TG-43 Dose Calculation Research Articles

SU‐FF‐T‐13: Monte Carlo Calculation of the TG‐43 Dosimetric Parameters of a New BEBIG Ir‐192 HDR Source

Purpose: High dose rate (HDR) brachytherapy is a highly extended practice in clinical brachytherapy today. Quality dose rate distribution datasets of the HDR sources used in a clinical treatment are required. Because of the different source designs, a specific dosimetry dataset is required for each source model. In the recently published BRAPHYQS‐ESTRO Report, an overview of available dosimetric data for all HDR Ir‐192 sources is given, pointing out the lack of data for one of the sources, the used by the BEBIG MultiSource afterloading system (BEBIG GmbH, Germany). The purpose of this study is to obtain detailed dose rate distributions in liquid water media around this source. Material and methods: The MC code GEANT4 was used to estimate dose rate in water and air‐kerma strength around the Ir‐192 source. All the details of the stainless steel encapsulated BEBIG HDR 1.1 mm in external diameter has been included in the simulation. Results: A complete dosimetric dataset for the BEBIG Ir‐192 HDR source is presented. TG43 dosimetric functions and parameters have been obtained as well as a 2‐D rectangular dose rate table, consistent with the TG43 dose calculation formalism. Conclusion: The dosimetric parameters and functions obtained for the BEBIG HDR source have been compared with that obtained in the literature for others HDR sources, showing that the use of specific datasets for this new source is justified. This dataset can be used as input in the TPS and to validate its calculations. As policy of BRAPHYQS‐ESTRO task group, this dataset will be incorporated to the website: http://www.uv.es/braphyqs available to users in excel format.

SU‐FF‐T‐14: Monte Carlo Derivation of TG‐43 Dosimetric Parameters for Radiation Therapy Resources and 3M Cs‐137 Sources

Purpose: In clinical brachytherapy dosimetry a detailed dose rate distribution of the radioactive source in water is needed to make a quality treatment planning. Two Cs‐137 sources are considered in this study the Radiation Therapy Resources 67–800 source (Radiation Therapy Resources Inc., Valencia, CA) and the 3M model 6500/6D6C source. Material and methods: A complete dosimetric dataset for both sources has been obtained by means of the Monte Carlo GEANT4 code. Results: Dose rate distributions are presented in two different ways, following the TG43 formalism and in a 2D rectangular dose rate table. Conclusion: This 2D dose rate table is helpful for the TPS quality control and it is fully consistent with the TG43 dose calculation formalism. In this wok, several improvements to the previously published data for these sources have been included: the source asymmetries were taken explicitly into account in the MC calculations, TG43 data were derived directly from MC calculations, the data radial range was increased, the angular grid in the anisotropy function was increased and TG43 data is now consistent with the along and away dose rate table as is recommended by the TG43 Update.

Monte Carlo and experimental derivation of TG43 dosimetric parameters for CSM‐type Cs‐137 sources

In this study, complete dosimetric datasets for the CSM2 and CSM3 Cs-137 sources were obtained using the Monte Carlo GEANT4 code. The application of this calculation method was experimentally validated with thermoluminescent dosimetry (TLD). Functions and parameters following the TG43 formalism are presented: the dose rate constant, the radial dose functional, and the anisotropy function. In addition, to aid the quality control process on treatment planning systems, a two-dimensional (2D) rectangular dose rate table (the traditional along-away table), coherent with the TG43 dose calculation formalism, is given. The data given in this study complement existing information for both sources on the following aspects: (i) the source asymmetries were considered explicitly in the Monte Carlo calculations, (ii) TG43 data were derived directly from Monte Carlo calculations, (iii) the radial range of the different tables was increased as well as the angular resolution in the anisotropy function, including angles close to the longitudinal source axis. The CSM2 source TG-43 data of Liu et al. [Med. Phys. 31, 477-483 (2004)] are not consistent with the Williamson 2D along-away data [Int. J. Radiat. Oncol., Biol., Phys. 15, 227-237 (1988)] at distances closer than approximately 2 cm from the source. The data presented here for this source are consistent with this 2D along-away table, and are suitable for use in clinical practice.

The effect of patient inhomogeneities in oesophageal 192Ir HDR brachytherapy: a Monte Carlo and analytical dosimetry study

The effect of patient inhomogeneities surrounding the oesophagus on the dosimetry planning of an upper thoracic oesophageal 192Ir HDR brachytherapy treatment is studied. The MCNPX Monte Carlo code is used for dosimetry in a patient-equivalent phantom geometry and results are compared in terms of isodose contours as well as dose volume histograms with corresponding calculations by a contemporary treatment planning system software featuring a full TG-43 dose calculation algorithm (PLATO BPS version 14.2.4). It is found that the presence of patient inhomogeneities does not alter the delivery of the planned dose distribution to the planning treatment volume. Regarding the organs at risk, the common practice of current treatment planning systems (TPSs) to consider the patient geometry as a homogeneous water medium leads to a dose overestimation of up to 13% to the spinal cord and an underestimation of up to 15% to the sternum bone. These findings which correspond to the dose region of about 5–10% of the prescribed dose could only be of significance when brachytherapy is used as a boost to external beam therapy. Additionally, an analytical dosimetry model, which is efficient in calculating dose in mathematical phantoms containing inhomogeneity shells of materials of radiobiological interest, is utilized for dosimetry in the patient-equivalent inhomogeneous phantom geometry. Analytical calculations in this work are in good agreement with corresponding Monte Carlo results within the bone inhomogeneities of spinal cord and sternum bone but, like treatment planning system calculations, the model fails to predict the dose distribution in the proximal lung surface as well as within the lungs just as the TPS does, due to its inherent limitation in treating lateral scatter and backscatter radiation.

Comparison of TG-43 dose calculations to pinpoint ion chamber and diamond detector measurements

Brachytherapy is an area of radiation therapy where the availability of high-resolution detectors with a low energy-dependence is of great importance. The suitability of two detectors is investigated in this study. Measurements in the proximity of an Ir-192 source were performed with a diamond detector type 60003 and a pinpoint chamber type 31006 (both PTW-Freiburg). For comparison, dose values were calculated with the dose calculation formalism recommended by the task group 43 (Nath et al 1995 Med. Phys. 22 209–34).

Review of AAPM task group no. 43 recommendations on interstitial brachytherapy sources dosimetry

In 1995, the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) published its recommendations on the dosimetry of interstitial brachytherapy sources. The report recommended the use of a new dose calculation formalism based on measured quantities. The formalism in modular form permits the computation of doses in two dimensions for 103Pd, 125I, and 192Ir sources. The TG-43 dose calculation formalism introduced new and updated quantities such as air kerma strength, dose rate constant, radial dose function, anisotropy function and anisotropy factor. The dose rate obtained using the TG-43 dose calculation formalism and updated source dosimetry data can be expected to be different from some of the currently used systems by as much as 17%. For the same treatment and implementing the TG-43 dosimetry with point source approximation, the widely prescribed dose of 160 Gy for 125I permanent implants using model 6711 sources changes to 144 Gy. In addition to the dose calculation formalism, TG-43 report also stated that the air kerma strength provided by NIST is estimated to be approximately 7–10% higher than it should be, due to low energy photon contamination for 125I. This difference has not been accounted for in the TG-43 report.