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Objective: To evaluate the dose-response relationship (D90 >144 Gy: probable absence of biochemical failure) in patients with prostate cancer treated by iodine-125 (I-125) brachytherapy., Material and Methods: From May 1999 to December 2006, 150 patients were treated by I-125 brachytherapy. The median follow-up was 60 months. All patients had clinical stage T1-T2, PSA < or =10 ng/ml, Gleason Scores < or = 3+3=6, IPSS >14 ml/sec. and prostate weight <50 gr. Implantation was ultrasound-guided, using a real-time technique and loose seeds of I-125 (dose 160 Gy). After 30 days, a post-implantation assessment was performed by pelvic CT scan for a definitive evaluation of the D90. All patients were subjected to clinical evaluation, PSA dosage and compilation of IPSS and IEFF questionnaires. In the event of biochemical failure (ASTRO), a prostate biopsy was performed. A D90 >144 Gy was considered the cut-off in order to predict the absence of biochemical failure., Results: Biochemical failure was observed in 9 patients: 5 with positive and 4 with negative prostate biopsies. The D90 >144 Gy cut-off was not achieved in 18 patients at the post-implantation assessment, however only 2 of them (one of whom had a positive biopsy) had biochemical failure (11.1%). On the other hand, only 2 of the 9 patients with biochemical failure had a D90 < 144 Gy while 6 patients had D90 >150 Gy, 5 with positive prostate biopsies., Conclusions: In our experience, the D90 >144 Gy cut-off does not seem to predict, in a reliable way, the control of prostate cancer following brachytherapy. Limitations of the analysis were the number of the patients, the learning curve, dosimetry processing and the relatively short follow-up.