Perturbation of TG‐43 parameters of the brachytherapy sources under insufficient scattering materials

Mehdi Zehtabian,Sedigheh Sina,Reza Faghihi,Ali Meigooni

doi:10.1120/jacmp.v14i3.4228

Abstract

In the recommendations of Task Group #43 from American Association of Physicists in Medicine (AAPM TG43), methods of brachytherapy source dosimetry are recommended, under full scattering conditions. However, in actual brachytherapy procedures, sources may not be surrounded by full scattering tissue in all directions. Clinical examples include high‐dose‐rate (HDR) brachytherapy of the breast or low‐dose‐rate (LDR) brachytherapy of ocular melanoma using eye plaque treatment with 125I and 103Pd. In this work, the impact of the missing tissue on the TG‐43–recommended dosimetric parameters of different brachytherapy sources was investigated. The impact of missing tissue on the TG‐43–recommended dosimetric parameters of 137Cs, 192Ir, and 103Pd brachytherapy sources was investigated using the MCNP5 Monte Carlo code. These evaluations were performed by placing the sources at different locations inside a 30×30×30 cm3 cubical water phantom and comparing the results with the values of the source located at the center of the phantom, which is in a full scattering condition. The differences between the thickness of the overlying tissues for different source positions and the thickness of the overlying tissue in full scattering condition is referred to as missing tissue. The results of these investigations indicate that values of the radial dose function and 2D anisotropy function vary as a function of the thickness of missing tissue, only in the direction of the missing tissue. These changes for radial dose function were up to 5%, 11%, and 8% for 137Cs, 192Ir, and 103Pd, respectively. No significant changes are observed for the values of the dose rate constants. In this project, we have demonstrated that the TG‐43 dosimetric parameters may only change in the directions of the missing tissue. These results are more practical than the published data by different investigators in which a symmetric effect of the missing tissue on the dosimetric parameters of brachytherapy source are being considered, regardless of the implant geometry in real clinical cases.PACS number: 87.53.JW

Highlights

165 Zehtabian et al.: Perturbation of Task Group 43 (TG-43) parameters available during the preparation of the original report
These results indicate the values of radial dose function of 103Pd at a distance of 5 cm from the source center on the side away from the missing tissue is larger by up to 7.78% than the side with missing tissue
TG-43 dosimetric parameters of the brachytherapy sources are normally designed for a phantom geometry that provides full scattering conditions

Summary

Introduction

165 Zehtabian et al.: Perturbation of TG-43 parameters available during the preparation of the original report. It should be noted that all of these investigations were based on the assumptions that the brachytherapy source was placed at the center of a phantom of varying size, with the same amount of the phantom materials in all directions These assumptions do not resemble many clinical procedures such as interstitial and balloon brachytherapy implants in breast, interstitial implant of head and neck, and 125I and 103Pd eye plaque therapy. The impact of the thickness of missing tissue at one side of the implant on the dose distribution of the treatment volume remained unresolved

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Conclusion