Patent Number: 
Section: claims

1. A collimator for a radiation beam comprising:a first multi-leaf collimator set;a second multi-leaf collimator set at an acute angle to the first multi-leaf collimator set; anda third multi-leaf collimator set at an acute angle to the second multi-leaf collimator set. 2. A collimator according to claim 1 in which each multi-leaf collimator set includes a pair of leaf banks mutually opposed to each other. 3. A collimator according to claim 1 in which the acute angle between the first and the second multi-leaf collimator set is the same as the acute angle between the second and the third set. 4. A collimator according to claim 1 in which at least one of the acute angles is 60°. 5. A collimator according to claim 1 in which the leaves of the multi-leaf collimator closest to the radiation source are deeper in the direction of the radiation than the leaves of a multi-leaf collimator more distant from the radiation source. 6. A collimator according to claim 1 in which the leaves of the multi-leaf collimator furthest from the radiation source are shallower in the direction of the radiation than the leaves of a multi-leaf collimator closer to the radiation source. 7. A collimator according to claim 1 in which the tips of the leaves of the multi-leaf collimators are rounded. 8. A collimator according to claim 7 in which the radius of curvature of the tips of the leaves of the multi-leaf collimator closest to the radiation source is greater than the radius of curvature of the tips of the leaves of a multi-leaf collimator more distant from the radiation source. 9. A collimator according to claim 7 in which the radius of curvature of the tips of the leaves of the multi-leaf collimator furthest from the radiation source is less than the radius of curvature of the tips of the leaves of a multi-leaf collimator closer to the radiation source. 10. A collimator according to claim 1, in which the first multi-leaf collimator is closest to the radiation source, the third multi-leaf collimator is furthest from the radiation source, and the second multi-leaf collimator is between the first and third multi-leaf collimators.