Patent Number: 
Section: claims

1. A radiotherapeutic apparatus comprising:a source of radiation emitting a beam of therapeutic radiation having a width, anda collimator comprising a plurality of moveable elements arranged to limit the beam width by a variable amount, the collimator being moveable along an arc centered substantially on the radiation source and comprising means for moving the beam in a direction transverse to the beam width including means for sweeping the beam to deliver a therapeutic dose of radiation across a two-dimensional area of a patient. 2. A radiotherapeutic apparatus according to claim 1 in which the elements of the collimator are adapted to be adjusted as the collimator is moved. 3. A radiotherapeutic apparatus according to claim 1 in which the collimator comprises an array of moveable elements each adapted to selectively block a beam segment. 4. A radiotherapeutic apparatus according to claim 3 in which the elements are arranged in a linear array. 5. A radiotherapeutic apparatus according to claim 3 in which the elements are arranged in a 2*n array. 6. A radiotherapeutic apparatus according to any one of claims 3 to 5 in which the beam of radiation is emitted along a path passing through part of the array of moveable elements, and the moveable elements can take up one of two available positions, one located within the beam path and thus adapted to block the beam segment, and one located outside the beam path. 7. A radiotherapeutic apparatus according to claim 6 in which the collimator is adapted to move transverse to both a beam axis and an array length. 8. A radiotherapeutic apparatus according to claim 1 or claim 2 in which the collimator is a multi-leaf collimator. 9. A radiotherapeutic apparatus according to claim 8 in which the collimator moves transverse to both a beam axis and a length of leaves of the multi-leaf collimator. 10. A radiotherapeutic apparatus according to claim 1 or claim 2 in which the collimator comprises elongate leaves which can be moved into a path of the beam by a desired length. 11. A radiotherapeutic apparatus according to claim 10 in which two arrays of leaves are provided, one on either side of the beam, each having a plurality of leaves disposed in a generally parallel arrangement. 12. A radiotherapeutic apparatus according to claim 11 in which the collimator moves transverse to both a beam axis and a length of the leaves. 13. A radiotherapeutic apparatus according to claim 10 in which the collimator moves transverse to both a beam axis and a length of the leaves.