Radiotherapy apparatus comprises a source of radiation mounted on a chassis, that is rotatable about a rotation axis and the source being adapted to emit a beam of radiation along a beam axis; a patient support, moveable along a translation axis; a set of magnetic coils located on either side of the beam, for establishing a magnetic field along a first direction; the translation axis, the rotation axis, and the first direction being substantially parallel; and further comprising a multi-leaf collimator fixed in its orientation with respect to the source of radiation, the multi-leaf collimator comprising a plurality of elongate leaves disposed with their longitudinal directions substantially aligned with the first direction and movable in that direction between a withdrawn position in which the leaf lies outside the beam, an extended position in which the leaf projects across the beam.