SU‐E‐T‐203: Patient Specific IMRT QA Analysis Comparison of Four Commercially Available Systems

Abstract

Purpose: IMRT patient specific QA is often used across institutions to provide a quantitative measure of the accuracy by which a treatment plan is delivered. The aim of this project is to evaluate the performance of four commercially available devices while varying the acceptance criteria of dose difference and distance to agreement (DTA). Methods: Ten VMAT and 10 static field IMRT plans were evaluated. All the plans were optimized using the Pinnacle v9.2. Measurements were performed on a Varian 2100CD linac equipped with millennium 80MLC. The gamma index was chosen as the primary metric for the evaluation of the agreement between the planned and measured dose distributions. The gamma index was calculated for each plan using 10% and 20% dose thresholds, 3%/3mm, 2%/2mm. The analysis was performed for the composite plans as well as field‐by‐field using the software provided with each IMRT QA systemResults: Octavius II showed higher passing gammas for 3%3mm and 2%2mm compared to the other detectors in all the IMRT field‐by‐field comparison. In the IMRT true composite plans, the gamma index values (3%/3mm) were: 89.5, 87.3, 89.2, 88.8 for Delta4, IBA Matrixx, Octavius4D and PTW Octavius II, respectively. For the same IMRT plans with the beams delivered with the gantry in the upright position, the gamma indices were 89.5, 98.4, 94.0, 100. For the VMAT plans, the gamma index values were 93.7, 95.0, 91.2, and 97.5. The gamma index values from the evaluation of the VMAT plan using a dose threshold of 20% were 91.8, 95.1, 88.6, and 97.3, respectively. Conclusion: The detectors response is very similar for the three treatment techniques examined. Stricter gamma index criteria reduced the passing rate of the plans by about 20%. The differences between the detectors did not show any trend depending on method, site, or mode of delivery.