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Margin Reduction Via 4D-Robust Optimization for VMAT Lung Radiotherapy

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K Yang

K Yang*, H Chen , L De Souza Lawrence , J McGlade , K Andreou , J Noonan , F Mourtada , Christiana Care Hospital, Newark, DE


SU-I-GPD-T-244 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate if zero-planning margin is adequate for lung treatment with 4D-robust optimization in VMAT radiotherapy.

Methods: The CIRS dynamic thorax phantom and a representative lung patient 4D-CT images (3-mm slice thickness) are imported into RayStation V5.0. Hybrid deformable registration was used to propagate physician-drawn CTV and OAR contours from end-exhalation phase to the remaining phases and average CT. Partial arc VMAT plans were generated and normalized to deliver 60 Gy in 30 fractions to 95% CTV on average CT. The beam geometry and optimization parameters were identical except 4D-robust planning (Plan A) utilizes CTV on all 10 phase images while ITV based planning optimizes on either PTV (Plan B with 5mm ITV to PTV margin) or ITV (Plan C) on average image. All plans were recalculated on all 10 phases and on average CT with ±5mm position uncertainty in S/I, A/P and L/R direction.

Results: Plan A spared dose to the ipsilateral lung, esophagus, cord and heart more effectively when there was large breathing motion, or when larger ITV to PTV margin was used. Compared to Plan B/C, Plan A further reduced V20,Lung, V10,Lung and DMean,Lung by 3-5%, 3-10% and 1.4-2Gy respectively. Both planning strategies are robust against breathing motion up to 3cm, and the differences in D95,CTV, D90,CTV and DMean,CTV are negligible (0.3±0.2Gy, 0.3±0.2Gy and 0.2±0.1Gy respectively). Given a 5-mm post positioning uncertainty, margin is necessary for ITV based planning as the D90,CTV and DMean,CTV decreased by 0.2±1.2Gy and 1.8±0.3Gy respectively in Plan C. While more variation is also observed in Plan A as excursion increases, D90,CTV and DMean,CTV stays within 0.3±0.7Gy and 0.1±0.7Gy.

Conclusion: 4D-robust optimization holds the potential to safely reduce planning margin to zero for lung patient with moderate to large breathing motion and 5mm post positioning uncertainty based on our preliminary study.

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