Program Information
Effect of Small Field Output Factor Measurement Uncertainty On Patient Specific IMRT QA as a Function of Plan Complexity
E Laugeman*, J Evans , D Fiedler , S Kim , Virginia Commonwealth University Medical Center, Richmond, VA
Presentations
MO-B-BRD-8 (Monday, March 9, 2015) 10:00 AM - 12:00 PM Room: Ballroom D
Purpose:
To evaluate the effect of small field output factor measurement errors on IMRT plan dosimetry and to determine if patient specific IMRT QA is able to catch these errors.
Methods:
Output factors (OF) were measured for our 6MV Varian Trilogy LINAC with a 0.6cc Farmer ion chamber oriented perpendicular to the beam to mimic a worst case OF measurement scenario. Due to volume averaging and non-equilibrium conditions, this curve differs from our clinical OF curve measured with a CC01 ion chamber for field sizes below 4x4cm². Both OF curves were utilized in Pinnacle Treatment Planning System to re-calculate patient-specific IMRT QA planar phantom dose (IBA MatriXX MultiCube) for a total of 15 clinical IMRT patient plans in four sites: brain, spinal cord, head and neck, and lung. The absolute difference between the two dose maps were calculated using OmniPro I’mRT software. Additionally, each plan was re-optimized with the goal of increasing plan complexity to increase the role of small field OF errors (i.e. increased number of segments and decreased segment size) and the OF phantom dose comparison repeated.
Results:
The two OF curves differed for the 1, 2, 3cm square field sizes, by 50%, 15%, 1.5%, respectively. Differences in plan dose were much smaller for both the initial clinical patient plans and the increased complexity plans; more than 98% of the pixels had a dose difference of less than 1%. The pixels with differences >1% appeared mainly on the high-gradient dose regions.
Conclusion:
The error in calculated dose for a Varian Trilogy LINAC calculated with Pinnacle TPS when using incorrectly measured small field OF values is very small. Due to the poor spatial resolution of the detector and the use of 3%/3mm gamma passing criteria, this error is not significant enough to be detected with routine patient-specific IMRT QA.
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