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Robust Quality Assurance (QA) Method Using a Single Phantom to Determine Radiation and Imaging Isocenters

H Kang*, R Patel , J Roeske , Loyola university Medical Center, Maywood, IL


(Sunday, 7/29/2018) 4:00 PM - 4:55 PM

Room: Karl Dean Ballroom A1

Purpose: To develop an integrated method to determine the coincidences of Optical Surface Monitoring System (OSMS, Varian, CA), kV and MV imaging, and radiation isocenters using a single phantom setup.

Methods: After aligning the OSMS cube phantom to the OSMS isocenter in real-time, CBCTs are acquired at different isocenters and registered to CT using online software. Without moving the phantom, orthogonal kV and MV images are simultaneously acquired at the four cardinal angles for OSMS software to analyze kV and MV isocenters. Then, MV images of a BB embedded in the phantom are also acquired at different gantry, collimator and couch angles to determine the radiation isocenter similar to the Winston-Lutz test. The accelerator and couch are automatically controlled in the TrueBeam Developer Mode during data acquisition. The radiation isocenter is then determined from the BB positions using commercial software. All the images are acquired weekly to track the isocenter location and uncertainties of all the imaging modalities utilized for frameless SRS treatment. The differences in isocenter locations between all imaging and OSMS are measured in 6 degrees, and the difference between radiation isocenter and OSMS isocenter is measured in 3 translations.

Results: The discrepancy of isocenters of all imaging modalities and the radiation isocenter is within 0.5mm and 0.2° determined in data acquired over 6 weeks. The OSMS isocenter is shifted towards the gantry in the longitudinal direction by 0.3mm on average relative to CBCT and radiation isocenters. The measured discrepancies are consistent from week to week. The whole process takes 25 minutes including data analyses.

Conclusion: A robust QA method is developed using a single phantom which is set up once and then not moved between different imaging modalities. Further development including full automation of data transfer and analyses to determine various isocenters is planned.


Not Applicable / None Entered.


TH- External beam- photons: Quality Assurance - Linear accelerator

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