The dose received by 90% of the prostate volume (D90) is the key parameter of dosimetric analysis in prostate brachytherapy. The aim of this analysis was to identify preimplant factors affecting prostate D90 after transperineal interstitial prostate brachytherapy with loose 125I seeds. We reviewed the records of 210 patients who underwent transperineal interstitial prostate brachytherapy with loose 125I seeds for clinical T1/T2 prostate cancer at our institution. Patients who received supplemental external-beam radiation therapy were excluded. One hundred and nine patients (51.9%) received neoadjuvant hormonal therapy (NHT). One month after seed implantation, postimplant computed tomography and dosimetric analysis were performed. Univariate and multivariate analyses were carried out to identify preimplant factors affecting postimplant prostate D90. The postimplant prostate D90 values ranged from 123.3 to 234.1 Gy (mean ± standard error, 177.1 ± 1.4 Gy). Postimplant prostate D90 differed significantly between patients who had and had not undergone NHT (P = 0.001). In addition, simple regression analyses showed positive correlations with the estimated preimplant prostate D90, preimplant prostate volume by transrectal ultrasound (TRUS), total radioactivity, number of needles, and number of seeds. On stepwise multiple regression analysis, postimplant prostate D90 showed significant negative correlations with NHT and preimplant prostate volume by TRUS, and a significant positive correlation with total radioactivity. In conclusion, NHT, preimplant prostate volume by TRUS, and total radioactivity are significant preimplant factors affecting postimplant prostate D90 in prostate cancer patients treated with transperineal interstitial prostate brachytherapy with loose 125I seeds.
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