Purpose: Electronic portal imaging devices have established their usefulness as two-dimensional dosimetry systems in radiotherapy. However, their performance monitoring as a routinely used clinical dosimeter has not been thoroughly examined in literature. The purpose of this research is to monitor stability and reproducibility of the dosimetric response of the EPID over an extended time frame.
Methods: Weekly measurements of the entire two-dimensional aS1200 EPID (Varian Medical Systems) dose response against an ion chamber positioned on central axis and an ion chamber array (MatriXX, IBA Dosimetry) were carried out. Weekly measurements of calibration fields (flood and dark) and defective pixel counts were run in parallel. The measurements were performed on two clinically used Varian TrueBeam linear accelerators. Both imaging modes (continuous and integrated), and all available beam energies (6X, 6FFF, 10X, 10FFF and 23X) were tested. Measurements took place over 18 months between September 2019 and February 2021, except when temporarily suspended from March - May 2020 due to the initial shutdown caused by the covid-19 pandemic.
Results: The entire EPID response to the large open ‘flood’ field and its ratios to the ion chamber reading and the ion chamber array demonstrated good constancy with 80% of all ratios falling within 1% from their mean values over the period of observation, and 100% of measurements within 2%. Measured dark and flood fields demonstrated good constancy with the individual pixel signal variations below 2.0% over the entire 18 months period for both imaging modes and all energies tested. The defective pixel count increased by 4% and 8% depending on the acquisition mode.
Conclusion: The long-term dosimetric behavior of the aS1200 EPID was monitored and shown to be stable and reproducible for routine clinical dosimetry.
Funding Support, Disclosures, and Conflict of Interest: Source of funding: Varian Medical Systems research grant "Investigation of aS1200 Electronic Portal Imaging Device (EPID) for absolute, water-equivalent dosimetry applications, and monitoring of system performance". No conflicts of interest