Algorithms for sequencing multileaf collimators

In delivering radiation therapy for cancer treatment, it is desirable to deliver high doses of radiation to the target tumor, while permitting a low dosage on the surrounding healthy tissues. In recent years, the development of intensity modulated radiation therapy (IMRT) has made this possible. IMRT may be delivered by several techniques. The delivery of IMRT with a multileaf collimator (MLC) requires the delivery of radiation from several beam orientations. The intensity profile for each beam direction is described as a MLC leaf sequence, which is developed using a leaf sequencing algorithm. Important considerations in developing a leaf sequence for a desired intensity profile include maximizing the monitor unit (MU) efficiency (equivalently minimizing the beam-on time) and minimizing the total treatment time subject to the leaf movement constraints of the MLC model. In this work, we present a systematic study of the optimization of leaf sequencing algorithms and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of MU efficiency under most common leaf movement constraints that include minimum and maximum leaf separation, leaf interdigitation and tongue-and-groove. We also develop algorithms to split large intensity modulated fields into two or three subfields.