Radiotherapy beam alignment

A method for aligning the treatment beam 22 in a radiotherapy system 2 having a radiation source 6 and a device for imaging 26 from the beam mounted on a rotatable chassis 24 so as to be rotatable around the longitudinal axis Y of a patient couch 5 on which a fiducial phantom 30 is placed. Images of the phantom are obtained from a variety of angles; the position of the source is determined from analysis of each one; and the centres of rotation of the source are calculated to define the isocentre of the system. Preferably, the chassis incorporates an MRI apparatus 4 and the method includes adjusting the trajectory of the source to locate the isocentre at the centre of the MRI volume. Markers in the phantom may be visible to MRI, enabling alignment of the source with the imaging field. The method may also be used to align a collimator or to reduce asymmetry in the beam intensity, and may include a second imaging device.