Real-world examples of sensitivity failures of the 3%/3mm pass rate metric and published action levels when used in IMRT/VMAT system commissioning

Abstract

In IMRT/VMAT system commissioning (as with any system), quality is improved by striving for tight tolerances of stringent metrics of accuracy. For 5 cases, passing rates for 3%/3mm gamma analysis were generated following the TG119 instructions. Subsequently, more stringent/sensitive criteria combined with advanced volumetric dose analysis were applied, and in each case significant systematic errors were clearly identified despite the high 3%/3mm passing rates. In 4 of 5 cases, the error was easily remedied. These real-world examples of observed "false negatives" (insensitivities) point towards the inappropriateness of the 3%/3mm gamma passing rate metric as the basis for acceptance testing/commissioning of the IMRT/VMAT delivery chain.