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 rotateable 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 receive a
treatment plan in which the arc is divided into a plurality of notional arc-segments, and specifying the
total dose for the arc-segment and a start and end MLC position. It then controls the source in accordance with that plan over an first arc-segment such that at least one of the rotation speed and
dose rate are constant and the multi-leaf
collimator changes shape, and a second arc segment such that at least one of the rotation speed and
dose rate are constant at a level different to the constant level adopted during the first arc-segment. It achieves this by calculating the total time required for the arc segment for a plurality of factors including an MLC leaf movement from a prescribed position at the start of the arc-segment to a prescribed position at the end of the arc-segment, at a maximum leaf speed, rotation of the source from the start to the end of the arc-segment at a maximum source rotation speed, delivery of the dose at a
maximum dose rate per time, selecting the factor dictating the longest time, and controlling the apparatus so that the selected factor operates at its respective maximum and the remaining factors are operated at a reduced rate selected to match that longest time.