Room: Exhibit Hall | Forum 8
Purpose: A 2D/3D registration technique by acquiring a pair of orthogonal images are used for initial patient setup or verification after re-positioning with a kV-CBCT procedure. The two orthogonal images can be acquired with kV-kV or kV-MV projected images. The two orthogonal kV-MV images can be acquired without need of additional gantry rotation. The purpose of this study is to compare the image dose to patient between using kV-kV and kV-MV registration techniques.
Methods: The comparisons were done using for typical acquisition protocols with Varian OBI system on a TrueBeam Linac. The MV images were acquired with a 2.5 MV beam and kV images were acquired by using clinical default protocols that are site specific with default kVp and mAs settings. These default image protocols were set by the manufacturer with optimized image parameters. The Monte Carlo simulations were used in the imaging dose calculations. Image acquisitions evaluated in this study include typical head, chest and pelvis sites.
Results: Imaging doses(D50 of the DVHs) to the eyes, heart and bladder from representative orthogonal pairs of kV-MV (kV-kV) images are 1.0 cGy (0.1 cGy), 0.5 cGy (0.07) and 0.5 cGy (0.06 cGy) for head, chest and pelvis image acquisitions respectively.
Conclusion: A 2D/3D registration with a kV-MV pair images results approximately 10 times doses than with a kV-kV pair images. However, a kV-MV pair images still results much lower dose than a pair of conventional MV-MV portal images. The main benefit of kV-MV pair images is its fast acquisition of the pair images because no gantry rotation is needed. However, the doses could be significantly reduced by using kV-kV image, which becomes very important in pediatric case. In all cases the imaging doses are well within the dose threshold of 5% target dose as recommended by the AAPM TG-180 report.