Evaluation of the Dosimetric Parameters for ^<125>I Brachytherapy Determined in Prostate Medium using CT Images

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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. PACS number: 87.00.00; 87.55.dk; 87.55.K-; 87.56.B-
2010-09-16