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The Impact of Cine Magnetic Resonance Image Gating in Breath Hold Pancreas Cancer Radiotherapy

Y Yang*, L Portelance, N Dogan, A Pollack, University of Miami, Miami, FL


(Sunday, 7/14/2019) 5:00 PM - 6:00 PM

Room: Stars at Night Ballroom 2-3

Purpose: Cine MRI enables real-time monitoring of tumor motion. However, cine MRI gating causes beam on/off latency which could impact the dose delivered to the tumor and critical structures. This study is to investigate the Impact of cine MRI gating in pancreas cancer patients treated with breath hold.

Methods: This retrospective study was performed based on sagittal cine MRI recorded with 4Hz frame rate through entire treatment from 17 patients. Patients were treated using MRI-guided stereotactic radiotherapy to a dose of 35-50Gy in 5 fractions. A GTV tracking boundary of 3-5 mm was used. The radiation beam turned off when>5% of the tracking target moved out of the tracking boundary. Dose overshoot occurs when beam remains on when >5% of the tracking target is outside of the tracking boundary. Cine MRI was analyzed in Matlab with an in-house developed program. The target-out percentage was extracted from each frame. Treatment time, beam duty cycle, total beam-on events, beam-on events of <4s duration, and particularly, dose overshoot, were calculated and averaged over all patients.

Results: The mean treatment time, beam duty cycle, total beam-on events, and percentage of beam-on events <4s are 38.4±13.2min, 62.2±12.8%, 195.7±120.4, and 59.8±16.6%, respectively, per fraction. The beam-off latency caused by image acquisition was about 0.375s and occurred in every beam-on event, resulting in 5.1±3.0% dose overshoot. This considerable dose overshoot was mainly caused by the surprisingly large number of beam-on events.

Conclusion: The dose overshoot caused by beam-off latency in cine MRI gating can decrease the dose delivered to the PTV, and increase the dose delivered to the critical structures surrounding the tumor. It can be mitigated by implementing patient-initiated beam-on control to reduce the number of unnecessary beam-on events and/or faster MRI acquisition to reduce the latency.


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