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TH‐A‐9A‐10: Prostate SBRT Delivery with Flattening‐Filter‐Free Mode: Benefit and Accuracy
Author(s) -
Li T,
Yuan L,
Sheng Y,
Wu Q
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4889580
Subject(s) - truebeam , image guided radiation therapy , nuclear medicine , context (archaeology) , prostate cancer , medicine , flattening , physics , dosimetry , radiation therapy , beam (structure) , linear particle accelerator , optics , surgery , geology , paleontology , cancer , astronomy
Purpose: Flattening‐filter‐free (FFF) beam mode offered on TrueBeam™ linac enables delivering IMRT at 2400 MU/min dose rate. This study investigates the benefit and delivery accuracy of using high dose rate in the context of prostate SBRT. Methods: 8 prostate SBRT patients were retrospectively studied. In 5 cases treated with 600‐MU/min dose rate, continuous prostate motion data acquired during radiation‐beam‐on was used to analyze motion range. In addition, the initial 1/3 of prostate motion trajectories during each radiation‐beam‐on was separated to simulate motion range if 2400‐MU/min were used. To analyze delivery accuracy in FFF mode, MLC trajectory log files from an additional 3 cases treated at 2400‐MU/min were acquired. These log files record MLC expected and actual positions every 20ms, and therefore can be used to reveal delivery accuracy. Results: (1) Benefit. On average treatment at 600‐MU/min takes 30s per beam; whereas 2400‐MU/min requires only 11s. When shortening delivery time to ˜1/3, the prostate motion range was significantly smaller (p<0.001). Largest motion reduction occurred in Sup‐Inf direction, from [−3.3mm, 2.1mm] to [−1.7mm, 1.7mm], followed by reduction from [−2.1mm, 2.4mm] to [−1.0mm, 2.4mm] in Ant‐Pos direction. No change observed in LR direction [−0.8mm, 0.6mm]. The combined motion amplitude (vector norm) confirms that average motion and ranges are significantly smaller when beam‐on was limited to the 1st 1/3 of actual delivery time. (2) Accuracy. Trajectory log file analysis showed excellent delivery accuracy with at 2400 MU/min. Most leaf deviations during beam‐on were within 0.07mm (99‐percentile). Maximum leaf‐opening deviations during each beam‐on were all under 0.1mm for all leaves. Dose‐rate was maintained at 2400‐MU/min during beam‐on without dipping. Conclusion: Delivery prostate SBRT with 2400 MU/min is both beneficial and accurate. High dose rates significantly reduced both treatment time and intra‐beam prostate motion range. Excellent delivery accuracy was confirmed with very small leaf motion deviation.