A multi-leaf collimator for a radiotherapy apparatus comprises at least one array of laterally-spaced elongate leaves, each leaf being driven by an associated motor connected to the leaf via a drive means so as to extend or retract the leaf in its longitudinal direction, the drive means comprising a sub- frame on which at least a subset of the motors are mounted, the sub-frame being mounted at a location spaced from the leaf array in a direction transverse to the lateral and longitudinal directions, and including a plurality of leadscrews disposed longitudinally, each being driven by a motor and being operatively connected to a leaf thereby to drive that leaf. This greater space efficiency, and allows the drive system to be modular. A plurality of the motors mounted on the subframe can be mounted at a first longitudinal end, and the remainder of the motors mounted on the subframe are mounted at a second, opposing, longitudinal end. Those leadscrews not at an edge of the array are preferably neighboured on either lateral side by one leadscrew driven by a motor mounted at the same longitudinal end and a second leadscrew driven by a motor mounted at the opposite longitudinal end. This results in the motors being arranged in pairs with a gap between which provides space for mounting the motors. The pairs of motors can be arranged one above the other to allow the necessary clearances, meaning that the leadscrews will be mounted in the subframe at one of two spacings from the leaf, with laterally neighbouring leadscrews being mounted at alternating spacings. The leadscrews can be mounted within a bore in the subframe. Alternate leaves can be driven by a corresponding lower subframe. The drive means can further include a threaded member on the leadscrew. This can urge a laterally extending lug, thereby to drive the leaf. The lug can engage with a recess on the leaf edge. It can be held in machined slot in the subframe that slot can be machined with non-parallel sides to as
