Brachytherapy sources are calibrated in terms of Reference Air-Kerma-Rate (RAKR):
where (d) is the air-kerma-rate measured at a distance d and dref is the reference distance (1 m). This quantity is defined in air, corrected for air attenuation and scattering.
Two different approaches have been used by the two laboratories participating in this comparison to measure this quantity. BNM-LNHB is using an indirect method. The RAKR of the source is measured with a cavity ionization chamber using a technique originally developed by Goetsch et al in 1991. Several improvements have been implemented in order to reduce the uncertainty levels. The calibration coefficient of the cavity ionization chamber for 192-Ir spectrum is determined indirectly by interpolation from x-rays (250 kV), 137-Cs and 60-Co calibrations. Therefore this is not a direct method, since it relies on x-rays, 137-Cs and 60-Co national references. However, the implementation of this technique is relatively simple and can lead to accurate results if a flat energy response function chamber is used.
As opposed to this, NPL developed a primary method for calibrating HDR 192-Ir sources based on the use of a volume-calibrated spherical ionization chamber. Recently a bilateral comparison of French and USA brachytherapy dosimetric standards has been conducted between BNM-LNHB and the University of Wisconsin Accredited Dosimetric Calibration Laboratory (UWADCL). UWADCL and BNM-LNHB are using basically the same technique to establish their national standards for HDR brachytherapy. The comparison concluded to an excellent agreement since the measured discrepancies were found to be less than 0.3%. It would be very interesting now for NPL and BNM-LNHB to perform a bilateral comparison in order to validate the recently developed standard of NPL and to compare it to todays international consensus.
Final Report 2008-03-10
The comparison has ceen completed and the report can be downloaded here>>.
An indirect comparison has been made of the air kerma standards for high dose rate (HDR) 192Ir brachytherapy sources at the Laboratoire National Henri Becquerel (LNHB) and the National Physical Laboratory (NPL).
The measurements were carried out at both laboratories between November and December 2004. The comparison was based on measurements using well-type transfer ionization chambers and two different source types, Nucletron microSelectron HDR Classic and version 2.
The results show the reported calibration coefficients to agree within 0.47% to 0.63% which is within the overall standard uncertainty of 0.65% reported by both laboratories at the time of this comparison. Following this comparison, some of the NPL primary standard correction factors were re-evaluated resulting in a change of 0.17% in the overall correction factor. The new factor was implemented in May 2006. Applying the revised chamber factor to the measurements reported in this comparison report will reduce the difference between the two standards by 0.17%.