A radiotherapy system comprising: a radiotherapy device comprising a head and a moveable gantry that carries the head; a reference detector; computing hardware configured to perform operations comprising: receiving a beam measurement signal from a field detector and a reference signal from the reference detector, the field detector generating a beam measurement signal representative of a radiation beam directed from the head of the radiotherapy device during operation of the radiotherapy device and being positioned within the radiation beam, the reference detector generating a reference signal; normalizing the beam measurement signal based on the reference signal, and outputting a normalized beam measurement; wherein the reference detector is positioned outside of the radiation beam during operation of the radiotherapy device, characterized in that the reference detector is carried on the moveable gantry so as to move therewith, wherein the head is configured to direct the radiation beam away from a first side of the movable gantry and the reference detector is carried on a second side of the movable gantry opposite the first side. a radiotherapy device comprising a head and a moveable gantry that carries the head; a reference detector; computing hardware configured to perform operations comprising: receiving a beam measurement signal from a field detector and a reference signal from the reference detector, the field detector generating a beam measurement signal representative of a radiation beam directed from the head of the radiotherapy device during operation of the radiotherapy device and being positioned within the radiation beam, the reference detector generating a reference signal; normalizing the beam measurement signal based on the reference signal, and outputting a normalized beam measurement; wherein the reference detector is positioned outside of the radiation beam during operation of the radiotherapy device, characterized in that the reference detector is carried on the moveable gantry so as to move therewith, wherein the head is configured to direct the radiation beam away from a first side of the movable gantry and the reference detector is carried on a second side of the movable gantry opposite the first side. receiving a beam measurement signal from a field detector and a reference signal from the reference detector, the field detector generating a beam measurement signal representative of a radiation beam directed from the head of the radiotherapy device during operation of the radiotherapy device and being positioned within the radiation beam, the reference detector generating a reference signal; normalizing the beam measurement signal based on the reference signal, and outputting a normalized beam measurement; wherein the reference detector is positioned outside of the radiation beam during operation of the radiotherapy device, characterized in that the reference detector is carried on the moveable gantry so as to move therewith, wherein the head is configured to direct the radiation beam away from a first side of the movable gantry and the reference detector is carried on a second side of the movable gantry opposite the first side.