New national air-kerma-strength standards for I-125 and Pd-103 brachytherapy seeds

Abstract

The new U.S. measurement standard for the air-kerma strength from low-energy photon-emitting brachytherapy seed sources is formally described in detail. This instrument-based standard was implemented on 1 January 1999, with its salient features and the implications of differences with the previous standard given only through a series of informal communications. The Wide-Angle Free-Air Chamber (WAFAC) is specially designed to realize air kerma from a single-seed source emitting photons with energies up to about 40 keV, and is now used to measure the wide variety of seeds used in prostate-cancer therapy that has appeared in the last few years. For the two 125I seed models that have been subject to both the old and new standards, the new standard reduces the air-kerma strength by 10.3 %. This change is mainly due to the removal of the influence on the measurement of the Ti K x rays produced in the source encapsulation, a component with no clinical significance.

Metrological support of measurements in brachytherapy

“…There are several primary standards in dosimetry of iridium-192 and iodine-125 radiation sources for brachytherapy (the USA, Germany, Italy [10][11][12][13]). The expanded uncertainty of K S (or  K  ) is from 1,8 % to 2,0 % for these primary standards [10][11][12].…”

“…The expanded uncertainty of K S (or  K  ) is from 1,8 % to 2,0 % for these primary standards [10][11][12]. In Russia such a primary standard is not yet developed therefore the measuring instruments traceable to the national primary standard for x-rays and gamma radiation air-kerma GET 8-2011 are used for measurements with brachytherapy sources [14].…”

Calculation Studies of Spatial Distribution of the Absorbed Dose Rate for Various Seeds

“…Относительный спектр фотонов, выходящих из микроисточника «model 6711», на оси, перпендикулярной оси микроисточника, в зависимости от материала покрытия слоя I-125 в сравнении с данными NIST [8] Table 6. Relative spectrum of photons emerging from the microsourse «model 6711», on the axis perpendicular to the axis of the microsource, depending on the material of the coating layer I-125 in comparison with the data of NIST [8] Показано, что, при соблюдении стандартных условий по составу окружающей среды, водяного фантома и воз-духа с 40% влажностью, полученные результаты выходят за пределы допустимых отличий в 3% от рекомендован-ных данных. Тогда как вариации только толщины нане-сения радиоактивного материала в диапазоне 0-10 мкм меняют дозиметрические характеристики в пределах 15-20%.…”

NCS Report 20: Dosimetry and quality control of brachytherapy with low-energy photon sources (I-125)

“…In this evaluation differences due to the intersource effect approaching 10% were observed, however dependent on the seed arrangement. [33]. A photograph of the WAFAC is shown in Figure 11.…”

“…The combined relative standard uncertainty of a calibration of a single seed performed with the WAFAC is estimated to vary between 0.8 and 2.1% (k=1) [33,35]. This measurement uncertainty includes the Type B uncertainties of several correction factors, most of which are fixed for the different seed types.…”