SU‐E‐T‐172: Portal Dosimetry of Gated VMAT; with and Without Gating

Purpose: To study duty cycle and respiratory period influence on VMAT delivery using portal dosimetry. Methods: For this study, we selected 12 ARC fields from four different gated VMAT plans with Varian, TrueBeam 6 MV photon beam and HD120 MLC. For Planning and portal dose prediction we used Varian, Eclipse (Version 10.0) AAA and PDIP algorithms respectively. We acquired integrated portal images with and without gated arc fields delivered to EPID at SID 140cm. We simulated respiratory periods with Quasar, Motion Phantom. For gated delivery, we used planned duty cycle for patient respiratory trace, and 6 sec per breath (SPB) and duty cycles 40%, 30%, 20% and 10% with 3SPB. Eclipse, Portal dosimetry software was used for Gamma analyses of measured and predicted portal dose images. In addition, we compared with and without gated portal dose images of same arc. MLC log file analyses of delivered arcs were done with Mobius, Dose lab software. Gamma passing criteria is to have gamma<1 for greater than 90% data points. Results: For all arcs, predicted vs. measured dose distributions passed with criteria ΔD = 3% & DTA = 3mm, excluding few 10% duty cycle measurements. For these, pass criteria is ΔD = 5% & DTA = 4mm. Similarly, with and without gated portal dose image comparison passed the criteria of ΔD = 1% & DTA = 1mm excluding few 10% duty cycle measurements. For these pass criteria is ΔD = 2% & DTA = 2mm. For all delivered arcs, Leaf error 95 percentile is < 0. 1mm and error RMS is below 0.05mmConclusion: Except 10 % duty cycle, the gating influence on VMAT delivery is minimal. Therefore, gated VMAT QA can be done without gating. Duty cycles larger than 20% are recommended to minimize delivery errors.