Patent Number: 
Section: claims

1. A radiotherapy apparatus comprising;a means for producing a beam of radiation that is directed along a beam axis and has a width in first and second directions transverse to the beam axis;a multi-leaf collimator for selectively limiting the width of the beam in at least the first direction;a block collimator for selectively limiting the width of the beam in at least the second direction;wherein the block collimator comprising a diaphragm moveable into and out of the beam and having a thickness that varies in the direction of the beam axis and in which the diaphragm has a spine region extending from a rear part thereof towards the front edge that is of greater thickness than at least one region displaced laterally with respect thereto. 2. The radiotherapy apparatus according to claim 1 in which the diaphragm has a front edge of greater thickness than at least one region behind the front edge. 3. The radiotherapy apparatus according to claim 1 in which the spine region extends from the rearmost part of the diaphragm. 4. The radiotherapy apparatus according to claim 1 in which the spine region extends to the front edge of the diaphragm. 5. The radiotherapy apparatus according to claim 1 in which the spine region is straight. 6. The radiotherapy apparatus according to claim 1 in which the spine region is a central region of the diaphragm. 7. The radiotherapy apparatus according to claim 1 in which the width of the spine region increases towards the front edge of the diaphragm. 8. The radiotherapy apparatus according to claim 1 in which the first and second directions are mutually transverse. 9. A radiotherapy apparatus comprising a multi-leaf collimator and a block collimator, the block collimator comprising a diaphragm with variable thickness and a spine region in which the spine region has a greater thickness than at least one region displaced laterally with respect thereto. 10. The radiotherapy apparatus according to claim 9, further comprising a control means for the multi-leaf collimator, arranged to extend leaves of the multi-leaf collimator to shadow regions of the beam that are blocked by a relatively thinner section of the diaphragm. 11. A block collimator for use in radiotherapy apparatus comprising a diaphragm moveable into and out of a beam, and having a thickness in the direction of the beam axis that varies and in which the diaphragm has a spine region extending from a rear part thereof towards the front edge that is greater thickness than at least one region displaced laterally with respect thereto. 12. The block collimator according to claim 11 in which the diaphragm has a front edge of greater thickness than at least one region behind the front edge. 13. The block collimator according to claim 11 in which the spine region extends from the rearmost part of the diaphragm. 14. The block collimator according to claim 11 in which the spine region extends to the front edge of the diaphragm. 15. The block collimator according to claim 11 in which the spine region is straight. 16. The block collimator according to claim 11 in which the spine region is a central region of the diaphragm. 17. The block collimator according to claim 11 in which the width of the spine region increases towards the front edge of the diaphragm. 18. A radiotherapy apparatus comprising;a means for producing a beam of radiation directed along a beam axis and having a width in first and second directions transverse to the beam axis;a multi-leaf collimator for selectively limiting the width of the beam in at least the first direction;a block collimator for selectively limiting the width of the beam in at least the second direction;the block collimator comprising a diaphragm moveable into and out of the beam and having a width that varies transverse to the direction of movement andcomprising a central spine region and a wider front edge. 19. The radiotherapy apparatus according to claim 18 in which the wider front edge extends across substantially the entire width of the beam in the first direction.