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Long-term two-dimensional pixel stability of EPIDs used for regular linear accelerator quality assurance

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Abstract

The long-term stability of three clinical electronic portal imaging devices (EPIDs) was studied to determine if longer times between calibrations can be justified. This would make alternatives to flood-field calibration of EPIDs clinically feasible, allowing for more effective use of EPIDs for dosimetry. Images were acquired monthly for each EPID as part of regular clinical quality assurance over a time period of approximately 3 years. The images were analysed to determine (1) the long-term stability of the EPID positioning system, (2) the dose response of the central pixels and (3) the long term stability of each pixel in the imager. The position of the EPID was found to be very repeatable with variations less than 0.3 pixels (0.27 mm) for all imagers (1 standard deviation). The central axis dose response was found to reliably track ion chamber measurements to better than 0.5%. The mean variation in pixel response (1 standard deviation), averaged over all pixels in the EPID, was found to be at most 0.6% for the three EPIDs studied over the entire period. More than 99% of pixels in each EPID showed less than 1% variation. Since the EPID response was found to be very stable over long periods of time, an annual calibration should be sufficient in most cases. More complex dosimetric calibrations should be clinically feasible.

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Acknowledgments

We are grateful to Prof. J Siebers for useful initial discussions relating to this study. The first author is supported by a National Health and Medical Research Council grant #569211.

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Authors and Affiliations

  1. School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, NSW, 2308, Australia

    B. W. King & P. B. Greer

  2. Department of Radiation Oncology, Calvary Mater Newcastle Hospital, Newcastle, NSW, 2310, Australia

    L. Clews & P. B. Greer

  3. RMIT University, Melbourne, VIC, 3000, Australia

    L. Clews

Authors
  1. B. W. King
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  2. L. Clews
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  3. P. B. Greer
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Correspondence to B. W. King.

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King, B.W., Clews, L. & Greer, P.B. Long-term two-dimensional pixel stability of EPIDs used for regular linear accelerator quality assurance. Australas Phys Eng Sci Med 34, 459–466 (2011). https://doi.org/10.1007/s13246-011-0106-0

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  • DOI: https://doi.org/10.1007/s13246-011-0106-0

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