Evaluation of Argus IMRT for routine IMRT QA

Full text: A new method for performing IMRT QA using DynaLog files has been evaluated. A new software product has been developed to provide automated and time saving QA for IMRT. The use of DynaLog files provides for immediate analysis of MLC leaf motions and positions and linac dose rates when compared to imported fluence maps from supported inverse treatment planning systems. With this analysis the need for film has been eliminated and the time spent performing IMRT QA has been greatly shortened. Dynamic log files (DynaLog) are recorded data of the actual dose fraction and MLC leaf positions versus the expected, taken at 50 ms interval snapshots throughout a dynamic treatment. Argus takes this stored information and compares actual and expected leaf motions, performs statistical analysis, simulates leaf motions, and performs a graphical analysis of individual leaves or gaps. These analyses show which individual leaves failed, where in their travel they failed, and can aid in predicting the failure of the individual MLC motors. Individual leaf analysis compares actual to expected leaf position, actual to expected leaf velocity, and actual to expected gap size to within a tolerance specified by the user. DMLC Plan Comparison and DMLC Field Comparison are two features which analyse fluence maps. DMLC Plan comparison allows the entire treatment plan to be delivered and analysed, comparing the derived data from the treatment plan to the data from the DynaLogs. Included in the analysis is fluence difference, gamma evaluation, leaf analysis, and gap analysis. Fluence difference is a pixel by pixel subtraction between the fluence calculated based on the actual leaf positions and fluence calculated based on the expected leaf positions. The gamma evaluation follows the algorithm of D. Low, et. al., where they describe regions where dose distributions disagree with measurement. However, Argus uses this for fluence analysis rather than dose, still utilizing the 3% and 3 mm tolerances. DMLC Field comparison is useful when analysis of a single field is needed rather than the entire plan. The DMLC Field Verification test is also designed to be a regular MLC test procedure in that it has the ability to graph the results in order to take an in depth look at trends and distributions of motor warnings and failures, fluence differences, gamma differences, and histogram analysis. The treatment planning system can use the DICOM RT transfer to export fluence maps for import into Argus for Fluence Map Cross-Comparison. Fluence Map Cross-Comparison allows close comparison of fluence maps from the treatment planning system to those produced by the DynaLog files. Up to four of the treatment plan optimal fluence and actual fluence, test DynaLog expected fluence and derived fluence, and reference DynaLog expected fluence and derived fluence can all be compared to one another with an analysis of the fluence difference and gamma evaluation. Argus provides a unique feature which allows the conversion of the DynaLogs to MLC Shaper files that can be imported into supported treatment planning systems. Once the Shaper MLCs have been imported into a verification plan, they can be converted to actual fluences and the plan can be recalculated using the new information on actual leaf positions and motions. This verification plan can then be compared to the treatment plan via plan subtraction to obtain the dose difference. Argus IMRT has provided a streamlined approach to IMRT QA by providing the tools necessary to test, analyse, and report in one thorough and detailed package. By eliminating the need for film analysis, time has been reduced significantly. With IMRT as a mainstream form of treatment, it is important that physicists are not bogged down with excessive testing and paperwork. (author)