Comparison of Pixel Sensitivity Map Methods for EPID Dosimetry Measurement
M Ahmed1, N Dang2*, V Leandro Alves2, J Siebers2, (1) Vanderbilt University Medical Center, Nashville, TN, (2) University of Virginia Health System, Charlottesville, VA,
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PO-GePV-T-176 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To intercompare EPID imagers PSMs created with a vendor-developed shift-based calibration method with a method that does not require shifting the imager.
Methods: The pixel sensitivity map (PSM) is used to convert the raw EPID signal output to a relative dosimetry response through correcting for pixel-specific response variations. In this study, we compared two different PSM calculation methods. The first method calculates the PSM via different image sets of large-overlapping-field irradiations acquired with the EPID. The second one is estimates the underlying fluence by fitting EPID images with 2D polynomials and then extracting PSM by division with raw images. Two EPID imaging systems aS1000 and aS1200 were utilized for this comparison. Raw signals from different field sizes of 2x2, 5x5, 10x10, and 15x15 cm2 and flood field at source to detector distance (SDD) of 150 and 180 cm were taken. The inline and crossline profiles of calculated PSMs were compared and a 2D relative difference map was calculated to estimate the discrepancy between the two methods.
Results: Both methods show good agreement in the center of the imager (relative difference up to 4%), however, discrepancies become more visible at the border with a maximum of 8%. Central crossline profiles of corrected images show good agreement, while central inline profiles show major deviations.
Conclusion: The proposed PSM calculation method can be used in replacement for the current shifting method within some degree of agreement.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by Varian Medical Systems. We would like to thank Varian engineers who helped finalizing the analysis of this work.
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