MULTILEAF COLLIMATOR LEAF SEQUENCING ALGORITHM FOR INTENSITY-MODULATED BEAMS WITH MULTIPLE STATIC SEGMENTS

The ''stop and shoot'' method of producing intensity modulation using combinations of static multileaf collimator (MLC) segments has a numbe r of advantages including precise dose delivery, easy verification, an d general availability. However, due to the potential limitation of pr olonged treatment time, it is essential to keep the number of required segments to a reasonable number. We propose an algorithm to minimize the number of segments for an intensity modulated field. In this algor ithm, the sequence of delivery intensity is proposed to be a series of powers of 2, depending on the maximum intensity level in the matrix. The MLC leaf position sequence is designed directly on the two-dimensi onal intensity matrix to irradiate the largest possible area in each s egment. The algorithm can be applied directly to MLC systems with diff erent motion constraints. This algorithm has been evaluated by generat ing 1000 random 15 x 15 cm intensity matrices, each having from 3 to 1 6 intensity levels. Five clinical intensity modulated fields generated from the NOMOS CORVUS planning system for a complex clinical head and neck case were also tested with this and two other algorithms. The re sults of both the statistical and clinical studies showed that for all the intensity matrices tested, the proposed algorithm results in the smallest number of segments with a moderately increased monitor units. Thus it is well-suited for use in static MLC intensity modulation bea m delivery. For MLC systems with interleaf motion constraint, we prove mathematically that this constraint reduces the tongue and groove eff ect at the expense of an increase of 25% in the number of segments. (C ) 1998 American Association of Physicists in Medicine.