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Fluence-Weighted Gating Considerations in MR-Guided Radiotherapy

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T Mazur

T Mazur*, Y Wang , H Li , S Mutic , O Green , Washington University School of Medicine, St. Louis, MO


TU-FG-FS2-12 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 2

Purpose: To investigate alternative schemes for gating treatment in MR-guided radiotherapy that incorporate the overlap between instantaneous fluence and tracked targets on real-time, planar imaging.

Methods: Cine MR images were acquired for 180 fractions spanning 9 patients being treated for abdominal cancers on a 60Co-based, MR-guided radiotherapy system. Sagittal images were acquired at 4 frames per second with 3.5x3.5 mm2 in-plane resolution and 7 mm slice thickness. In-plane GTV boundaries were retrospectively tracked on all cine acquisitions. Planned treatments were then decomposed into individual segments. For each segment, flux per volume was calculated within a frame-of-reference defined by the delivery system. Cine acquisitions were ultimately registered to this frame of reference, thus allowing for calculated per-segment flux within the imaging plane to be determined. The feasibility and potential efficacy of “fluence”-weighted schemes for treatment gating were then investigated.

Results: The mean number of segments per treatment was 120. For each patient and segment, flux thresholds – relative to the maximum calculated flux within the PTV – for covering specific fractions of the in-plane (imaging) PTV were calculated. For example, on average 95% of the in-plane PTV was covered by <1% of the maximum in-PTV flux. For just 3.2% of segments was 95% coverage achieved by greater than 50% of the maximum PTV fluence. Variation in cumulative in-plane PTV fluence with choice of imaging location was then quantified by shifting and rotating the imaging plane. Lastly, gating performance was retrospectively compared between fluence-weighted schemes and the current clinical implementation based solely on relative target motion.

Conclusion: Per segment beam flux can readily be mapped onto cine imaging for implementing a gating protocol that considers instantaneous fluence. To optimally incorporate fluence into gating decision-making, imaging location and orientation should be carefully considered to capture meaningful target motion where instantaneous fluence is significant.

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