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 substantially planar in the depth direction of the block and non-parallel to the front face. 2. 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. 3. 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. 4. The collimator according to claim 1, wherein the at least one main rear face is at an angle to the front face of between 10° and 80°. 5. The collimator according to claim 1, wherein the at least one main rear face is at an angle to the front face of between 30° and 60°. 6. The collimator according to claim 1, wherein the two side faces are substantially planar or parallel to each other. 7. The collimator according to claim 1, wherein at least one of the top face and bottom face are substantially planar. 8. The collimator according to claim 7, wherein the top face and bottom face are parallel and planar to each other. 9. 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. 10. The collimator according to claim 1, wherein the block further includes a web material disposed against the at least one rear main face. 11. The collimator according to claim 1, wherein the block has a uniform thickness in the depth direction. 12. The collimator of claim 1, whereinthe block has a minimum thickness in a direction defined from the front face tothe at least one main rear face, the minimum thickness being determined based on a speed of movement of leaves of a multi-leaf collimator. 13. A radiotherapy apparatus comprising:a beam source and an associated primary collimator defining a rectangular aperture for a radiation beam emitted by the beam source, the aperture having a long dimension and a short dimension;a multi-leaf collimator with leaves moveable across the short dimension, 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 a beam emanating from the beam source, the block collimator further having:a front face defining leading edge of the block when the block moves into the beam;at 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 substantially planar in the direction of the depth of the block and non-parallel to the front face,wherein the block collimator is moveable in a direction aligned with the long dimension. 14. A method of operation of a radiotherapy apparatus according to claim 13, wherein the movement of the leaves and the collimator block are controlled so as to match the profile of a rear face of the collimator block extending in the direction of the radiation beam. 15. The method according to claim 14, wherein movement of the leaves and/or the collimator block is controlled so as to account for acceleration and/or deceleration of the moving leaves and/or collimator block.