Abstract: A radiation therapy apparatus includes a multi-leaf attenuator having a first plurality of radiation attenuating leaves and a second plurality of radiation attenuating leaves. The first plurality of leaves is spaced with gaps between adjacent leaves within a radiation beam so that the leaves and the gaps therebetween divide the radiation beam into rays. The second plurality of leaves is disposed directly under gaps in the first plurality of leaves so that each ray of the beam can be occluded by a leaf in either the first or second plurality. The gaps eliminate interference between leaves; the staggering prevents radiation leakage between leaves. Each leaf, in both groups, may be moved between a first open state on one side of the beam, a closed state within the beam thus occluding one ray of the beam, and a second open state on the other side of the beam.