Intensity modulation with the "step and shoot" technique using a commercial MLC: A planning study

Purpose/Objective: For complex planning situations where organs at risk (OA R) surrounding the target volume place stringent constraints, intensity-mod ulated treatments with photons provide a promising solution to improve tumo r control and/or reduce side effects. One approach for the clinical impleme ntation of intensity-modulated treatments is the use of a multileaf collima tor (MLC) in the "step and shoot' mode, in which multiple subfields are sup erimposed for each beam direction to generate stratified intensity distribu tions with a discrete number of intensity levels. In this paper, we examine the interrelation between the number of intensity levels per beam for vari ous numbers of beams, the conformity of the resulting dose distribution, an d the treatment time on a commercial accelerator (Siemens Mevatron KD2) wit h built-in MLC. Methods and Materials: Two typical, clinically relevant cases of patients w ith head and neck tumors were selected for this study. Using the inverse pl anning technique, optimized treatment plans are generated for 3-25 evenly d istributed coplanar beams as well as noncoplanar beams. An iterative gradie nt method is used to optimize a physical treatment objective that is based on the specified target dose and individual dose constraints assigned to ea ch organ at risk (brain stem, eyes, optic nerves) by the radiation oncologi st. The intensity distribution of each beam is discretized within the inver se planning program into three to infinitely many intensity levels or strat a. These stratified intensity distributions are converted into MLC leaf pos ition sequences, which can be subsequently transferred via computer link to the linac console, and can be delivered without user intervention. The qua lity of the plan is determined by comparing the values of the objective fun ction, dose-volume histograms (DVHs), and isodose distributions. Results: Highly conformal dose distributions can be achieved with five inte nsity levels in each of seven beams. The merit of using more intensity leve ls or more beams is relatively small. Acceptable results are achievable eve n with three levels only. On average, the number of subfields per beam is a bout 2-2.5 times the number of intensity levels. The average treatment time per subfield is about 20 s. The total treatment time for the three-level a nd seven-beam case with a total of 39 subfields is 13 min. Conclusion: Optimizing stratified intensity distributions in the inverse pl anning process allows us to achieve close to optimum results with a surpris ingly small number of intensity levels. This finding may help to facilitate and accelerate the delivery of intensity-modulated treatments with the "st ep and shoot" technique. (C) 1999 Elsevier Science Inc.