Abstract
Purpose: To compare the impact of MLC width on the dose conformity, doses received by the normal brain tissue, and other critical structures for single isocenter (SI) and multiple isocenter (MI) stereotactic IMRT (SRT) treatments for patients with at least four brain tumors. Methods: Six patients (three with seven and three with four lesions) planned in BrainLab iPlan were evaluated using Varian HD MLC (2.5mm leaf-width) and Millennium-120 MLC (5mm leaf-width). Prescribed doses in all plans were 25 Gy delivered in five fractions normalized to 98% dose covering 98% of the target volume. Results: All plans were judged clinically acceptable. The average Paddick conformity index for 2.5mm vs 5mm SI and MI plans were (0.48±0.22 vs. 0.38±0.21, p<.01) and (0.49±0.18 vs. 0.39±0.16, p<.01), respectively. The average percent volumes of normal brain receiving, V12, V10, and V8 Gy for (2.5mm vs 5mm) SI plans were (15.3±11.5 vs. 17.8±13.2, p=0.04), (20.8±14.8 vs. 23.5±15.7, p=0.01), (28.1±18.0 vs. 30.7±18.1, p=0.05), respectively. For MI plans these values were respectively (11.8±8.3 vs. 13.2±8.9, p<0.01), (16.3±11.5 vs. 17.7±11.9, p=0.01), and (22.4±15.5 vs. 23.8±15.4, p=0.01). The average mean and maximum brainstem doses for (SI and MI) plans were (7% and 5%) and (6% and 2%) lower in 2.5mm plans than 5mm plans, respectively. For (SI and MI) plans the average mean and maximum hippocampus doses were (11% and 11%) and (7% and 5%) lower in 2.5mm plans than 5mm plans, respectively. The average mean and maximum chiasm doses for (SI and MI) plans were (5% and 30%) and (7% and 28%) lower in 2.5mm plans than 5mm plans, respectively. Conclusion: MI stereotactic IMRT treatment plans, especially with HD MLC, were capable of achieving significantly higher dose conformity, lower normal brain tissue doses, and lower doses to critical structures than SI plans for four or more intracranial tumors.
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