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

Purpose: The dose calculation in most of the current treatment planning systems are based on the recommendations of TG‐43U1 in water phantom not considering the heterogeneity effects. The purpose of this study is developing a method for heterogeneity corrections for Cs‐137 brachytherapy sources based on pre‐calculated MC simulations and interpolation. Methods: To simulate the effect of phantom heterogeneity on dose distribution around Cs‐137 sources, spherical water phantoms were simulated in which 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. Tally cells used in this study, were spherical shells with 0.1cm thickness at different distances from 0.1cm to 10cm. The dose at each tally cell around the Cs‐137 point source was obtained by tally type F6. Results: The results of the simulations indicate that for the bone layers with the same thickness, located at different distances, the patterns of dose difference are different. The percentage differences between the dose in heterogeneity section with the dose at the same position inside the homogeneous water phantom are 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 % dose difference for each unknown configuration (different thickness of inhomogenity at different distances from the source) was estimated using the 2D interpolation of MATLAB. According to the results, the algorithm used in this study, is capable of dose estimation with the percentage difference of less than 4%. Conclusion: The algorithm developed using the results of Monte Carlo simulations and the dose Interpolation, can be used in treatment planning systems for heterogeneity corrections.