SU-E-T-539: Developing a Method for Dose Heterogeneity Corrections for Cs-137 Brachytherapy Sources

Abstract

Background Most of the current brachytherapy treatment planning systems (TPS) use the TG-43U1 recommendations for dosimetry in water phantom, not considering the heterogeneity effects. Objective The purpose of this study is developing a method for obtaining correction factors for heterogeneity for Cs-137 brachytherapy sources based on pre-calculated MC simulations and interpolation. Method To simulate the effect of phantom heterogeneity on dose distribution around Cs-137 sources, spherical water phantoms were simulated in which there were spherical shells of bone with different thicknesses (0.2cm to 1.8cm with 0.1cm increment) at different distances (from 0.1cm to 10cm, with 0.5cm increment) from the source center. The spherical shells with 0.1cm thickness at different distances from 0.1cm to 10cm were used as tally cells. The doses at these cells were obtained by tally types F6, *F8, and *F4.The results indicate that the percentage differences between the doses in heterogeneity sections with the dose at the same positions inside the homogeneous water phantom vary when the distance of bone section from the source center increases, because of decreasing the average energy of photons reaching the bone layer. Finally, the results of Monte Carlo simulations were used as the input data of MATLAB software, and the percentage dose difference for each new configuration (i.e. different thickness of inhomogenity at different distances from the source) was estimated using the 2D interpolation of MATLAB. ResultsAccording to the results, the algorithm used in this study, is capable of dose estimation with high accuracy. Conclusion The developed method using the results of Monte Carlo simulations and the dose interpolation can be used in treatment planning systems for heterogeneity corrections.

Estimation and Evaluation of Tissue Inhomogeneity Effect on Dose Distribution for High Dose Rate Iridium 192 Source Using Monte Carlo Simulation and Film Dosimetery

“…Also, the radioactive sources that generally are utilized for brachytherapy such as 125 I, 103 Pd, 137 Cs and 192 Ir emit lower energy photons than those used in external radiotherapy. This cause more importance of inhomogeneity effect in brachytherapy treatments [10,11]. One of the most important moot points in brachytherapy, as external radiotherapy method, is dose distribution in patient's body.…”

“…Hence, accuracy of dose calculation in brachytherapy is a high-priority and effective factor to patient treatment that strongly depends on the mass effective attenuations coefficients used in TPS algorithm [3,12]. On the other hand, dose and dose rate distribution are pertained to probability of photoelectric and compton interactions of the photons [10]. This probability in homogenous medium depends on photon energies, atomic number, density and electron density.…”

“…where S k is air Kerma strength, Λ is dose rate constant, G L is geometric function, g(r) is radial dose function and F(r, θ) is anisotropic function. This formulism also assumed that patient body tissue in and around the implant region is an unbounded homogeneous medium water equivalent and mass energy absorption coefficient as well as dose distributions curves are functions of this information [4,10,15,16]; while, human body is not a homogenous medium and has been formed of different layers of fat, muscle, bone, etc. Each of them has a different density and atomic number.…”