SU-E-T-43: The Effects of Image Resolution and Noise on the Gamma Dose Distribution Comparison Method for IMRT QA

Abstract

Purpose: To investigate the effects of image noise and resolution on gamma analysis results for clinical IMRT quality assurance (QA) treatment plans. Methods: For each clinical IMRT plan, a hybrid QA plan was created in Pinnacle3 using a solid water phantom and delivered with a Varian Clinac 21EX. A transverse dose plane was measured using Kodak EDR2 radiographic film. Gamma analyses were performed using OmniPro-I'mRT software (IBA Dosimetry, Germany), comparing the calculated dose distribution (reference) versus the measured dose distribution (evaluated). Each IMRT QA film was digitized using three different image resolutions (71, 142, and 285 dpi). In order to study the potential for noise (e.g., from the film, developer, or digitizer) to mask an error, a 4 mm positional shift and various noise levels (normally distributed with 1, 2, and 3% standard deviation) were introduced to the calculated dose distributions, and gamma analyses were performed using these noisy distributions as the measured dose distributions. Results: Our analysis showed that the percentage of passing pixels increased with both increasing image resolution and image noise. Doubling the film image resolution increased the passing rate by an average of 0.1% for ±5%/3mm acceptance criteria, 0.8% for 3%/3mm, and 2.6% for 2%/2mm for the six clinical IMRT QA plans in this study. Increasing the noise from 0% to 3% increased the passing rate of the shifted images by an average of 1.5% for the 5%/3mm acceptance criteria, 4.6% for 3%/3mm, and 8.9% for 2%/2mm. Conclusions: Image noise and high scanning resolution artificially increased the percentage of passing pixels in gamma analysis. This effect was more pronounced for tighter γ criteria (i.e. 2%/2mm). In designing IMRT QA protocols, it is important to be aware of the fact that gamma analysis is sensitive to these two parameters.