Non-Uniform Irradiation of the Asi-1200 Electronic Portal Imaging Detector Due to Portal Dosimetry Quality Assurance of SRS/SBRT Plans
Abstract
Purpose
Increased use of portal imaging for quality assurance of SRS/SBRT plans results in a higher workload for the center detector elements compared to the peripheral elements. This study aims to evaluate the long-term stability of the EPID under frequent SRS/SBRT QA use and to assess any spatially dependent degradation of detector performance.
Methods
All measurements were conducted with a Varian Truebeam Edge and aSi-1200 EPID. Daily 6 MV beam uniformity was measured with MPC over a 1-year period. A 6 MV 10x10 cm2 open field image was delivered daily. Five regions of interest were defined, one in the center and the remaining 4 cm away from the center in each cardinal direction. Image intensity values were recorded over a 10-week period in which MPC uniformity exhibited an upward trend between baselines. These were compared using a two-way repeated-measures ANOVA. Correlation was calculated between pixel value change and uniformity change. Portal dosimetry for a single-met SRS plan was measured bi-weekly to observe changes in resulting gamma values. Results will continually be recorded over a 6-month period.
Results
Over a one-year period, uniformity values were observed to increase following baseline establishment and increased at an accelerated rate as the year progressed. Visual changes in flood-field images were observed over time, with the central region showing greater deviation relative to the baseline image. Statistically significant differences in pixel values were found between central and peripheral detector regions over the 10-week period. No correlation was found between pixel value change and uniformity. Additionally, no significant changes were observed in gamma values from five preliminary SRS-QA measurements.
Conclusion
Some evidence of non-uniform panel degradation has been observed based on flood-field images and increased uniformity deviation from baseline over time. However, these changes have yet to show any clinically significant impacts.