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Mitigation of Interplay Effects Using a Novel Robust, Delivery-Efficient and Continuous Spot Scanning Proton Arc Therapy for Lung Cancer

X Li

X Li*, P Kabolizadeh , J Zhou , A Qin , D Yan , T Guerrero , I Grills , C Stevens , X Ding , Beaumont Health System, Royal Oak, MI


SU-K-108-11 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 108

Purpose: Spot scanning proton arc therapy (SPArc) is a novel planning and treatment technique with potential to deliver proton therapy via continuous arc trajectories efficiently. We hypothesize that SPArc can result in diminishing the negative dosimetric impacts of respiratory tumor motion in lung cancer treatment compared to traditional intensity modulated proton therapy (IMPT) with limited number of beam angles.

Methods: Six lung patients with average internal target volume (ITV) of 331cc (range 77-735 cc) and average tumor motion of 0.82 cm (range: 0.53 - 1.35 cm) were planned. Both SPArc and IMPT plans were developed on the average phase scan of 4D CT scan using robust optimization with same uncertainty parameters to achieve an optimal coverage to 99% of the ITV with 66 Gy (RBE) in 33 fractions. To evaluate the interplay effect, ten scenarios of single-fraction 4D dynamic doses via different starting breathing phases were calculated using spot switching time of 0.2 ms and energy layer switching time of 0.9s without re-paintings. The doses encompassing 95% (D95%) and 1% (D1%) of ITV were compared for both plans.

Results: SPArc significantly improved the target coverage D95% (p < 0.01) and reduced target volume hotspots D1% (p = 0.01). More specifically, for all patients planned in SPArc group, the median D95% reached at least 95% of prescription and the median D1% were less than 115% of prescription. In contrast, only 50% of the patients in the IMPT group maintained at least 95% of prescription coverage or hotspots less than 115% of prescribed dose.

Conclusion: SPArc maintained 95% ITV of prescription coverage for 100% of the evaluated plans, even using a single fraction delivery simulation, compared to multi-field IMPT which achieved this only 50% of the time. SPArc has the potential to be implemented into clinical practice to treat lung cancer.

Funding Support, Disclosures, and Conflict of Interest: A patent based on this work has been filed.

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