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Determination of Linear Mapping Factors to Convert Corrected EPID Pixel Values for Dynamic Plan QA with Portal Dosimetry and MOSAIQ

M Schmidt

M Schmidt*, N Knutson , F Liu , M Price , Rhode Island Hospital, Providence, RI


PO-BPC-Exhibit Hall-11 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: This works explains the method by which the electronic portal imaging device (EPID) on a Varian Medical Systems treatment machine may be used for fluence-based patient specific IMRT quality assurance while utilizing MOSAIQ record and verify system.

Methods: When an integrated image is transferred from the Varian treatment console to the MOSAIQ R&V, some of the DICOM tags essential for Portal Dosimetry are modified or removed. Two of those tags, R-intercept and R-slope, contain the mapping from relative pixel values to absolute dosimetry units calibrated upon commissioning of the amorphous silicon (aSi) panel. This work utilizes fields of various MU (50MU-200MU) and field size (5cmx5cm – 20cmx20cm) to define a linear relationship between the slope and intercept values with the MU of the delivered field and introduces a simple tool to restore the R&V images to an acceptable format for analysis. These fields were extracted with and without the use of the R&V system and compared using the Portal Dosimetry application after making the necessary alterations to the R&V DICOM images.

Results: These images were compared using a gamma analysis (1%/1mm) and dose difference criteria with a region of interest selection of the field size with an additional 1cm margin. All images were able to achieve a passing pixel percentage within their ROI of greater than 93% with only one image not gaining 100% pass rate. The average dose difference for all images relative to their maximum value was 0.26%.

Conclusion: The determination of pixel to calibrated unit mapping derived from a linear mapping relationship with the MU delivered proves effective in reinstating the absolute dosimetry of the integrated images imported into MOSAIQ.

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