A positioning QA procedure for 2D/2D (kV/MV) and 3D/3D (CT/CBCT) image matching for radiotherapy patient setup

Abstract

A daily QA procedure for Varian’s on-board imager (OBI) and portal vision (PV) device was developed. The procedure was to check: (1) the two imaging device’s isocenters (digital graticules) coincides with linac’s isocenter (to a pre-specified accuracy) and (2) the positional difference detected by 2D/2D image matching can be reliably transferred to couch motion. A cube isocenter phantom was used and two sets of 1 mm metal balls (bb) were placed on the phantom to define two separate spatial points (locations.) The first set defined the isocenter at the phantom center and the second set defined a point 1.5cm anterior, 1.5cm inferior, and 1.5cm right lateral from the isocenter. An axial scan of the phantom was acquired (2mm slice thickness and 1mm spacing) from an AcQSim CT. The procedure starts with setting up the phantom at linac’s isocenter (lasers) and taking AP MV and Lat KV images. The accuracy of each device’s isocenter was obtained by checking the distance between the central bb and the digital graticule on the image. If the accuracy is acceptable, the central bbs in the DRR images were manually moved to overlay to the off-axis bbs in the KV/MV images. The shifts obtained were documented and then applied to remotely move the couch. The room lasers can also check the accuracy of the couch shift. This procedure is simple (takes ~10-15 min) and was used in our institution for more than 2 years. For our OBI/PV system, the coincidence of imager’s and linac’s isocenter was better than 1.5 mm and the couch shift’s accuracy was better than 2mm. This procedure can be a supplementary to the general OBI QA program recently reported (1).