Comparison of TrueBeam Portal Dosimetry Results Across a Large Health System
J Donaghue1*, S Gajdos2, L Muhieddine Mossolly3, (1) Cleveland Clinc Fairview, Rocky River, OH, (2) Cleveland Clinic Akron General, Strongsville, OH, (3) Cleveland Clinic Foundation, Strongsville, OH
Presentations
PO-GePV-T-218 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Establish the dosimetric fluence differences between the Varian Truebeams across the health system
Methods: The Varian Truebeams portal dosimetry systems were calibrated individually. 6MV and 10MV photon beams were intercompared with an SID that was set to 100 cm. A selection of commissioning plans that consisted of simple beams configurations, TG-119 plans, and TG-244 plans were used. All of the plans maintained segmentation and MU settings across all energies. All data was normalized to a 10x10 measurement to account for different possible outputs between machines. There were two different EPID types used based on machine age, in particular both the AS1000 and AS1200 EPIDs were used. The open source software PyMedPhys was used to run gamma comparisons using 2%/1mm and a dose threshold of 10%.
Results: Excellent agreement was found across all machines for the VMAT (modulated) plans with the AS1000 (6X: 99.9%±0.26%, 10X:99.9%±0.14%). It was noted that not all of the portal dosimetry calibrations used the same profile. This was revealed in the simple/open field measurements, but modulation tended to mask this information. A straight forward comparison between the AS1000 and AS1200 EPIDs was not easily available. Using per field normalization yielded excellent results, but direct dosimetric differences were not available this way.
Conclusion: Excellent agreement between all EPID types exists with the modulated fields. When commissioning portal dosimetry to improve reproducibility it is best to use the same base profile between machines (if valid for that machine). Some data points needed to be thrown out because the values reached a max value that surpassed the 16 bit value supplied. It is unclear if this is software or EPID limitations.
Keywords
Portal Imaging, Intensity Modulation, Equivalent Fields
Taxonomy
TH- External Beam- Photons: portal dosimetry, in-vivo dosimetry and dose reconstruction
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