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Direct megavoltage photon calibration service in Australia

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Abstract

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) maintains the Australian primary standard of absorbed dose. Until recently, the standard was used to calibrate ionisation chambers only in 60Co gamma rays. These chambers are then used by radiotherapy clinics to determine linac output, using a correction factor (k Q ) to take into account the different spectra of 60Co and the linac. Over the period 2010–2013, ARPANSA adapted the primary standard to work in megavoltage linac beams, and has developed a calibration service at three photon beams (6, 10 and 18 MV) from an Elekta Synergy linac. We describe the details of the new calibration service, the method validation and the use of the new calibration factors with the International Atomic Energy Agency’s TRS-398 dosimetry Code of Practice. The expected changes in absorbed dose measurements in the clinic when shifting from 60Co to the direct calibration are determined. For a Farmer chamber (model 2571), the measured chamber calibration coefficient is expected to be reduced by 0.4, 1.0 and 1.1 % respectively for these three beams when compared to the factor derived from 60Co. These results are in overall agreement with international absorbed dose standards and calculations by Muir and Rogers in 2010 of k Q factors using Monte Carlo techniques. The reasons for and against moving to the new service are discussed in the light of the requirements of clinical dosimetry.

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Acknowledgments

The authors wish to thank staff from the primary standards laboratories who performed comparisons with ARPANSA during the development of this service (BIPM, NMIJ and NRC), and Rebecca Day from the Wellington Blood and Cancer Centre in New Zealand, who first raised our awareness of the possibility of using audit data to produce Fig. 4. The Australian Clinical Dosimetry Service is a joint initiative between the Department of Health and ARPANSA. The Radiological Physics Center is supported by grant CA 10953 from the National Cancer Institute, NIH, DHHS.

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Authors and Affiliations

  1. Australian Radiation Protection and Nuclear Safety Agency, 619 Lower Plenty Road, Yallambie, VIC, 3085, Australia

    D. J. Butler, G. Ramanathan, C. Oliver, A. Cole, J. Lye, P. D. Harty, T. Wright & D. V. Webb

  2. Australian Clinical Dosimetry Service, 619 Lower Plenty Road, Yallambie, VIC, 3085, Australia

    J. Lye

  3. Radiological Physics Centre (now the Imaging and Radiation Oncology Core Quality Assurance Centre), M. D. Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX, 77030, USA

    D. S. Followill

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  1. D. J. Butler
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  2. G. Ramanathan
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  3. C. Oliver
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  4. A. Cole
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  5. J. Lye
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  6. P. D. Harty
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  7. T. Wright
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  8. D. V. Webb
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  9. D. S. Followill
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Correspondence to D. J. Butler.

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Butler, D.J., Ramanathan, G., Oliver, C. et al. Direct megavoltage photon calibration service in Australia. Australas Phys Eng Sci Med 37, 753–761 (2014). https://doi.org/10.1007/s13246-014-0293-6

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