SU‐E‐T‐415: Investigation of VMAT Patient Specific Quality Assurance Action Levels

Purpose: To examine the appropriateness of patient specific IMRT quality assurance (QA) action levels for use in VMAT QA. 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 Pinnacle treatment planning system, IMRT and VMAT treatments were planned. The plans were delivered and the resulting dose distributions were measured (1) in 2 planes and at 3–4 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%/3mm. Measured point doses were analyzed using percent difference. Differences between IMRT QA and VMAT QA results were tested for significance using a Student's t‐test. Results: The radiochromic film results showed averages of 98.9%±1.0% and 99.1%±0.9% (p=0.47) of measured doses within 3%/3mm of calculated doses for IMRT and VMAT plans, respectively. Ion chamber results showed average differences between measured and calculated point doses of − 0.95%±1.5% and −1.8%±1.8% (p<0.01) for IMRT and VMAT plans, respectively. The diode array results showed averages of 98.7%±0.5% and 98.6%±0.7% (p=0.70) of measured doses within 3%/3mm of calculated doses for IMRT and VMAT plans, respectively. Conclusion: Differences between IMRT QA and VMAT QA results were not statistically significant for planar doses measured with radiochromic film and 2D diode array. The average difference between measured and calculated point doses was smaller in IMRT plans (p<0.01); however, the average point dose differences for both IMRT and VMAT were less than 2%. These results suggest that current action levels used in our clinic for evaluating IMRT QA may be applicable to VMAT QA. This work is supported in part by a sponsored research agreement with Elekta, Inc.