Predicting radial dose function of LDR brachytherapy sources in the calcified soft tissues through Monte Carlo simulation

This study aimed to investigate the effect of tissue calcification on the dose distribution of low-energy brachytherapy seeds used in the treatment of prostate and breast tumors. To achieve this, simulations of the IR-¹²⁵I and¹³¹Cs-1 seeds were conducted using the MCNPX Monte Carlo (MC) code. Brachytherapy dosimetric parameters, such as the dose rate constant (Λ) and radial dose function (g (r)), were calculated in water following the TG-43U1 protocol and validated by comparing the results with data reported in other literature. The findings revealed significant uncertainty in the g(r) for calcified tissue of the prostate, adipose, and glandular breast compared to normal tissue. The g(r) value showed a notable increase in the vicinity of the seeds (within 1 cm from the center of the seeds), followed by a decrease. This trend was observed in all three tissues. The higher the percentage of calcification in the tissue, the more pronounced the changes. The g(r) increased by a factor of 266 and 102 for IR-¹²⁵I, and¹³¹Cs-1 seeds, respectively, in the 100% calcified prostate. The trend of g(r) changes aligned with the trend of changes in the mass attenuation coefficients calculated for the tissues at different calcification percentages. An exponential function (Aexp(-r/t)) was derived to predict g(r) across various calcification percentages. Additionally, the insights gained from this study may apply to other soft tissues.