Commissioning experience with cone beam CT for image guided radiation therapy

Abstract

This paper reports on the commissioning of the Elekta Cone Beam CT system which was performed as one of the first sites in the US to install a ?regular?, off-the-shelf Elekta Synergy accelerator system. A QA procedure is presented as a guide for other users. The commissioning was sixfold: (1) System safety (2) Geometric accuracy ? Agreement of MV and kV beam isocenters, (3) Image quality (4) Algorithm accuracy (5) Dose to patient (6) Quality assurance procedures. The system passed the safety tests and the agreement of the isocenters was found to be within 1 mm. Using a precisely moved skull phantom, the reconstruction and alignment algorithm was found to be accurate within 1mm and 1 degree in each dimension. Of 12 measurement points spanning a 9x9x15cm3 volume in a RANDO phantom the average agreement in the x, y, and z coordinates was 0.10mm, -0.12mm, 0.22mm (SD: 0.21mm, 0.55mm, 0.21mm; Largest deviations: 0.6mm, 1.0mm, 0.5mm, respectively). The larger deviation for the y-component can be partially attributed to the CT slice thickness of 1 mm along that direction. Dose to the patient depends on the machine settings and patient geometry. To monitor dose consistency air kerma (output) and half value layer (beam quality) are measured for a typical clinical setting. Air kerma was 6.3cGy (120kVp, 40mA, 40ms per frame, 360° scan, S20 field of view), half value layer 7.1 mm Al (120 kV, 40 mA).
We suggest to perform items (1), (2) and (3) on a monthly basis and (4) and (5) annually. In addition we devised a daily QA to verify the agreement of MV and kV isocenter using a simple phantom with three small steel balls.