A radiotherapy system comprising a support for a patient undergoing radiotherapy treatment, a gantry rotatable about an axis, a source of radiation mounted on the gantry and producing a beam of radiation directed towards a target region of the patient, a collimator coupled to said radiation source, the collimator comprising a plurality of movable, beam-limiting elements, to collectively define a shaped aperture through which the radiation beam passes, a portal imager mounted on the gantry opposite the radiation source for detecting the radiation after it has passed through the patient and generating corresponding images, and associated circuitry for controlling at least the gantry, the source, the collimator, and the portal imager, collating detected data comprising a plurality of images acquired including images at a plurality of angles of rotation of said gantry and images at a plurality of collimator shapes; generating a three-dimensional image of the target region based thereon.