Described is a method of determining whether repair or replacement of an electron gun of a radiotherapy device should be scheduled. The radiotherapy device comprises a linear accelerator and is configured to provide therapeutic radiation to a patient. The radiotherapy device comprises a vacuum tube comprising the electron gun, a waveguide configured to accelerate electrons emitted by the electron gun toward a target to produce said radiation. The radiotherapy device comprises also comprises a current sensor, the current sensor being configured to provide signals indicative of current supplied to the electron gun. The method comprises receiving a current value, processing the current value, and based on the processing of the current value, determining whether repair or replacement of the electron gun should be scheduled. Processing the current value comprises determining whether the current value meets at least one threshold criterion, and determining whether the current value has changed by at least a threshold amount in a particular time period.