Program Information
Improving Multi-Target Intracranial SRS Plan Evaluation with Scripting
C Mayo*, D Roberts , K Younge , K Lam , K Younge , J Mikell , D Litzenberg , P Archer , B Yanke , J Wilkinson , University of Michigan, Ann Arbor, MI
Presentations
SU-F-FS2-4 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: Four Seasons 2
Purpose: Transitioning, away from a fixed beam toward a VMAT approach for multi-target SRS, we developed a standardized algorithmic approach for treatment planning, and a script based evaluation application characterizing high, intermediate and low dose regions proximal to targets and throughout the brain. For this study the evaluation script was used to compare metrics for treated fixed and VMAT based plan to quantify benchmark norms.
Methods: Dual purpose structures used for optimization and evaluation include 5 mm thick shells around the PTV (HDRing) and around the HDRing (MDRing) to control/measure dose fall off around the targets and Brain – (PTV+ 5 mm) to quantify for low dose regions. Plans were examined for 33 patients (26 Fixed/7 VMAT) treating 81 ( 42 fixed/29 VMAT) targets.
Results: Conformality index (CI) decreased rapidly with increasing volume plateauing by 4 cc. No significant (p = 0.33) difference in average volume of targets treated was observed for VMAT (1.1 ±0.87 cc) vs Fixed (1.39 ± 1.19cc). Difference in CI was significant ( p = 0.007) for VMAT ( 1.19 ± 0.25) vs Fixed (1.34 ± 0.25). Effective gradients (GrEff) were calculated using V100%[cc] and V50%[cc] in the HDRing and MDRings. Gradients (%/cm) were steeper (p<<0.001) for Fixed (149.7 +- 19.3) vs VMAT (124.8+- 14.6) plans. Gradients decreased with increasing target volume (TV) converging near 4 cc for Fixed ( 167.6 – 12.9 * TV) and VMAT (127.0 – 1.95 * TV). Low and intermediate dose in the brain > 5 mm from targets were similar. Median values of volume at dose (VxGy[cc]) for VMAT vs Fixed were V12Gy[cc] : 0.2 cc vs 0.1 cc, V5Gy[cc]: 95.6cc vs 22.0cc with plans treating average 4 vs 2 targets.
Conclusion: Development of standardized algorithmic approach to optimization plus script based metrics calculation improved SRS planning process and evaluation.
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