Intensity-modulated radiation therapy with dynamic multileaf collimators

Intensity-modulated radiotherapy (IMRT) has been considered as a means of providing dose distributions that conform to concave target volumes. For computer-controlled multileaf collimators (MLCs) to be used to modulate x-ray beams, some procedure must be used to determine the sequence of leaf positions used to produce the desired modulation. This article derives and compares four leaf-sequencing algorithms. MLC leaf sequencing can be accomplished by representing the areal intensity modulation of a beam with a series of beam profiles. A velocity-modulation equation for computing the modulation required for a one-dimensional profile, described originally using more extensive algebra, is derived using a graphic approach. The velocity-modulation approach is compared with an equal incremental step-and-shoot approach derived by Bortfeld and Boyer. An areal step-and-shoot technique derived by Xia and Verhey is introduced and compared with the profile-by-profile methods. Finally, an approach is considered using multiple repeated arcs developed by Yu. This wide variety of methods can yield an approach to IMRT that conforms to the engineering constraints imposed by the design of a particular linear accelerator.