Commissioning experience with cone-beam computed tomography for image-guided radiation therapy

This paper reports on the commissioning of an Elekta cone-beam computed tomography ( CT) system at one of the first U. S. sites to install a regular, off-the-shelf Elekta Synergy ( Elekta, Stockholm, Sweden) accelerator system. We present the quality assurance ( QA) procedure as a guide for other users. The commissioning had six elements: ( 1) system safety, ( 2) geometric accuracy ( agreement of megavoltage and kilovoltage beam isocenters), ( 3) image quality, ( 4) registration and correction accuracy, ( 5) dose to patient and dosimetric stability, and ( 6) QA procedures. The system passed the safety tests, and 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 1 mm and 1 degree in each dimension. Of 12 measurement points spanning a 9 x 9 x 15-cm volume in a Rando phantom ( The Phantom Laboratory, Salem, NY), the average agreement in the x, y, and z coordinates was 0.10 mm, -0.12 mm, and 0.22 mm [ standard deviations ( SDs): 0.21 mm, 0.55 mm, 0.21 mm; largest deviations: 0.6 mm, 1.0 mm, 0.5 mm] respectively. The larger deviation for the y component can be partly attributed to the CT slice thickness of 1 mm in 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 6.3 cGy ( 120 kVp, 40 mA, 40 ms per frame, 360-degree scan, S20 field of view); half value layer was 7.1 mm aluminum ( 120 kV, 40 mA). We suggest performing items 1, 2, and 3 monthly, and 4 and 5 annually. In addition, we devised a daily QA procedure to verify agreement of the megavoltage and kilovoltage isocenters using a simple phantom containing three small steel balls. The frequency of all checks will be reevaluated based on data collected during about 1 year.