Direct calibration of a reference standard against the air kerma strength primary standard, at 192Ir HDR energy

Air kerma and absorbed dose standards for reference dosimetry in brachytherapy

This article reviews recent developments in primary standards for the calibration of brachytherapy sources, with an emphasis on the currently most common photon-emitting radionuclides. The introduction discusses the need for reference dosimetry in brachytherapy in general. The following section focuses on the three main quantities, i.e. reference air kerma rate, air kerma strength and absorbed dose rate to water, which are currently used for the specification of brachytherapy photon sources and which can be realized with primary standards from first principles. An overview of different air kerma and absorbed dose standards, which have been independently developed by various national metrology institutes over the past two decades, is given in the next two sections. Other dosimetry techniques for brachytherapy will also be discussed. The review closes with an outlook on a possible transition from air kerma to absorbed dose to water-based calibrations for brachytherapy sources in the future.

Development of a water calorimetry-based standard for absorbed dose to water in HDR 192Ir brachytherapy

PURPOSE

The aim of this article is to develop and evaluate a primary standard for HDR 192Ir brachytherapy based on 4 degrees C stagnant water calorimetry.

METHODS

The absolute absorbed dose to water was directly measured for several different Nucletron microSelectron 192Ir sources of air kerma strength ranging between 21,000 and 38,000 U and for source-to-detector separations ranging between 25 and 70 mm. The COMSOL MULTIPHYSICS software was used to accurately calculate the heat transport in a detailed model geometry. Through a coupling of the "conduction and convection" module with the "Navier-Stokes incompressible fluid" module in the software, both the conductive and convective effects were modeled.

RESULTS

A detailed uncertainty analysis resulted in an overall uncertainty in the absorbed dose of 1.90% (1 sigma). However, this includes a 1.5% uncertainty associated with a nonlinear predrift correction which can be substantially reduced if sufficient time is provided for the system to come to a new equilibrium in between successive calorimetric runs, an opportunity not available to the authors in their clinical setting due to time constraints on the machine. An average normalized dose rate of 361 +/- 7 microGy/(h U) at a source-to-detector separation of 55 mm was measured for the microSelectron 192Ir source based on water calorimetry. The measured absorbed dose per air kerma strength agreed to better than 0.8% (1 sigma) with independent ionization chamber and EBT-1 Gafchromic film reference dosimetry as well as with the currently accepted AAPM TG-43 protocol measurements.

CONCLUSIONS

This work paves the way toward a primary absorbed dose to water standard in 192Ir brachytherapy.

On the accuracy of techniques for obtaining the calibration coefficient N(K) of 192Ir HDR brachytherapy sources

The accuracy of interpolation or averaging procedures for obtaining the calibration coefficient N(K) for 192Ir high-dose-rate brachytherapy sources has been investigated using the EGSnrc Monte Carlo simulation system. It is shown that the widely used two-point averaging procedure of Goetsch et al. [Med. Phys. 18, 462 (1991)] has some conceptual problems. Most importantly, they recommended, as did the IAEA, averaging A(wall)N(K) values whereas one should average 1/N(K) values. In practice this and other issues are shown to have little effect except for Goetsch et al.'s methods for determining A(wall) values. Their method of generalizing the A(wall) values measured in one geometry to other geometries is incorrect by up to 2%. However, these errors in A(wall) values cause systematic errors of only 0.3% in 192Ir calibration coefficients. It is shown that A(wall) values need not be included in the averaging technique at all, thereby simplifying the technique considerably. It is demonstrated that as long as ion chambers with a flat response are used and/or very heavily filtered 250 kV (or higher) beams of x rays are used in the averaging, then almost all techniques can provide adequate accuracy.

Monte Carlo aided room scatter corrections in the air-kerma strength standardization of 169Yb and 60Co brachytherapy sources

For accurate evaluation of air-kerma strength, S(k), of 169Yb and 60Co brachytherapy sources, the present study reports Monte Carlo (MC) based corrections for (1) room scatter, and (2) departure from constant room scatter for rooms of various sizes. Correction for exponential attenuation of effective primary in air is also reported for the above sources. Values of S(k) per contained mCi, S(k)/A(c) predicted by MC calculations for 169Yb source (model X1267) with and without Ti K x-rays are 1.302 +/- 0.03% (this value is in excellent agreement with the published value reported by Piermattei et al) and 1.260 +/- 0.03% cGy cm2 h(-1) mCi(-1) respectively, and in the case of Cathetron 60Co source the value of S(k)/A(c) is 11.015 +/- 0.01% cGy cm2 h(-1) mCi(-1). It is observed that depending upon the position of the source with respect to the surrounding concrete scattering surfaces and set of d values, the assumption of constant room scatter has resulted in overestimation of S(k) that varied between 0.30% and 1.5% for the 169Yb source and only between 0.10% and 0.20% for the 60Co source.