A radiotherapy apparatus has a source 110 mounted on a rotatable gantry 104 such as a drum, a portal imaging device (such as an EPID 118) also mounted to the gantry and moveable relative to the source, and a control unit (132, figure 8) arranged to coordinate the movement of both to keep the imaging device in the beam as the source moves to different treatment positions. The apparatus is especially for the treatment of targets lying away from the isocentre of the apparatus. Preferably the beam passes through a multi-leaf collimator (MLC) 116. The EPID may be connected to a gantry arm 120 by a series of servo-controlled linkages 122 arranged transverse to the beam direction to provide movement of the EPID in the transverse plane. Adjustment towards or away from the source may also be provided. Movement of the EPID may be predictive or it may react to the collimated beam approaching the periphery of the imaging area. The movement ensures that the image is valid for dosimetry purposes and prevents damage to the EPID as a result of exposure of more sensitive (or less shielded) parts to the beam.
