Dynamic X-ray compensation for conformal radiotherapy by means of multi-leaf collimation.

The application of a multiple fixed field technique employing individually shaped and intensity-modulated beams makes it possible to produce dose distributions of high conformity even in the case of concave target volumes. With the technique presented here arbitrary intensity-modulated beams for the practical solution of the inverse problem can be generated. It is also possible to omit wedges, blocks and compensators in conventional radiotherapy. A continuous unidirectional sweep of independently computer-controlled leaves of a multi-leaf collimator is used to modulate the primary uniform beam. A new algorithm is introduced that computes the leaf trajectories. Also, a method is presented that accounts for leaf penumbra and transmission, which causes the generated fluence distribution to deviate from the desired fluence distribution. An optimization algorithm minimizing this deviation is described. The algorithm calculating the leaf trajectories, as well as the method considering penumbra and transmission and the successive optimization technique are used to calculate examples. Treatment times are calculated and compared to those needed when using compensators. A relationship between the treatment time and the maximum leaf speed is also deduced. To achieve good performance the maximum leaf speed should be no less than 20 mm/s.