Evaluation of linear accelerator performance standards using an outcome oriented approach.

Radiation therapy, along with other branches of medicine, is moving towards a firmer basis in evidence to optimally utilize resources. As new treatment technology and strategies place greater demands on quality assurance resources, the need to objectively evaluate equipment and process performance standards from the perspective of predicted clinical impact becomes more urgent. This study evaluates the appropriateness of recommended quality control tolerance and action levels for linear accelerators based on the calculated dosimetric impact of suboptimal equipment performance. A method is described to quantify the dosimetric changes, as reflected by the changes in the outcome surrogate, equivalent uniform dose (EUD), of machine performance deviations from the optimal, specifically in the range of tolerance and action levels promulgated by the Canadian Association of Provincial Cancer Agencies (CAPCA). Linear accelerator performance deviations were simulated for the treatment of prostate, breast, lung, and brain using 3D conformal techniques, and the impact evaluated in terms of the changes in the EUD of the target volumes and two principal organs at risk (OARs) per site. The eight key performance characteristics examined are: Output constancy, beam flatness, gantry angle, collimator angle, field size indicator, laser alignment (three directions) and, by inference, the optical distance indicator. Currently accepted CAPCA tolerance levels for these eight performance characteristics are shown to maintain average EUD deviations to within 2% for the targets and 2 Gy for the OARs. However, within the 2% or 2 Gy range, the recommended tolerance levels are found to have markedly different effects on the EUDs of the structures of interest.