Experimental determination of the dose rate constant for selected¹²⁵I- and¹⁹²Ir-brachytherapy sources

Abstract

In 2008, the European project ‘T2.J06, Increasing cancer treatment efficacy using 3D brachytherapy’ was launched. One of the main goals of the Joint Research Project was the experimental determination of the dose rate constant Λ to allow the linkage between the air kerma strength or reference air kerma rate currently used for the source characterization and the ‘new’ absorbed dose rate to water with an uncertainty of <3% (k = 1) for some selected brachytherapy sources. The results obtained by five National Metrology Institutes (NMI) for four different types of brachytherapy sources are presented and compared with consensus data published in the literature. A further goal of the project was to develop a calibration chain for the transfer of the new reference quantity to the end user, minimizing the uncertainty. A first direct calibration in terms of absorbed dose rate to water of a secondary standard and the dissemination to the hospitals is presented.

Re-evaluation of the correction factors for the GROVEX

“…Selbach et al reported on the experimental determination of the dose rate constant of Λ = Ḋw,1 / Kδ,100 = 1.021(17) • 10 4 at PTB for seeds of type Bebig I25.S16 [15]. As the exact date of the changes to the setup is unknown, no conclusion can be drawn of the impact of the present results on this work.…”

Air kerma and absorbed dose standards for reference dosimetry in brachytherapy

“…Several comparisons of brachytherapy standards have already been completed, 34,42,43,90,92 and generally good agreement was found between the different measurement techniques and methods. The brachytherapy standards working group at the Bureau International des Poids et Mesures (BIPM) in Sèvres, France, has set up an ongoing key comparison for HDR 192 Ir air kerma rate primary standards.…”

“…The new absorbed dose standards, together with the existing air kerma standards at the NMIs, which participated in the EMRP T2.J06 project, allowed an experimental determination of the dose rate constant L for different LDR 125 I and HDR 192 Ir sources as the ratio _ Dðr 0 ; u 0 Þ= _ K R with a significant decrease in the associated relative uncertainty to ,5% (k 5 2). 90,91 As a consequence of the reduced measurement uncertainty, medical physicists will be able to characterize brachytherapy sources more accurately with traceably calibrated secondary standards, such as well-type ionization chambers, and this will potentially improve the brachytherapy treatment outcome.…”

European research projects for metrology in Brachytherapy and External Beam Cancer Therapy

“…Once these new BT absorbed dose standards were completed, experimental determinations of the dose rate constant were carried out for the BT sources used. These determinations of were carried out with a significant decrease in the associated standard uncertainty to less than 2.5% (at the level of one standard deviation) being based on measurements by the KR and the Ḋw,1cm standards operating at the participating national laboratories [13]. The different values of determined with the standards of the same group (LDR or HDR) also allowed a preliminary comparison among these standards.…”