A radiotherapeutic apparatus comprises a source able to emit a beam of therapeutic radiation along a beam axis, a multi-leaf collimator arranged to collimate the beam to a desired shape, wherein the source is rotatable about a rotation axis that is substantially orthogonal and intersects with the beam axis thereby to describe an arc around that axis, and further comprises a control means able to control the dose/time rate of the source, the rotation speed of the source, and the multi-leaf collimator position. The control means is arranged to control the source in accordance with a treatment plan over first and second arc-segments such that at least the multi-leaf collimator changes shape at a different rate per degree in the second arc-segment as to the first arc-segment.