SU‐E‐J‐119: Geometric Accuracy Verification for Gated RapidArc Using Gated CBCT Reconstructed From Continuously Triggered KV Images

Abstract

Purpose: To develop a method using continuously triggered kV images acquired during gated RapidArc to reconstruct a gated CBCT for geometric accuracy verification of a moving target. Methods: A gated RapidArc treatment was planned to be delivered to a commercial breathing simulation phantom embedded with metallic markers using a Varian TrueBeam™ linear accelerator. The phantom was setup to the treatment position using 2D/2D match followed by marker match IGRT procedure. Continuously triggered on‐board‐imager kV images were taken immediately before each MV beam on cycle at various gantry angles during the gated RapidArc delivery. The kV images obtained were imported into an in‐house developed software to reconstruct a gated CBCT corresponding to the beam on triggering phase. The gated CBCT was then imported into a commercial treatment planning system. The geometric displacement of the target in the gated CBCT from its counterpart in the planning CT was determined through image registration based on marker alignment. The entire process was repeated 10 times using a variety of target movement magnitudes in the range of 1cm to 3cm and periods in the range of 4s to 8s. Results: The magnitude of the translational shifts of the target at the beam on triggering phase was 0.18+/−0.11mm in left/right direction, 0.38+/−0.15mm in anterior/posterior direction and 0.56+/−0.19mm in the superior/inferior direction. The composite 3D translational shift was 0.74+/−0.12mm. Conclusion: Gated CBCT reconstruction could be a practical and efficient way of utilizing continuously triggered kV images acquired during gated RapidArc treatments to verify geometric accuracy of a moving target.