Program Information
An Enhanced Gamma Index for Patient Specific VMAT QA
R Patel*, A Sethi , Loyola Univ Medical Center, Maywood, IL
Presentations
TH-AB-FS1-3 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: Four Seasons 1
Purpose: The limitations of conventional gamma-index for patient specific pre-treatment IMRT QA have been well documented. We evaluate the sensitivity of gamma-index to planning and delivery errors for multi-met SRS treatment and propose a simple method to improve its effectiveness.
Methods: VMAT based SRS plans were created for a specially designed multi-met phantom with 5 targets ranging in volume from 2.1cc to 8.1cc. VMAT plans with four 6MV arcs were created for two cases: single-met and multi-met targets. The isocenter for multi-met plan was located at the center of mass of all targets. Linear and angular shifts in isocenter(up to 3mm and 2deg) were simulated for treatment delivery. Three gamma indices (3%/3mm, 2%/2mm and 1%/1mm) at 95% pass criteria were used to evaluate plan versus delivered dose agreement. An enhanced version of gamma-index that employs target dose conformality and spatial dose comparison between planned and delivered dose was designed and tested.
Results: For the single-met SRS, a small positional error of 1mm produced >10% reduction in minimum target dose. However, delivery errors as large as 3mm would not be detectable with the 2%/2mm gamma criteria. For the case of multi-met SRS, delivery errors of up to 2 mm were not captured by 2%/2mm gamma. However angular shifts (> 1 degree) showed greater sensitivity to our gamma analysis. When gamma index was coupled with spatial dose information, its detection efficiency was significantly improved. A 3mm isocenter shift resulted in a 30% reduction in minimum target dose and a 10% reduction in target volume coverage.
Conclusion: Routinely accepted gamma pass criteria of 3%/3mm in evaluating VMAT based SRS QA is inadequate for detecting common treatment delivery errors. An enhanced version of gamma test that is coupled with spatial dose information was evaluated and found clinically useful.
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