Patent Number: 
Section: abstract

A radiotherapy apparatus comprises a first collimator and a second collimator, the first collimator being a multi-leaf collimator, the second collimator comprising a plurality of slits having a width which is a fraction of the width of the leaves of the first collimator, the first and second collimators being aligned such that each slit of the second collimator is associated with a leaf of the first collimator. A first irradiation is made, during which the first collimator will define the outer edge of the irradiation pattern, and the second collimator will serve to narrow the effective width of each leaf of the first collimator. This narrowing is a simple function of the relative widths of the slits of the second collimator and the leaves of the first. This will leave gaps in between the slits of the second collimator, which can then be filled by moving one or more of the patient, first and second collimators, so as to irradiate an area omitted in the first irradiation. In this second irradiation, the positions of the leaves of the first collimator are adjusted as necessary. This process is then repeated until the entire target area has been irradiated. Suitable values for the fraction are {fraction (1/2, 1/3, 1/4)}, or ⅕.