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Object. The purpose of this study was to determine the effect of static and dynamic collimator optimization when using a micromultileaf collimator (mMLC) in dynamic-arc stereotactic radiosurgery (SRS) by evaluating the dose to healthy peritumoral tissue. Methods. Thirty patients previously treated for intracranial lesions with the BrainLAB mMLC underwent retro- spective replanning. Three collimator optimization strategies were compared for a simulated SRS treatment plan, as follows: Strategy 1, static collimation fixed at 90˚ throughout arcs; Strategy 2, static collimator settings optimized for each arc; and Strategy 3, dynamic collimator settings optimized every 10˚ throughout treatment arcs. Dose-volume histograms for a 0.7-cm shell of healthy peritumoral tissue were quantitatively compared. Collimator optimization schemes (Strategies 2 and 3) significantly decreased the volume of peritumoral tissue that is irradiated when compared with static collimation at 90˚ (Strategy 1). The volume was reduced by 40.6% for Strategy 2 (95% confidence interval (CI) 6 11) and by 47.1% for Strategy 3 (95% CI 6 8.1) at the 95% isodose; by 28.4% for Strategy 2 (95% CI 6 4.9) and 39.1% for Strategy 3 (95% CI 6 6) at the 90% isodose; and by 18.2% for Strategy 2 (95% CI 6 8.1) and 25.4% for Strategy 3 (95% CI 6 7.1) at the 80% isodose. Serial collimator optimization through- out the treatment arcs (Strategy 3) reduced the mean volume of peritumoral tissue irradiated when compared with sta- tic collimator optimization (Strategy 2), by 16.1% (95% CI 6 1.5) at 95% isodose, by 11.7% (95% CI 6 1) at 90% isodose, and by 8.2% (95% CI 6 1.2) at 80% isodose regions. In specific cases, linear or polynomial functions were formulated to optimize collimator settings dynamically throughout treatment arcs. Conclusions. Dynamic collimator optimization during arc-based SRS decreases the volume of healthy peritumoral tissue treated with high doses of radiation and appears to be an effective method of improving target conformality. This study is the first step toward determination of a smoothing function algorithm to allow for true dynamic collimation during SRS. KEY WORDScollimator optimizationstereotactic radiotherapyradiosurgery

Dynamic collimator optimization compared with fixed collimator angle in arc-based stereotactic radiotherapy: a dosimetric analysis