Determination of the reference air kerma rate for brachytherapy sources and the related uncertainty

Abstract

Different methods exist to determine the air kerma calibration factor of an ionization chamber for the spectrum of a 192Ir high-dose-rate (HDR) or pulsed-dose-rate (PDR) source. An analysis of two methods to obtain such a calibration factor was performed: (i) the method recommended by [Goetsch et al., Med. Phys. 18, 462-467 (1991)] and (ii) the method employed by the Dutch national standards institute NMi [Petersen et al., Report S-EI-94.01 (NMi, Delft, The Netherlands, 1994)]. This analysis showed a systematic difference on the order of 1% in the determination of the strength of 192Ir HDR and PDR sources depending on the method used for determining the air kerma calibration factor. The definitive significance of the difference between these methods can only be addressed after performing an accurate analysis of the associated uncertainties. For an NE 2561 (or equivalent) ionization chamber and an in-air jig, a typical uncertainty budget of 0.94% was found with the NMi method. The largest contribution in the type-B uncertainty is the uncertainty in the air kerma calibration factor for isotope i, N(i)k, as determined by the primary or secondary standards laboratories. This uncertainty is dominated by the uncertainties in the physical constants for the average mass-energy absorption coefficient ratio and the stopping power ratios. This means that it is not foreseeable that the standards laboratories can decrease the uncertainty in the air kerma calibration factors for ionization chambers in the short term. When the results of the determination of the 192Ir reference air kerma rates in, e.g., different institutes are compared, the uncertainties in the physical constants are the same. To compare the applied techniques, the ratio of the results can be judged by leaving out the uncertainties due to these physical constants. In that case an uncertainty budget of 0.40% (coverage factor=2) should be taken into account. Due to the differences in approach between the method used by NMi and the method recommended by Goetsch et al., an extra type-B uncertainty of 0.9% (k= 1) has to be taken into account when the method of Goetsch et al. is applied. Compared to the uncertainty of 1% (k= 2) found for the air calibration of 192Ir, the difference of 0.9% found is significant.

Comparison of air‐kerma strength determinations for HDR 192Ir sources

“…In recent years, publications by Mainegra-Hing and Rogers 4 and van Dijk et al 5 have provided alternatives to the interpolation methodology used in the original standard. The consensus from those publications was that the interpolation should be based on parameters that are proportional to ion chamber response.…”

“…During recent years, the methods on which the original stand-ard was based have been critiqued, and improved methods have been suggested by several researchers. The original standard used a linear interpolation between A wall weighted air-kerma calibration coefficients, but the works of Mainegra-Hing and Rogers 4 and van Dijk et al 5 have shown that it would be more appropriate to use an interpolation based on the inverse of the air-kerma calibration coefficients and without the inclusion of A wall weighting. In addition, van Dijk et al suggested an interpolation using the spectral lines of 192 Ir.…”

Measurement of absorbed dose‐to‐water for an HDR 192Ir source with ionization chambers in a sandwich setup

“…Some issues related to this method have been discussed and improvements to this method have been proposed. [9][10][11][12] On the other hand, to date, the determination of absorbed dose in water or plastic phantoms for HDR 192 Ir brachytherapy sources has been reported for direct inphantom measurements using detectors such as Fricke dosimeters, 13,14 thermoluminescent dosimeters, 3,4 radiophotoluminescent glass dosimeters, 15 ionization chamber dosimeters, [16][17][18][19][20][21][22] diode dosimeters, 3 metal-oxide semiconductor field-effect transistor dosimeters, 23 and Gafchromic film dosimeters. 21,22,24,25 Absorbed dose standards for HDR 192 Ir sources based on water and graphite calorimeters have recently been developed in the framework of the European Metrology Research Programme (EMRP) from 2008 to 2011.…”

Evaluation of Wall Correction Factor of INER's Air-Kerma Primary Standard Chamber and Dose Variation by Source Displacement for HDR192Ir Brachytherapy

“…To replace the interpolation techniques between air-kerma calibration coefficients of an ionization chamber for 192 Ir [5, 6], the Institute of Nuclear Energy Research (INER, Taiwan) has recently developed a spherical graphite-walled cavity ionization chamber as the primary standard for direct measurement of HDR 192 Ir brachytherapy source strength to provide a traceable source calibration. Our primary standard is derived from measurements using the graphite-walled chamber, based on the Bragg-Gray theory.…”