A QA programme for a radiographic image-guidance system

As image guided systems become routinely available, we propose a comprehensive acceptance protocol and quality assurance (QA) programme. Since image guidance is likely to transform the practice and process of radiation medicine, image guided systems can be thought of as a QA tool for the delivery of radiation medicine. The image guided system under study consists of a radiographic imaging system combined with a linear accelerator (Synergy, Elekta Oncology Systems, Crawley, UK). The imaging system consists of a retractable conventional X ray tube and a 41x41 cm2 amorphous silicon flat panel detector. This assembly is mounted at 90 deg. from the treatment beam central axis and shares the accelerator isocentre. The radiographic system can acquire radiographic images, perform fluoroscopic imaging, or reconstruct cone-beam computed tomographic (CBCT) images. During installation, users verify the electrical and mechanical safety, geometric reproducibility and calibration, radiographic, fluoroscopic, and megavoltage image quality, the reconstructed CBCT image quality, database and data transfer safety and integrity, and the accuracy of the intended clinical process. To maintain the image guided system performance, we have designed a QA programme with specified test frequencies and tolerances. As several aspects of acceptance and QA testing are extensively covered in the literature, we focus on those tests germane to CBCT, including geometric calibration, image quality, and accuracy of clinical processes. Since the kilovoltage beam does not coincide with the treatment beam, the kilovoltage system geometry must be calibrated such that the three dimensional imaging matrix represents accurately to the treatment beam geometry. Geometric calibration involves measurement of the voxel size and scale, the alignment of the kilovoltage and megavoltage beam axes, and compensation for component flex as the accelerator gantry rotates around its axis. This is achieved by taking radiographs of a ball bearing placed at the isocentre through a complete gantry rotation. The travel of the ball bearing with respect to the image matrix [longitudinal (v) and medial (u) axes] is plotted as a function of gantry angle defining a 'flexmap'. The stability and reproducibility of these calibration flexmaps has been assessed on one unit over nine months. The relative flex motions are within 1.5 mm, and are reproducible, within 0.5 mm. The second part of the QA programme focuses on the stability and reproducibility of the technical components of the system. First, the programme verifies that the X ray generator settings (kVp, HVL, mA, ms) are accurate and linear. Second, the properties of the flat panels are refreshed such that variation in individual dark pixel performance, pixel gains and defects are accounted for to ensure optimal image quality. Tracking these parameters may indicate when a panel is nearing the end of its useful life. Image quality is assessed using phantoms commonly encountered in diagnostic CT imaging (CatPhan, the Phantom Laboratories, Salem, NY) to compare image artefacts, noise, and high-contrast and low-contrast resolutions with baseline values. A single imaging session can assess simultaneously image quality, pixel size, and scale. Radiographic image guidance has the potential to profoundly affect radiotherapy practice and processes by reducing or eliminating geometric variations from radiation medicine. For example, our radiographic guidance system has been used to assess the accuracy of target position for our hypofractionated, stereotactic radiotherapy lung program. For our first ten patients, displacements of 4.7±5.3 mm, 7.3±9.4 mm, and 5.6±4.8 mm were required to move the target to the machine isocentre, in the medio-lateral, anterior-posterior, and cranio-caudal directions, respectively. After setup correction, the accuracy of image guidance reduced to 2.0±1.8 mm, 2.7±2.2 mm, and 2.1±2.3 mm in the corresponding directions. A quality assurance programme for radiographic image guidance periodically asses ses the stability and accuracy of its components. In turn, a reliable image-guidance system can be a powerful tool to assure the quality of external beam radiation medicine