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Adaptive SBRT Planning for Interfraction Motion

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V Wu

V Wu*, M Epelman , K Brock , M Feng , R Ten Haken , M Matuszak , University of Michigan, Ann Arbor, MI


WE-AB-209-3 (Wednesday, August 3, 2016) 7:30 AM - 9:30 AM Room: 209

Purpose: Interfraction changes in abdominal SBRT patient anatomy are challenging to address with image guidance alone because avoidance of an overlapping high-priority gastrointestinal normal structure may come at the expense of tumor coverage. This work investigates the benefit of anticipating interfraction motion and exploiting favorable geometries at the time of treatment through dose adaptive planning.

Methods: Beyond no change, six scenarios of duodenum shifts per fraction are generated (±1 cm XYZ) and sample five-fraction paths (combinations of no and/or the indicated changes) are simulated. Under the baseline (nominal) plan, generated assuming no uncertainty, three scenarios result in overdosing the duodenum while the others result in safe or reduced dose. The nominal plan under-doses the PTV (36.1 Gy-EQD2) due to duodenum proximity. Three adaptive strategies are considered: i) Standard IGRT: for duodenum overdose, patient’s isocenter is shifted to satisfy the duodenum dose tolerance; nominal plan is delivered otherwise; ii) Scaling: nominal plan scaled up/down in favorable/unfavorable patient geometries; iii) Mixed: shifting as in i) but boosting otherwise. If max duodenum dose (limit 6 Gy/Fx) is exceeded, dose distributions are shifted or scaled down, depending on the strategy considered. In all adaptive fractions, critical structure dose limits are satisfied, even if the PTVs are under-dosed.

Results: Generally, scaling dose allows better PTV dose (gEUD in Gy-EQD2); however, in some scenarios, shifting is better than scaling down dose. In a path with 2 fractions of the duodenum not moving and 3 fractions of an unfavorable duodenum position, scaling dose results in the best coverage, with 36.8 Gy-EQD2. In another path with 2 favorable and 3 unfavorable duodenum positions, the mixed strategy achieves the highest coverage with 41.0 Gy-EQD2.

Conclusions: Anticipating interfraction motion and exploiting favorable geometries through dose adaptive scaling can improve overall tumor coverage by compensating for conservative IGRT in unfavorable fractions.

Funding Support, Disclosures, and Conflict of Interest: NIH-P01-CA059827

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