A multi-leaf
collimator for a radiotherapy apparatus comprises a plurality of elongate leaves mounted in a
carriage, the
carriage being mounted on a substrate, wherein the leaves are independently moveable relative to the
carriage in a longitudinal direction, and the carriage is moveable in that direction relative to the substrate, and a control apparatus arranged to receive a
signal representing leaf positions relative to the substrate and being arranged to control the leaf positions relative to the carriage and the carriage positions relative to the substrate so as to achieve those leaf positions relative to the substrate. By allowing the carriage to be driven concurrently with the leaves, the speed of movement of the carriage can be added to that of the leaves. In this way, where leaves are required to make a long
traverse they can do so more quickly. Most MLCs have a means for sensing the current positions of the leaves relative to the substrate, such as an optical or mechanical positional feedback
system. The control apparatus can therefore compare the current leaf positions to the signalled leaf positions, and move the leaves and the carriage accordingly. Each leaf can be moved according to a difference between the current leaf position and the signalled leaf position, and the carriage can be moved according to an average difference between the current leaf positions and the signalled leaf positions. A corresponding method is also disclosed.