SU-F-P-62: The Sensitivity of Routine IMRT QA Metrics to Monitor Unit Errors




To assess the sensitivity of routine IMRT QA metrics in relative or absolute dose analysis modes to monitor unit (MU) errors.


Monitor units for two IMRT plans were altered by scaling them uniformly as well as by introducing random errors. The uniformly scaled plans were created by increasing or decreasing the total MUs by 5, 10, or 20% and the random ones were created by modifying two fields’ MUs to varying degrees. The composite plans were delivered to a MatriXX ion chamber array and evaluated using OmniPro I’mRT software. The analysis was done both in relative and absolute dose modes. Isodose overlay, Gamma Index, percent difference, and X and Y profiles were evaluated. The Gamma Index pass criterion was 3%/3mm for 95% of the points and that of percent difference was ±3% for 90 or 95% of the points depending on plan type.


Analysis of the QA of the plans without and with induced errors indicates using one single metric, like Gamma, may not point to MU errors and often more than one analysis tool is needed to detect errors. In addition, analyzing the QA results in relative mode may not detect errors as well. Overall, the random MU errors were detected by analyzing X and Y profiles and the uniformly scaled ones were detected by a combination of metrics.


Using a single IMRT QA metric such as Gamma analysis to analyze IMRT QA may not detect errors such as MU discrepancies due to data entry or transfer in cross vender treatment planning and record and verify system environments. Hence, a combination of analysis matrices should be used, preferably in absolute dose mode.