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Motion Mitigation in Active Scanning Proton Therapy for Lung Cancer: A 4D Monte Carlo Study

C Grassberger

C Grassberger1 2*, S Dowdell1, G Sharp1, H Paganetti1, (1) Massachusetts General Hospital, Boston, MA, (2) Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland

SU-F-500-4 Sunday 4:00PM - 6:00PM Room: 500 Ballroom

Purpose: To study the efficacy of various rescanning techniques and gating in the mitigation of motion interplay effects in hypofractioned lung treatments employing actively scanned protons.

Methods: 5 lung cancer patients with varying tumor size (21.1-82.3cc) and motion amplitude (2.9-30.6mm) were simulated employing 4D Monte Carlo simulations and deformable image registration. We investigated 2 different spot sizes (σ~12mm and ~3mm), 3 different rescanning techniques (layered, volumetric, breath-sampled volumetric) and respiratory gating. Changes in the resulting dose distribution in the target were determined using equivalent uniform dose (EUD), dose homogeneity, minimum and mean dose, in the lung through V₅, V₂₀ and mean lung dose (MLD).

Results: For patients with motion amplitudes <20mm, rescanning successfully maintains EUD within the target. For the large spot size 2 paintings are sufficient, while at least 6 are required for the small spot size. Rescanning also reduces the dependence on the initial breathing phase. The average variation of the EUD with initial breathing phase is reduced from 4% without rescanning to 2.5% for 2x- and 0.7% for 10x-rescanning. Breath-sampling the timing of the paintings is effective, leading to EUDs that are, on average, 5.3% higher than for the same number of consecutive rescans. Volumetric rescanning is sensitive to synchronization effects with the breathing cycle, reducing its effectiveness in 4 out of 50 scenarios studied, while no such effects were observed for layered rescanning. Rescanning has no significant effect on normal lung V₂₀, V₅ and MLD, though it reduces maximum lung dose by 2.2Gy(RBE) on average. Gating improves normal lung sparing, reducing V₂₀ and MLD by up to 10% and 7.4Gy(RBE).

Conclusion: Rescanning offers the possibility to mitigate interplay effects in the target up to a motion of 20mm in this patient cohort. Gating performs similar or better, additionally significantly sparing lung tissue.

Funding Support, Disclosures, and Conflict of Interest: This project was supported by the National Cancer Institute Grant No. R01 CA111590

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