A method of operating a radiotherapy system comprising: a gantry that is rotatable around a treatment location a radiotherapy beam generator located on the gantry; a radiotherapy imaging system located on the opposite side of the treatment location; and at least one component that is static with respect to the treatment location; wherein a beam path is defined between the radiotherapy beam generator and the radiotherapy imaging system so as to pass through the static component; the method comprising: obtaining a reference image using the radiotherapy imaging system with the gantry in a predetermined position so as to provide the beam generator, the imaging system and the static component in reference positions, the reference image including the static component; rotating the gantry relative to the static component; returning the gantry to the predetermined position; obtaining a further image using the radiotherapy imaging system with the gantry in the predetermined position, the further image including the static component; and determining changes in the relative positions of the radiotherapy beam generator, the radiotherapy imaging system, and the static component from differences in the position of the static component in the reference image and in the further image. a gantry that is rotatable around a treatment location a radiotherapy beam generator located on the gantry; a radiotherapy imaging system located on the opposite side of the treatment location; and at least one component that is static with respect to the treatment location; wherein a beam path is defined between the radiotherapy beam generator and the radiotherapy imaging system so as to pass through the static component; the method comprising: obtaining a reference image using the radiotherapy imaging system with the gantry in a predetermined position so as to provide the beam generator, the imaging system and the static component in reference positions, the reference image including the static component; rotating the gantry relative to the static component; returning the gantry to the predetermined position; obtaining a further image using the radiotherapy imaging system with the gantry in the predetermined position, the further image including the static component; and determining changes in the relative positions of the radiotherapy beam generator, the radiotherapy imaging system, and the static component from differences in the position of the static component in the reference image and in the further image. obtaining a reference image using the radiotherapy imaging system with the gantry in a predetermined position so as to provide the beam generator, the imaging system and the static component in reference positions, the reference image including the static component; rotating the gantry relative to the static component; returning the gantry to the predetermined position; obtaining a further image using the radiotherapy imaging system with the gantry in the predetermined position, the further image including the static component; and determining changes in the relative positions of the radiotherapy beam generator, the radiotherapy imaging system, and the static component from differences in the position of the static component in the reference image and in the further image. <EOS>