beam. There is also provided a method for operating the system.