Evaluation of volumetric modulated arc therapy (VMAT) patient specific quality assurance

Abstract

Purpose: The purpose of this work was to perform a comprehensive comparison of fixed-beam intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) patient specific quality assurance (QA) results, in order to investigate the appropriateness of applying IMRT QA methods and action levels to VMAT treatment plans. Methods: QA measurements were evaluated for the test geometries provided in AAPM Task Group Report 119. The structure sets were copied onto a cylindrical water-equivalent phantom. Using the Philips Pinnacle³ treatment planning system, fixed-beam IMRT and VMAT treatment plans were constructed. The plans were delivered to the phantom and the resulting dose distributions were measured (1) in the coronal and sagittal planes and at high and low dose points in the cylindrical phantom using radiochromic film and ion chamber, respectively, and (2) using a commercial 2D diode array. Ion chamber and diode array measurements were taken five times each, and film measurements were taken three times. Measured planar doses were analyzed using gamma analysis with criteria of 3%/3 mm. Measured point doses were analyzed using percent difference. Differences between IMRT QA and VMAT QA results were tested for significance using the Wilcoxon rank-sum test. Results: The radiochromic film results showed averages of 98.9%±1.0% and 99.1%±0.9% of measured doses within 3%/3 mm of calculated doses for IMRT and VMAT plans, respectively. Ion chamber results showed average differences between measured and calculated point doses of -0.7%±1.5% and -1.7%±1.8% for IMRT and VMAT plans, respectively. The diode array results showed averages of 98.7%±0.5% and 98.6%±0.8% of measured doses within 3%/3 mm of calculated doses for IMRT and VMAT plans, respectively. The Wilcoxon rank-sum test found p values of p=1.00, p=0.27, and p=0.89 for the film, point dose, and diode array measurements, respectively. Conclusion: Differences between IMRT QA and VMAT QA results were not statistically significant for any of the measurement types. The measured differences between IMRT and VMAT QA results were small and likely do not constitute clinically significant differences. These results suggest that it is appropriate to apply IMRT QA methods and action levels to VMAT treatments.