Evaluation of the dosimetric parameters for 125I brachytherapy determined in prostate medium using CT images

In the present study, the prostate medium determined from the CT images of 149 patients was developed. The dosimetric parameters such as Λ, gL(r) and F(r, θ) used in TG-43U1-based calculation for an iodine-125 (125I) brachytherapy-source were examined using Monte Carlo code Geant4. Clinical dosimetry parameters such as the D90 were evaluated among a subgroup of 50 randomly selected patients who had been treated with permanent brachytherapy between January 2008 and December 2008 at the Tokyo Medical Center. The results show a slight difference in the dose rate constant Λ (within 1.0%). The radial dose function gL(r) exhibits a prominent difference in the region over 3 cm, and this difference is maintained within 2.9% in the region close to the source. The calculated values of F(r, θ) for the prostate medium were similar to values for water (within 1%), except in the longitudinal axis. A comparison of D90 values shows a systematic dose overestimation of 2.8±0.7 Gy in water, where the distribution of the differences can be seen with a spread of 1.8±0.3% compared to that in prostate medium. It was concluded that the introduction of any kind of tissue correction for the TG-43U1-based calculation was not necessary to allow for the differences in elemental compositions and densities between water and prostate medium. (author)