(Saturday, 4/4/2020) [Mountain Time (GMT-6)]
Purpose: To determine if using a gamma analysis is a reasonable alternative to traditional TG-40 tests for monthly QA by comparing passing rates for inline and crossline profiles with manufactured errors in flatness, symmetry, and energy.
Methods: Beam profiles with altered flatness were obtained by rotating the gantry angle from 0 to 5 degrees in 1-degree increments. Symmetry was altered by placing additional layers (2 mm to 15 mm) of solid water on half of the scan. Energy was altered by placing a copper plate in the beam over the entire field. Measurements of 10x10 and 20x20 cm fields were collected using the MatriXX ion chamber array at depths of 5 cm and 10 cm, using a 100 cm SSD setup. The experiment was performed with the following energies; 6x, 6FFF, 10x, 10FFF, and 15x. Data was analyzed using Quadrant, an in-house profile QA software.
Results: The flatness, symmetry, and energy results were evaluated using three different sets of gamma criteria (1%/1 mm, 2%/2 mm, 3%/3mm), and using traditional methods. The passing rates for gamma analysis are 88-93%, 95-98%, and 98-100% respectively for profiles with traditional flatness of 2% or less. Symmetry results are 24-79%, 94-98%, and 100% respectively for profiles with traditional symmetry of 3% or less. The results are 62-100%, 100%, and 100% respectively for energy profiles with central axis constancy of 2% or less.
Conclusion: The data from this study indicates using a 2%/2mm gamma criteria with a 5% failure threshold could serve as an effective replacement for traditional profile QA tests for determining flatness and symmetry. Energy constancy proved inconclusive.
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