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Source Measurement Justification HDR 1000 Plus Brachytherapy Well Chamber
ACR Standards for brachytherapy Physics requires a well chamber be used in order for an institution to be accredited.5 Some NRC agreement states have ratified a mandate that a well chamber be used for independent calibration of interstitial brachytherapy applications. Task Group 56 lists a calibrated well chamber and electrometer as instrumentation that is needed for any and all brachytherapy procedures.4 It is clear that this equipment is no longer an option for all brachytherapy physics programs. The well chamber is the recommended standard for HDR Ir calibrations as well. "Each HDR facility should acquire a suitable re-entrant chamber. The external beam ion chamber is an acceptable but not a recommended alternative."4 Dose calibrators typically used in nuclear medicine departments cannot be used for calibration of iodine or palladium seeds as the reading received will not be an accurate measurement. The problem is not the low activity, but low energy of iodine and palladium (28KeV, 22KeV) which do not adequately penetrate the thick walls of a dose calibrator. The nuclear medicine dose calibrator is optimized for reading Tc-99 which has an energy of 140 KeV and cannot adequately read energy levels as low as 28KeV.
Brachytherapy sources are assigned a calibration by the manufacturer. For example, for LDR Iodine the manufacturers stated calibration accuracy is only +/-5% random uncertainty of the mean output with a 7% uncertainty spread in each range.6 This means there could be a discrepancy in stated activity and the true value of your sources. Many manufacturers of brachytherapy seeds, including Amersham, recommend independent verification of their sources.6 "Every institution practicing brachytherapy shall have a system for measuring source strength with secondary traceability for all source types used in its practice. Prior to using newly received sources for treatment, the vendor-supplied calibrations must be verified as per Task Group No. 40 recommendations. The institution should compare the manufacturer's stated value with the institutions standard."4 Task Group 40 of the AAPM Radiation Therapy Committee recommends a tolerance of 3% for agreement between the institutional assay and the vendor's statement of source strength. The common experience has increasingly been a significant number of shipments found outside the recommended 3% tolerance.7 Without independent verification of your source strength this variance would not be discovered. These findings require that independent verification of the vendors stated value be performed. References1. Comprehensive QA for radiation oncology: Report of AAPM Radiation Therapy Committee Task Group 40. Med. Phys. 21(4):581-618, 1994. "However, it is the responsibility of the institution to verify that this (manufacturer's) calibration is correct. The institution should compare the manufacturer's stated value with the institution's standard." 2. A New Re-Entrant Ionization Chamber for the Calibration of... : Goetsch, Attix, DeWerd, Thomadsen. Int. Journal Radiation Oncology Biol. Phys. Vol. 24. pp. 167-170, 1991 " The manufacturer provides an activity calibration for each source with a stated accuracy of +/- 10%. No traceability of this calibration to any national standards laboratory has been demonstrated. Since the International Commission on Radiation Units and measurements has recommended an overall uncertainty of no more than +\-5% in absorbed dose delivered to the treatment volume, the manufacturer's calibration is not considered adequate by most institutions." 3. Physics: Calibration and Quality Assurance; Ezell, Hicks, and DeWerd, 7th International Brachytherapy Working Conference. 1992. "Calibration of all sources is necessary because some sources are delivered from the source manufacturer with +10% specification, One facilities experience has shown a variation of 15% in delivered sources." 4. Code of practice for brachytherapy physics: Report of AAPM Radiation Therapy Committee Task Group No. 56. Med. Phys. 24(10), : 1568-1569, 1582, 1594-1595, Oct. 1997. " Well-type ionization chambers make it easy to establish source calibrations with NIST traceability. Thus, for sources for which NIST provides calibration it is no longer necessary and should no longer be a practice to rely on source strengths quoted by the manufacturer." 5. ACR Standard for Brachytherapy Physics: Manually Loaded Sources. Res. 25, 1995. "Each facility should have instrumentation to independently verify the source strength provided by the manufacturer. This should be done with a well ionization chamber with a calibration directly traceable to NIST." 6. AMERSHAM product literature: 1997. "It is recommended that customers measure each source to validate the values stated on the (vendors) Test Report, prior to the sources being implanted into patients." 7.
Experience with Assays of 125I Seeds; Johnson, and Kline. Med. Phys. 24 (6): pg. 1019, June 1997. " A significant number of shipments have been outside the recommended 3% tolerance. Our supposition is that there has developed a systematic error in the calibration or "binning" system employed by the vendor. Also, based on our experience, the
tolerance recommended by the RTC TG-40 must be considered unreasonable for 125I seeds. NEW (Dec 2002) Brachytherapy Source Calibration Justification - SeeDOS Information Sheet (PDF) NEW (February 2003) New Brachytherapy source measuring and handling systems pdf If you are interested in any of the products mentioned here, please complete a Quotation Request Form or an Enquiry Form so that we may promptly respond to your detailed request.
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