Patent Number: 
Section: claims

1. Radiotherapy apparatus comprising a first collimator and a second collimator, the first collimator comprising a plurality of elongate leaves lying alongside each other, each being moveable longitudinally, the second collimator comprising a plurality of slits, the first and second collimators being aligned such that each leaf of the first collimator at least partially covers a slit of the second collimator, the slits having a width which corresponds to a fraction of the width of the leaves. 2. Radiotherapy apparatus according to  claim 1  wherein the first collimator is above the second. claim 1 3. Radiotherapy apparatus according to  claim 1 , wherein the slits are focused on the source of radiation within the apparatus. claim 1 4. Radiotherapy apparatus according to  claim 1 , in which the second collimator is movable relative to the first. claim 1 5. Radiotherapy apparatus according to  claim 1 , in which the first and second collimators are movable together relative to the patient. claim 1 6. Radiotherapy apparatus according to  claim 1 , adapted to allow a patient to be moved continuously relative to the source, the first collimator leaves being adjustable as the treatment progresses. claim 1 7. Radiotherapy apparatus according to  claim 1 , in which the fraction is one of {fraction (1/2, 1/3, 1/4)} or ⅕. claim 1 8. Radiotherapy apparatus according to  claim 1 , in which the second collimator is removable from the apparatus, thereby the enable exchange with an alternative second collimator. claim 1 9. Radiotherapy apparatus according to  claim 8  in which the alternative second collimator has a different fractional width. claim 8 10. Radiotherapy apparatus according to  claim 8 , in which the alternative second collimator has a different irradiatable area. claim 8 11. Radiotherapy apparatus according to  claim 1 , in which the second collimator is above the first. claim 1 12. Radiotherapy apparatus according to  claim 1 , in which the second collimator is above a light reflecting mirror. claim 1 13. Radiotherapy apparatus according to  claim 1 , in which the first collimator is below a light reflecting mirror. claim 1 14. Radiotherapy apparatus according to  claim 1 , in which the second collimator is positionable such that during an irradiation at least one slit is aligned with a pair of leaves in the first collimator. claim 1 15. Radiotherapy apparatus according to  claim 14  in which the centreline of the slit is aligned with the adjacent edges of the pair of leaves. claim 14 16. Radiotherapy apparatus according to  claim 1 , in which the pitch of the slits in the second collimator is approximately an integer multiple of the width of the leaves in the first collimator. claim 1 17. Radiotherapy apparatus according to  claim 1 , in which the pitch of the slits in the second collimator is approximately an integer multiple of the width of the leaves in the first collimator plus one half that width. claim 1 18. A kit of parts comprising the apparatus as defined above in  claim 1 , in combination with alternative second collimators exhibiting different fractional widths. claim 1 19. A method for performing radiotherapy comprising: aligning leaves of a first collimator with slits of a second collimator to produce at least one substantially planar beam of radiation;  sweeping the at least one substantially planar beam of radiation through the first and the second collimators and across a selected area of a patient lying in a plane transverse to that of the beam; and  modulating a width of the at least one substantially planar beam to map out an irradiated area on the patient. 20. The method of  claim 19  wherein the slits of the second collimator either allow completely or block entirely a portion of the at least one substantially planar beam. claim 19