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. <EOS>