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Effect of the Normalized Prescription Isodose Line On Target Dose Deficiency in Lung SBRT Based On Monte Carlo Calculation

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D Zheng

D Zheng*, Q Zhang, S Zhou, University of Nebraska Medical Center, Omaha, NE


SU-E-T-486 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the impact of normalized prescription isodose line on target dose deficiency calculated with Monte Carlo (MC) vs. pencil Beam (PB) in lung SBRT. RTOG guidelines recommend prescription lines between 60% and 90% for lung SBRT. How this affects the magnitude of MC-calculated target dose deficiency has never been studied.

Methods: Under an IRB-approved protocol, four lung SBRT patients were replanned following RTOG0813 by a single physicist. For each patient, four alternative plans were generated based on PB calculation prescribing to 60-90% isodose lines, respectively. Each plan consisted of 360o coplanar dynamic conformal arcs with beam apertures manually optimized to achieve similar dose coverage and conformity for all plans of the same patient. Dose distribution was calculated with MC and compared to that with PB. PTV dose-volume endpoints were compared, including Dmin, D5, Dmean, D95, and Dmax. PTV V100 coverage, conformity index (CI), and heterogeneity index (HI) were also evaluated.

Results: For all 16 plans, median (range) PTV V100 and CI were 99.7% (97.5-100%) and 1.27 (1.20-1.41), respectively. As expected, lower prescription line resulted in higher target dose heterogeneity, yielding median (range) HI of 1.26 (1.05-1.51) for all plans. Comparing MC to PB, median (range) D95, Dmean, D5 PTV dose deficiency were 18.9% (11.2-23.2%), 15.6% (10.0-22.7%), and 9.4%(5.5-13.6%) of the prescription dose, respectively. The Dmean, D5, and Dmax deficiency was found to monotonically increase with decreasing prescription line from 90% to 60%, while the Dmin deficiency monotonically decreased. D95 deficiency exhibited more complex trend, reaching the largest deficiency at 80% for all patients.

Conclusion: Dependence on prescription isodose line was found for MC-calculated PTV dose deficiency of lung SBRT. When comparing reported MC dose deficiency values from different institutions, their individual selections of prescription line should be considered in addition to other factors affecting the deficiency magnitude.

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