Patent Number: 
Section: claims

1. A method of providing intensity modulated radiation therapy to a target using a multileaf collimator (MLC), wherein the target comprises a region that moves periodically along a trajectory, comprising:determining the trajectory of the moving target region;determining a line segment that at least approximately matches said trajectory;orienting the MLC such that all of the leaves of the MLC are perpendicular to said line segment;delivering therapeutic radiation through an aperture in said MLC, wherein each of said aperture and said target are in a first position; anddelivering therapeutic radiation through an aperture in said MLC, wherein each of said aperture and said target are in a second position. 2. The method of claim 1, wherein the trajectory is linear and said line segment corresponds to said trajectory. 3. The method of claim 1, wherein the trajectory is a curve. 4. The method of claim 3, wherein the trajectory is obtained from deformable image registration or a point-specific movement model. 5. The method of claim 1, wherein said MLC aperture moves while said therapeutic radiation is delivered to the target. 6. The method of claim 5, wherein said MLC aperture is held in a fixed position each time said therapeutic radiation is delivered to the target. 7. An intensity modulated radiation therapy system for providing radiation therapy to a target, wherein the target comprises at least one region that moves periodically along a trajectory, comprising:a beam generator for creating a radiotherapy beam for irradiating the moving target region;a multi-leaf collimator positioned between said beam generator and said moving target region for controlling the shape and position of the beam;a control system having a computer-readable medium encoded with an algorithm including the steps of:projecting the trajectory of the moving target region onto a multi-leaf collimator plane; dividing the multi-leaf collimator plane into a plurality of thin slices substantially parallel to the movement of the target region; andgenerating a leaf sequence such that each point in a slice that receives the same fluence receives radiation at the same time. 8. The system of claim 7, wherein the trajectory is a line segment. 9. The system of claim 7, wherein the trajectory is a curve. 10. The system of claim 9, wherein the trajectory is obtained from deformable image registration or a point-specific movement model. 11. The system of claim 7, wherein generating the leaf sequence includes providing an aperture that is parallel to the movement of the target region. 12. The system of claim 11, further comprising controlling the time each slice is exposed to the aperture. 13. The system of claim 12, wherein each slice is exposed to the aperture for about the duration of one movement cycle. 14. The system of claim 13, further comprising providing a variable dose rate. 15. The system of claim 7, wherein the generated leaf sequence includes leaf movement that is perpendicular to the movement of the target region. 16. The system of claim 7, wherein the generated leaf sequence includes leaf movement that is not perpendicular to the movement of the target region. 17. A method of providing intensity modulated radiation therapy to a treatment volume using a multileaf collimator (MLC), wherein the treatment volume comprises a region that moves periodically along a trajectory, comprising:defining a line segment that corresponds to said trajectory;delivering radiation through a first elongate aperture in said MLC, when said aperture is in a first position; anddelivering radiation through a second elongate aperture in said MLC, when said aperture is in a second position;wherein said first and second elongate apertures define first and second elongate axes that are offset from said line segment by first and second angles, and wherein said first and second elongate apertures are determined such that said first and second angles are minimized. 18. The method of claim 17 wherein said first and second elongate apertures are determined based on additional parameters. 19. The method of claim 17, wherein the trajectory is linear and said line segment corresponds to said trajectory. 20. The method of claim 17, wherein the trajectory is a curve.