Patent ID: 8519370

Claim:
A multi-leaf collimator assembly, the assembly comprising: a first multi-leaf collimator having multiple leaves movable relative to a source of radiation and defining an aperture through which a first radiation beam field passes, the leaves of the first multi-leaf collimator having a first depth along a radiation beam direction; a second multi-leaf collimator having multiple leaves movable relative to the source of radiation, the leaves of the second multi-leaf collimator having a second depth along the radiation beam direction, the second depth being less than the first depth and insufficient to fully attenuate the first radiation beam field, wherein at least one leaf of the second multi-leaf collimator intersects a portion of the first radiation beam field to define a second radiation beam field comprising a first subfield and a second subfield, the second subfield resulting from the intersection of the at least one leaf of the second multi-leaf collimator with the first radiation beam field and having lower intensity than the first subfield.