Program Information
Intermediate Dose Calculation During Optimization Improves Plan Quality for Lung IMRT and SBRT
S Gardner*, A Doemer , B Miller , I Aref , M Ajlouni , B Movsas , I Chetty , Henry Ford Health System, Detroit, MI
Presentations
PO-BPC-Exhibit Hall-2 (Saturday, March 7, 2015) Room: Exhibit Hall
Purpose: To quantify the improvement in plan quality for lung IMRT plans utilizing intermediate dose calculation during IMRT optimization.
Methods: For this study, 10 conventionally-fractionated and 5 SBRT lung IMRT cases were analyzed. For comparison, the optimization was repeated (using same constraints) with intermediate dose calculation. Plan metrics for conventional plans were: conformity index (CI), PTV D(99%), and PTV D(10%). For SBRT plans: CI, PTV D(99%), Ratio of 50% isodose volume to PTV volume (50% Ratio), and the volume of tissue receiving the prescription dose outside the PTV (Vol. Rx-Outside). All dose values were scaled as a percentage of the prescription dose; all volumes are given in cc. For each case, the original plan (‘Original’) was compared to the plan with intermediate dose calculation (‘Optimized’).
Results: The intermediate dose calculation resulted in improvements in all plan quality metrics. For conventional IMRT lung cases, the mean CI was 1.21 (Original) and 1.06 (Optimized) [p<0.005]; the mean D(99%) was 96.8% (Original) and 98.0% (Optimized) [p<0.005]; the mean D(10%) was 109.3% (Original) and 104.0% (Optimized) [p<0.005]. For SBRT cases, the mean CI was 1.26 (Original) and 1.07 (Optimized) [p=0.010]; the mean 50% Ratio was 6.32 (Original) and 6.02 (Optimized) [p=0.019]; the mean D(99%) was 96.8% (Original) and 98.0% (Optimized) [p=0.016]; the mean Vol. Rx-Outside was 4.04 cc (Original) and 0.82 cc (Optimized) [p<0.009].
Conclusion: For optimized plans, the most substantial improvements were observed in PTV dose conformity and uniformity. Pencil-beam based dose calculation provides inaccurate results when used for beamlet calculations. Optimization using AAA produces a more accurate dose distribution and drives the optimizer based on a different intermediate solution. Therefore, use of intermediate dose calculation in lung IMRT plan optimization has the potential to simultaneously improve planning efficiency and plan quality.
Funding Support, Disclosures, and Conflict of Interest: Varian holds research agreements with Varian Medical Systems (Palo Alto, CA).
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