Radiotherapy apparatus typically employs a radiation source mounted on a gantry arm that is rotatable around a patient support. As it rotates, the gantry arm can exhibit very small deformations as a result of its weight and that of the source. These deformations can cause very small movements in the isocentre, which we would like to eliminate. We propose that a variable tilt is applied to the gantry depending on its rotational orientation. This tilt can act in opposition to the droop and return the radiation source to its correct location and orientation. This tilt could be achieved in one of a number of ways. A mechanical actuator within the drum or beneath the wheels on which it rotates could be driven in response to a rotation sensor in order to tilt the gantry and/or the drum. The drum itself could be made non-circular in at least some of the areas that are supported by the wheels, so that the rotation is not completely smooth but causes the drum to tilt as it rotates. Alternatively, the wheels can be arranged to lift and/or lower as the drum rotates. A mechanism within the wheels could use an eccentric mechanism in order to adjust the position of the wheels at they rotate. Generally, the wheels will have a much smaller radius than that of the drum, so to synchronise the movement of the wheels with the rotation of the drum, a gearing arrangement such as an epicyclic gear can be employed. The epicyclic gear can include the eccentric mounting. A further possibility is to make non-circular at least one of the circumferential bearing surface or surfaces by which the rotating drum is supported on wheels.
