Calibration of the NPL secondary standard radionuclide calibrator for¹⁹²Ir brachytherapy sources

Abstract

Safe and effective treatment with brachytherapy sources requires an accurate knowledge of the local tissue absorbed dose rate derived from the source reference air kerma rate. It is desirable that these air kerma rate measurements be traceable to national standards. The NPL has embarked on a programme that will enable the user to assay brachytherapy sources in a convenient manner prior to treatment. Calibration figures have been derived for the NPL secondary standard radionuclide calibrator for 192Ir brachytherapy sources manufactured by Amersham International plc. The calibration figures enable the user to accurately estimate the reference air kerma rate and activity of such sources by measuring the ionization chamber response. Calibration figures for other brachytherapy sources are also being derived.

Air kerma and absorbed dose standards for reference dosimetry in brachytherapy

“…The well chamber is traceably calibrated against the air kerma primary standard of the NPL. 18,19 The primary air kerma measurement is performed at 1-m distance from the brachytherapy seeds using a spherical ionization chamber with a thin carbon fibre wall and a volume of approximately 3 l. The 3-l transfer chamber is calibrated in X-ray beams with average energies of 25 and 33 keV in order to derive the calibration coefficient for the 125 I beam quality, which is taken as the average of both X-ray calibration coefficients. The 125 I seed is rotated at 45°intervals about its long axis, and the mean of the eight measurements is calculated to average out any anisotropy of the source spectrum.…”

Primary standards and dosimetry protocols for brachytherapy sources

“…Ir wires and pins. As for 125 I seeds, NPL provides air-kerma rate calibrations of 192 Ir wires and pins using their secondary standard radionuclide calibrator, which is traceable to the NPL air-kerma primary standard (Rossiter et al 1991, Sephton et al 1993. The primary air-kerma measurement is performed on a 10 mm length of each source design using the same 3 L chamber described above for 125 I seed calibrations.…”

Applications of the Italian protocol for the calibration of brachytherapy sources

“…The direct procedure for Kr determinations required the use of an optical bench to measure the chamber-source distances. A well-type ionization chamber enables accurate calibration of brachytherapy sources to be carried out without being time consuming (Goetsch et al 1991, Sephton et al 1993, Jones 1995. A Standard Imaging HDR-1000 ionization chamber has been calibrated in terms of the reference air kerma rate calibration factor N source K r .…”