Effect of photon energy spectrum on dosimetric parameters of brachytherapy sources

Abstract

Aim: The aim of this study is to quantify the influence of the photon energy spectrum of brachytherapy sources on task group No. 43 (TG-43) dosimetric parameters. Background: Different photon spectra are used for a specific radionuclide in Monte Carlo simulations of brachytherapy sources. Materials and methods: MCNPX code was used to simulate ¹²⁵I, ¹⁰³Pd, ¹⁶⁹Yb, and ¹⁹²Ir brachytherapy sources. Air kerma strength per activity, dose rate constant, radial dose function, and two dimensional (2D) anisotropy functions were calculated and isodose curves were plotted for three different photon energy spectra. The references for photon energy spectra were: published papers, Lawrence Berkeley National Laboratory (LBNL), and National Nuclear Data Center (NNDC). The data calculated by these photon energy spectra were compared. Results: Dose rate constant values show a maximum difference of 24.07% for ¹⁰³Pd source with different photon energy spectra. Radial dose function values based on different spectra were relatively the same. 2D anisotropy function values show minor differences in most of distances and angles. There is not any detectable difference between the isodose contours. Conclusion: Dosimetric parameters obtained with different photon spectra are relatively the same, however it is suggested that more accurate and updated photon energy spectra be used in Monte Carlo simulations. This would allow for calculation of reliable dosimetric data for source modeling and calculation in brachytherapy treatment planning systems.