Disclosed herein is a method of determining the nature of a fault in a radiotherapy device comprising a linear accelerator. The radiotherapy device is configured to provide therapeutic radiation to a patient. The radiotherapy device comprises a vacuum tube comprising an electron gun, a waveguide configured to accelerate electrons emitted by the electron gun toward a target to produce said radiation, and a flight tube. The electron gun is located at a first end of the vacuum tube and the flight tube is located at a second end of the vacuum tube. The radiotherapy device further comprises a first and a second sensor. The first sensor is configured to provide signals indicative of pressure at a first region inside the vacuum tube and the second sensor is configured to provide signals indicative of pressure at a second region inside the vacuum tube. The first region is closer to the first end of the vacuum tube than the second region is. The method comprises processing a first value derived from signals from the first sensor and a second value derived from signals from the second sensor. The first value is indicative of pressure at the first region inside the vacuum tube, and the second value is indicative of pressure at the second region inside the vacuum tube. Processing the first and second value comprises comparing the first value with a first threshold and comparing the second value with a second threshold; and, based on the processing of the signals, determining that the nature of the fault is associated with the flight tube.