Patent Number: 
Section: claims

1. A Collimator for a radiotherapy apparatus comprising:a block of radiation-attenuating material for moving into and out of a beam of therapeutic radiation during delivery of the therapeutic radiation, the block having:a top face and a bottom face, each transverse to a propagation direction of the beam and defining a depth direction extending from the top face to the bottom face; anda front face, at least one main rear face, and at least two side faces, each face extending from the top face to the bottom face, the side faces extending from the front face to the at least one main rear face,wherein the front face forms a leading edge when the block is moved into the beam, andwherein the at least one main rear face is non-parallel to the front face, andwherein a material is disposed against the at least one main rear face of the block. 2. The collimator according to claim 1, wherein the material is the same radiation-attenuating material. 3. The collimator according to claim 1, wherein the material is different from the radiation-attenuating material. 4. The collimator according to claim 1, wherein the at least one main rear face is substantially planar in the depth direction of the block. 5. The collimator according to claim 1, wherein the block has a uniform thickness in the depth direction. 6. The collimator according to claim 1, wherein the at least one main rear face comprises a single main rear face extending between the two side faces. 7. The collimator according to claim 1, wherein the at least one main rear face comprises two main rear faces forming a concave V-shape transverse to the propagation direction of the beam, in which each face of the V-shape rear face is non-parallel to the front face. 8. The collimator according to claim 7, wherein the material is disposed between the two main rear faces forming a concave V-shape, and the material has a lesser thickness than the block. 9. The collimator according to claim 1, wherein the two side faces are substantially planar or parallel to each other. 10. The collimator according to claim 1, wherein at least one of the top face and bottom face are substantially planar. 11. The collimator according to claim 10, wherein the top face and bottom face are parallel to each other. 12. The collimator according to claim 1, wherein the collimator is arranged to be mountable in a radiotherapy apparatus so as to be moveable back and forth in a direction transverse to the front face. 13. The collimator of claim 12, wherein the block has a minimum thickness in a direction defined from the front face to the at least one main rear face, the minimum thickness being determined based on a speed of movement of the collimator. 14. A radiotherapy apparatus, comprising:a radiation beam emitted by the beam source through an aperture;a multi-leaf collimator with leaves moveable across the aperture, and having a range of movement adequate to transport a tip of each leaf from one side of the aperture to the other side; anda block collimator having:a depth in a propagation direction of the radiation beam;a front face defining a leading edge of the block when the block moves into the beam; andat least one main rear face defining a trailing edge when the block moves into the beam,wherein the at least one main rear face is non-parallel to the front face and the block collimator is moveable across the aperture in a direction transverse to the movement of the leaves. 15. The radiotherapy apparatus of claim 14, wherein a material is disposed against the at least one main rear face. 16. The radiotherapy apparatus of claim 15, wherein the material is different from the radiation-attenuating material. 17. A method of operating a radiotherapy apparatus, the radiotherapy apparatus including a multi-leaf collimator with leaves moveable across an aperture through which a radiation beam is emitted, and a block collimator moveable across the aperture in a direction transverse to the movement of the leaves during delivery of the radiation, the method comprising:controlling movement of the leaves or the block collimator so as to match the leaves to the profile of a rear face of the block collimator extending in the direction of the radiation beam. 18. The method according to claim 17, wherein the block collimator comprises:a front face and at least one main rear face,wherein the front face forms a leading edge when the block is moved into the beam, the at least one main rear face is non-parallel to the front face. 19. The method according to claim 18, wherein the block collimator further comprises:a top face and a bottom face, each transverse to a propagation direction of the radiation beam and defining a depth direction extending from the top face to the bottom face;at least two side faces, each face extending from the top face to the bottom face, the side faces extending from the front face to the at least one main rear face; anda material disposed against the at least one main rear face. 20. The method according to claim 17, wherein controlling movement of the leaves or the block collimator includes accounting for acceleration or deceleration of the moving leaves or block collimator.