Purpose: To perform a comparison of the interim air-kerma strength standard for high dose rate (HDR) 192 Ir brachytherapy sources maintained by the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) with measurements of the various source models using modified techniques from the literature. The current interim standard was established by Goetsch et al. in 1991 and has remained unchanged to date. Methods: The improved, laser-aligned seven-distance apparatus of the University of Wisconsin Medical Radiation Research Center (UWMRRC) was used to perform air-kerma strength measurements of five different HDR192 Ir source models. The results of these measurements were compared with those from well chambers traceable to the original standard. Alternative methodologies for interpolating the 192 Ir air-kerma calibration coefficient from the NIST air-kerma standards at 137 Cs and 250 kVp x rays (M250) were investigated and intercompared. As part of the interpolation method comparison, the Monte Carlo code EGSnrc was used to calculate updated values of A wall for the Exradin A3 chamber used for air-kerma strength measurements. The effects of air attenuation and scatter, room scatter, as well as the solution method were investigated in detail. Results: The average measurements when using the inverse N K interpolation method for the Classic Nucletron, Nucletron microSelectron, VariSource VS2000, GammaMed Plus, and Flexisource were found to be 0.47%, À0.10%, À1.13%, À0.20%, and 0.89% different than the existing standard, respectively. A further investigation of the differences observed between the sources was performed using MCNP5 Monte Carlo simulations of each source model inside a full model of an HDR 1000 Plus well chamber. Conclusions: Although the differences between the source models were found to be statistically significant, the equally weighted average difference between the seven-distance measurements and the well chambers was 0.01%, confirming that it is not necessary to update the current standard maintained at the UWADCL.
The intrinsic energy dependence of TLD-100 is dependent on photon energy, exhibiting changes of 13%-15% for (125)I and (103)Pd sources relative to (60)Co. TLD measurements of absolute dose around (125)I and (103)Pd brachytherapy sources should explicitly account for the relative TLD intrinsic energy dependence in order to improve dosimetric accuracy.
Purpose: To characterize the component of the LiF:Mg,Ti TLD response to the low‐energy photons of 125normalI and 103Pd LDR brachytherapy sources and x‐ray beam qualities of M40 and M80, that cannot be predicted by cavity theory or Monte Carlo methods. To provide a methodology for determining accurate energy correction factors for experiments performed in a variety of scatter conditions and to provide an example of how to apply the results of this work will also be presented. Method and Materials: TLD‐100 chips were exposed to 125normalI and 103Pd LDR brachytherapy sources using the known geometry of the University of Wisconsin Variable Aperture Free Air Chamber. Dose calculations were based on primary determinations of air‐kerma strength and Monte Carlo simulations of the full irradiation and source geometry. For comparison purposes and to examine the effects of dosimeter size and scatter conditions, a series of x‐ray experiments were performed to compare the response of chips (3×3×0.89mm) to microcubes (1×1×1mm) and free in air irradiations to irradiations in a PMMA holder. Results: The results of the x‐ray experiments agreed well with the work of Nunn et al. (Med. Phys. 2006) and confirmed that the “solid‐state” component of the energy response was largely independent of irradiation geometry. The results of the 125normalI experiments exhibited good reproducibility for the single 125normalI source, but the 103Pd measurements seem to exhibit source‐to‐source variability. More experiments are needed for both isotopes to determine the nature of these observations. Conclusion: For all radiation qualities used in these experiments it was found that Monte Carlo simulations appear to underestimate the dose response of LiF:Mg,Ti when compared to measured response. As a result, the previously published values for the dose rate constants of LDR brachytherapy sources are likely overestimates.
Purpose: To present a compilation of a seven‐year study of measurements using several HDR afterloaders and to provide a comprehensive analysis of the various published methodologies for interpolating between NIST standards to determine the air‐kerma calibration coefficient for 192Ir. Ultimately an update of the current interim standard will be considered. Method and Materials: An acrylic apparatus for performing the seven‐distance measurement technique equipped with laser alignment was used to acquire all datasets. Measurements were performed with an Exradin A3 spherical ion chamber that had been calibrated at NIST for beam qualities of M250 as well as 137Cs. A total of four different afterloaders were measured during multiple trials and a comparison of the results was made to assess any trends in measurements due to source geometry. Recently published interpolation methods were compared to the method used in the original establishment of the interim standard in 1991 with proper accounting of the revisions in the NIST air‐kerma standards in 2003. Three different methods for solving the non‐linear system of equations were compared to assess stability and minimize uncertainty. Results: Depending on the interpolation method, deviations of −1.12% to −0.37% from the long‐standing air‐kerma calibration factor were observed. In comparing the measurements from the last seven years, (2000–2007), to the well chamber transfer standards, (1991), certain trends between various source models were identified, but the overall effect was found to be in the range of — 0.95% to 0.18%. Conclusion: Based on the data recorded it is reasonable to assume, given the uncertainty in the method, that a single calibration factor would indeed be appropriate for all source models. The possibility of a formal update to the standard will be considered.
The HU to RED curve is more dependent on the phantom model than CT scanner. The HU to RED curve from the GAMMEX phantom produced better agreement between Eclipse AAA calculations and measured dose distributions on a heterogeneity phantom than that from the CATPHAN. However, DVH and isodose data on patient plans show small differences for common treatment sites.
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