EPID-Based Patient-Specific Quality Assurance in VMAT Radiation Therapy Using Split Arcs

Electronic Portal Imaging Device (EPID) with Portal Dosimetry software (Varian Medical Systems) is a widely used and efficient strategy for performing quality control tests for radiation therapy treatments using Volumetric Modulated Arc Therapy (VMAT). However, a significant limitation is that we have a 3D dose representation projected in a 2D plane along the arcs, which brings many uncertainties to the analysis and is not recommended in many modern studies. This work aims to propose a quality control strategy for treatments with VMAT technique, using division of treatment arcs for quality control measures, in two, four and eight segments, as well as to analyze the best division of arcs in two clinical cases: prostate and head and neck cancers. We analyzed the results using the Portal Dosimetry regarding the required number of arc divisions and how it affects the measurements specificity and sensitivity based on gamma index analysis in Portal Dosimetry software, by inserting controlled errors into the Multileaf Collimator (MLC) positions. We conclude that the division into at least four segments and the careful analysis against the historical baseline results of the service can help to identify cases where some corrective action is necessary or to carry out even more segmented analysis, such as the division into eight parts. We also emphasize that this strategy becomes especially useful due to its practicality in clinical routine, as it is a practical and widely available tool.