a radiotherapy apparatus comprises a source for producing a beam of ionising radiation along an axis , the beam covering a maximum aperture of the source , a collimator for collimating the beam to produce a collimated beam covering a sub - part of the maximum aperture , a patient support positioned in the path of the beam , a rotatable gantry , on which the source is mounted , for rotating the source around the patient support thereby to deliver the beam from a range of directions , an imaging device located opposite the source and with the patient support between the source and the imaging device , and mounted on the gantry via a drive member allowing translational motion of the imaging device in at least one direction perpendicular to the axis , and a control unit adapted to control the drive member to move the imaging device within the maximum aperture and maintain coincidence between the imaging device and the sub - part of the maximum aperture . accordingly , the epid can be moved during the treatment in order to maintain the collimated field of the radiation beam within the bounds of the epid . this ensures that the image is valid and prevents damage to the epid as a result of exposure of more sensitive parts to the beam .