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Gating Performance Considerations in MR-Guided Radiation Therapy

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e omari

e omari, T Mazur , H Gach, O Green, Washington University in St. Louis, St. Louis, MO


MO-L-GePD-J(A)-2 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: Magnetic resonance guided radiation therapy (MRgRT) is being utilized for gating treatments based on real-time target tracking. In this study, we investigate both the dependence of delivery efficiency and tissue sparing on gating parameters used clinically and the impact of imaging uncertainties on gating performance.

Methods: Gating on the MRIdian system (ViewRay, Cleveland, OH) is currently performed by tracking a delineated target on planar, cine MR images. Tracking data for 9 patients treated for stomach cancer over 20 fractions were analyzed (totaling 180 data sets). We sampled the two parameters that currently determine treatment gating, including: 1) the margin around the GTV (between 0 and 3 cm) and the 2) fractional contour area allowed beyond this margin (FCABW) (between 0% and 50%). Treatment efficiency was calculated as the ratio between the time the radiation beam is on to the total delivery time. Normal tissue irradiation was likewise quantified based on target motion.

Results: Treatment time efficiency (η) varied from 2% to 100%. For all contour margins and FCABW >20%, treatment time efficiency was > 95%. For margins beyond 15 mm η was >98% regardless of FCABW. η was measured to be < 84% for the current clinical practice of 3 mm and 5% FCABW. Efficiency was largely insensitive to small changes about these parameters. For instance, η for 4 mm/5% was > 90% and for 5 mm/4% was >92%.

Conclusion: Gating in MRgRT must balance delivery efficiency and tissue sparing. For the 180 fractions analyzed, η can be improved by over 6% by increasing the gating margin by just 1 mm.

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