HDR brachytherapy seems to be a valid option for low and intermediate risk prostate cancer. The purpose is to determine acute toxicity of one 19.5 Gy fraction or two 14.5 Gy fractions of HDR brachytherapy monotherapy delivered on a one day schedule. We recruited 205 patients with low or intermediate-risk prostate cancer. 199 were eligible and randomized between one fraction of 19.5Gy (Arm 1) or two fractions of 14.5Gy (Arm 2) (BRP2 protocol, NCT03424694). In both arms treatments were delivered with a single implant on a one-day schedule. For all patients dosimetry was MRI based with dominant intra prostatic lesion boots allowed. Toxicity was evaluated at 1, 3, 6 weeks and 3 months after treatment. CTCAE v4.0, IPSS and IIEF-5, were used to evaluate toxicity at each visit and compared to baseline. paired t test, Pearson Chi-Square and Fisher Exact test were used. Statistical significance was 0.05. Median Follow-up is 20 (3-42) months. Pre-treatment median PSA for arm1 and arm 2 were 6.53±2.76 and 6.68±2.67 respectively (p=0.71). T stages for arm 1 and arm2 were T1c = 57.6% and 55%, T2a = 29.3% and 32%, T2b = 9.1% and 13%, T2c 4% and 0 % respectively. Gleason score were 6(3+3) 18.2% and 13% ,7(3+4) 54.5% and 58%, 7(4+3) 27.3% and 29% respectively (p=0.60). For arm 1 and arm 2, post treatment CTCAE urinary toxicity were: at 6 weeks grade 1 13% and 16.2% respectively (p=0.517), grade 2 only 1% and 3% (p=0.699). The most frequent symptoms at 6 weeks were urinary frequency. Urinary retention was 0% vs 12.5% respectively (p=0.699). At 12 weeks CTCAE urinary toxicity were 6.1% and 9% grade 1 and 2% and 1% grade 2 for arm 1 and 2 respectively (p=0.693). Urinary frequency was the most frequent symptom. Few patients had gastrointestinal toxicity. At 6 weeks 4% and 5% had CTCAE grade 1 (p=1). At 12 weeks 2 patients in arm 1 and 1 patient in arm 2 presented anal pain and hemorrhoids For sexual function IIEF-5 score for arm 1 and arm 2 were 12.98±6.77 and 14.85±6.78 at baleine compared to 12.63±6.9 and 12.21±6.59 at 3 months. There is a significant statistical decrease between IIEF-5 at baseline and at 12 weeks post treatment values in arm 2 (p<0.001). There is no difference for arm 1 (p=0.097). In this HDR brachytherapy as monotherapy protocol both arms, either the single fraction of 19.5Gy or 2 fractions of 14.5Gy, are well tolerated. Acute GI and GU toxicities are the same in both arms. IPSS score returns to baseline at 3 months. At this point he statistical difference on erectile scores in arm 2 has no clinical significance. Further report will follow.Abstract 2652; Table 11 fraction2 fractionspmeanSDmeanSDIPSS Baseline6.03.76.33.80.664IPSS 1 week post treatment11.47.711.87.90.711IPSS 3 week post treatment8.55.79.46.20.284IPSS 6 week post treatment7.05.27.75.00.393IPSS 3 months post treatment6.25.26.95.20.372baseline vs 12 weeksp=0.524p=0.182 Open table in a new tab