Abstract: Methods: The authors have assumed a theoretical Tm-170 cylindrical source encapsulated with stainless steel and typical dimensions taken from the currently available HDR Ir-192 brachytherapy sources. The dose-rate distribution was calculated for this source using the GEANT4 Monte Carlo (MC) code considering both photon and electron Tm-170 spectra. The AAPM TG-43 U1 brachytherapy dosimetry parameters were derived. To study general properties of Tm-170 encapsulated sources, spherical sources encapsulated with stainless steel and platinum were also studied. Moreover, the influence of small variations in the active core and capsule dimensions on the dosimetric characteristics was assessed. Treatment times required for a Tm-170 source were compared to those for Ir-192 and Yb-169 for the same contained activity. Results: Due to the energetic beta spectrum and the large electron yield, the bremsstrahlung contribution to the dose was of the same order of magnitude as from the emitted gammas and characteristic x rays. Moreover, the electron spectrum contribution to the dose was significant up to 4 mm from the source center compared to the photon contribution. The dose-rate constant Lambda of the cylindrical source was 1.23 cGy h(-1) U-1. The behavior of the radial dose function showed promise for applications in brachytherapy. Due to the electron spectrum, the anisotropy was large for r < 6 mm. Variations in manufacturing tolerances did not significantly influence the final dosimetry data when expressed in cGy h(-1) U-1. For typical capsule dimensions, maximum reference dose rates of about 0.2, 10, and 2 Gy min(-1) would then be obtained for Tm-170, Ir-192, and Yb-169, respectively, resulting in treatment times greater than those for HDR Ir-192 brachytherapy. Conclusions: The dosimetric characteristics of source designs exploiting the low photon energy of Tm-170 were studied for potential application in HDR-brachytherapy. Dose-rate distributions were obtained for cylindrical and simplified spherical Tm-170 source designs (stainless steel and platinum capsule materials) using MC calculations. Despite the high activity of Tm-170, calculated treatment times were much longer than for Ir-192.

Abstract: The introduction of model-based dose calculation algorithms (MBDCAs) in brachytherapy provides an opportunity for a more accurate dose calculation and opens the possibility for novel, innovative treatment modalities. The joint AAPM, ESTRO, and ABG Task Group 186 (TG-186) report provided guidance to early adopters. However, the commissioning aspect of these algorithms was described only in general terms with no quantitative goals. This report, from the Working Group on Model-Based Dose Calculation Algorithms in Brachytherapy, introduced a field-tested approach to MBDCA commissioning. It is based on a set of well-characterized test cases for which reference Monte Carlo (MC) and vendor-specific MBDCA dose distributions are available in a Digital Imaging and Communications in Medicine-Radiotherapy (DICOM-RT) format to the clinical users. The key elements of the TG-186 commissioning workflow are now described in detail, and quantitative goals are provided. This approach leverages the well-known Brachytherapy Source Registry jointly managed by the AAPM and the Imaging and Radiation Oncology Core (IROC) Houston Quality Assurance Center (with associated links at ESTRO) to provide open access to test cases as well as step-by-step user guides. While the current report is limited to the two most widely commercially available MBDCAs and only for Ir-192-based afterloading brachytherapy at this time, this report establishes a general framework that can easily be extended to other brachytherapy MBDCAs and brachytherapy sources. The AAPM, ESTRO, ABG, and ABS recommend that clinical medical physicists implement the workflow presented in this report to validate both the basic and the advanced dose calculation features of their commercial MBDCAs. Recommendations are also given to vendors to integrate advanced analysis tools into their brachytherapy treatment planning system to facilitate extensive dose comparisons. The use of the test cases for research and educational purposes is further encouraged.

Abstract: The goal of this study is to evaluate the ambient dose due to the transit of high dose rate (HDR) Co-60 sources along a transfer tube as compared to Ir-192 ones in a realistic clinical scenario. This goal is accomplished by evaluating air-kerma differences with Monte Carlo calculations using PENELOPE2011. Scatter from both the afterloader and the patient was not taken into account. Two sources, mHDR-v2 and Flexisource Co-60, (Elekta Brachytherapy, Veenendaal, the Netherlands) have been considered. These sources were simulated within a standard transfer tube located in an infinite air phantom. The movement of the source was included by displacing their positions along the connecting tube from z = – 75 cm to z = + 75 cm and combining them. Since modern afterloaders like Flexitron (Elekta) or Saginova (BEBIG GmbH) are able to use equally 192Ir and 60Co sources, it was assumed that both sources are displaced with equal speed. Typical HDR source activity content values were provided by the manufacturer. 2D distributions were obtained with type-A uncertainties (k = 2) less than 0.01%. From those, the air-kerma ratio Co-60/Ir-192 was evaluated weighted by their corresponding typical activities. It was found that it varies slowly with distance (less than 10% variation at 75 cm) but strongly in time due to the shorter half-life of the 192Ir (73.83 d). The maximum ratio is located close to the tube. It reaches a value of 0.57 when the typical activity of the sources at the time when they were installed by the vendor was used. Such ratio increases up to 1.28 at the end of the recommended working life (90 d) of the Ir-192 source. Co-60/Ir-192 air-kerma ratios are almost constant (0.51-0.57) in the vicinity of the source-tube with recent installed sources. Nevertheless, air-kerma ratios increase rapidly (1.15-1.29) whenever the Ir-192 is approaching the end of its life. In case of a medical event requiring the medical staff to access the treatment room, these ratios indicate that the dosimetric impact on the medical team will be lower, with a few exceptions, in the case of Co-60-based HDR brachytherapy as compared to Ir-192-based one when typical air-kerma strength values are considered.