A method of determining the nature of a fault in a radiotherapy device comprising a linear accelerator, the radiotherapy device being configured to provide therapeutic radiation to a patient, the radiotherapy device comprising: 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 being located at a first end of the vacuum tube and the flight tube being located at a second end of the vacuum tube; and a first and a second sensor, the first sensor being configured to provide signals indicative of pressure at a first region inside the vacuum tube and the second sensor being configured to provide signals indicative of pressure at a second region inside the vacuum tube, the first region being closer to the first end of the vacuum tube than the second region is; the method comprising: 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 being indicative of pressure at the first region inside the vacuum tube, and the second value being indicative of pressure at the second region inside the vacuum tube; wherein the first value is an average of signals received from the first sensor over a time period and the second value is an average of signals received from the second sensor over the time period, and wherein 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 whether the nature of the fault is associated with the flight tube. 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 being located at a first end of the vacuum tube and the flight tube being located at a second end of the vacuum tube; and a first and a second sensor, the first sensor being configured to provide signals indicative of pressure at a first region inside the vacuum tube and the second sensor being configured to provide signals indicative of pressure at a second region inside the vacuum tube, the first region being closer to the first end of the vacuum tube than the second region is; the method comprising: 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 being indicative of pressure at the first region inside the vacuum tube, and the second value being indicative of pressure at the second region inside the vacuum tube; wherein the first value is an average of signals received from the first sensor over a time period and the second value is an average of signals received from the second sensor over the time period, and wherein 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 whether the nature of the fault is associated with the flight tube. 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 being indicative of pressure at the first region inside the vacuum tube, and the second value being indicative of pressure at the second region inside the vacuum tube; wherein the first value is an average of signals received from the first sensor over a time period and the second value is an average of signals received from the second sensor over the time period, and wherein 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 whether the nature of the fault is associated with the flight tube.