Patent Number: 
Section: claims

1. 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. 2. The multi-leaf collimator assembly of claim 1 wherein the second multi-leaf collimator is positioned apart from and is movable relative to the first multi-leaf collimator. 3. The multi-leaf collimator assembly of claim 2 wherein the second multi-leaf collimator is rotatably movable relative to the first multi-leaf collimator. 4. The multi-leaf collimator assembly of claim 2 wherein the second depth is between 20 millimeters and 40 millimeters. 5. The multi-leaf collimator assembly of claim 2 wherein the second multi-leaf collimator is positioned below the first multi-leaf collimator relative to the source of radiation. 6. The multi-leaf collimator assembly of claim 1 wherein each leaf of the first multi-leaf collimator has a first thickness when measured in a direction perpendicular to an axis from the source of radiation to a treatment table, and wherein each leaf of the second multi-leaf collimator has a second thickness, the second thickness being no greater than half the first thickness. 7. The multi-leaf collimator assembly of claim 1 wherein the second subfield comprises a penumbra of the first radiation beam field. 8. The multi-leaf collimator assembly of claim 1 wherein the first and second multi-leaf collimators are permanently mounted to a linear accelerator. 9. The multi-leaf collimator assembly of claim 1 wherein the first and second multi-leaf collimators are removably mounted to a linear accelerator as a single unit. 10. The multi-leaf collimator assembly of claim 1 further comprising a controller for controlling movement of at least one leaf within the first multi-leaf collimator and two or more leaves of the second multi-leaf collimator such that the at least one leaf and the two or more leaves move in concert along a common axis. 11. A linear accelerator system for applying radiation treatment, the system comprising:a linear accelerator for generating a radiation beam;a first multi-leaf collimator having multiple leaves movable relative to a source of radiation and defining an aperture through which the radiation beam passes, thereby creating a first radiation beam field, 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. 12. The linear accelerator system of claim 11 wherein the second multi-leaf collimator is positioned apart from and movable relative to the first multi-leaf collimator. 13. The linear accelerator system of claim 12 wherein the second depth is between 20 millimeters and 40 millimeters. 14. The linear accelerator system of claim 12 wherein the second multi-leaf collimator is positioned below the first multi-leaf collimator relative to the linear accelerator. 15. The linear accelerator system of claim 11 wherein each leaf of the first multi-leaf collimator has a first thickness when measured in a direction perpendicular to an axis from the source of radiation to a treatment table wherein each leaf of the second multi-leaf collimator has a second thickness, the second thickness being no greater than half the first thickness. 16. The linear accelerator system of claim 11 wherein the second subfield comprises a penumbra of the first radiation beam field. 17. The linear accelerator system of claim 11 further comprising a controller for controlling movement of at least one leaf within the first multi-leaf collimator and two or more leaves of the second multi-leaf collimator such that the at least one leaf and the two or more leaves move in concert along a common axis. 18. A method of delivering radiation therapy, the method comprising:defining an aperture by adjusting a positioning of one or more leaves of a first multi-leaf collimator, wherein the one or more leaves of the first multi-leaf collimator have a first depth along a radiation beam direction;delivering a first radiation beam field through the aperture; andintersecting a portion of the first radiation beam field with at least one leaf of a second multi-leaf collimator to define a second radiation beam field comprising a first subfield and a second subfield, the at least one leaf 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 the second subfield results from the intersection of the at least one leaf of the second multi-leaf collimator with the first radiation beam field and has a lower intensity than the first subfield. 19. The method of claim 18 wherein adjusting the positioning of one or more leaves of the first multi-leaf collimator causes the at least one leaf of the second multi-leaf collimator to move in concert with the one or more leaves of the first multi-leaf collimator. 20. The method of claim 19 wherein an end of at least a first leaf of the second multi-leaf collimator is aligned with an end of the first multi-leaf collimator partially defining the aperture and at least a second leaf of the second multi-leaf collimator extends beyond the end of the first multi-leaf collimator into the first radiation beam field.