Source geometry factors for HDR¹⁹²Ir brachytherapy secondary standard well-type ionization chamber calibrations

Abstract

Well-type ionization chambers are used for measuring the source strength of radioactive brachytherapy sources before clinical use. Initially, the well chambers are calibrated against a suitable national standard. For high dose rate (HDR) (192)Ir, this calibration is usually a two-step process. Firstly, the calibration source is traceably calibrated against an air kerma primary standard in terms of either reference air kerma rate or air kerma strength. The calibrated (192)Ir source is then used to calibrate the secondary standard well-type ionization chamber. Calibration laboratories are usually only equipped with one type of HDR (192)Ir source. If the clinical source type is different from that used for the calibration of the well chamber at the standards laboratory, a source geometry factor, k(sg), is required to correct the calibration coefficient for any change of the well chamber response due to geometric differences between the sources. In this work we present source geometry factors for six different HDR (192)Ir brachytherapy sources which have been determined using Monte Carlo techniques for a specific ionization chamber, the Standard Imaging HDR 1000 Plus well chamber with a type 70010 HDR iridium source holder. The calculated correction factors were normalized to the old and new type of calibration source used at the National Physical Laboratory. With the old Nucletron microSelectron-v1 (classic) HDR (192)Ir calibration source, ksg was found to be in the range 0.983 to 0.999 and with the new Isodose Control HDR (192)Ir Flexisource k(sg) was found to be in the range 0.987 to 1.004 with a relative uncertainty of 0.4% (k = 2). Source geometry factors for different combinations of calibration sources, clinical sources, well chambers and associated source holders, can be calculated with the formalism discussed in this paper.

The SSDL-ININ calibration and measurement capability (CMC) for reference air kerma rate K˙R in HDR 192Ir for well chambers

“…Otherwise, if a general source design is considered, the value is increased [4]. Also, to reduce the uncertainty, primary metrology institutes have developed calibration protocols in terms of for well chambers to HDR [5].…”

“…Furthermore, compared to the US protocol AAPM TG 32 [4], these protocols set the reference temperature at 20 °C, not 22 °C, and also do not use the intensity of kerma quantity and its special unit 1 U≡μGym 2 h -1 .…”

“…, which here is assumed equivalent to the corrected chamber measurement , is obtained by first performing a series of simulations at different source dwell positions in the chamber and calculating the dose to the collecting volume for each dwell position, [4].…”

A Preliminary Study of Radiation Quality Correction Factor (Kq) Determination for Well-Type Ionization Chamber

“…The strength of the brachytherapy radiation source was determined by the amount of the reference air Kerma rate (RAKR). RAKR is the rate of air Kerma in the air at a distance of 1 meter (dref) corrected by attenuation and scattering [16]. Besides RAKR, another recommendation concerning the strength of the brachytherapy radiation source is in the amount of water kerma | 132 SPEKTRA: Jurnal Fisika dan Aplikasinya Volume 5 Issue 2, August 2020 strength (sK) i.e the rate of air kerma in the air at a distance d from the radiation source is corrected by attenuation and scattering and multiplied by the square of the distance (d 2 ) [17].…”

Source strength determination in iridium-192 and cobalt-60 brachytherapy: A European survey on the level of agreement between clinical measurements and manufacturer certificates

“…Additionally, differences in calibration coefficients for different types of 192 Ir sources are small compared to those for low energy sources [20] . RAKR measurement corrections due to source geometry, derived using Monte Carlo calculated factors, of about 0% to 2%, may be applied to further reduce RAKR measurement differences between various 192 Ir sources [21] . Such factors are not yet available for all source types (notably short length PDR sources) and all well-type chambers or the Krieger phantom setup.…”