Phase 1-2 study, comparing ultra-hypofractionnated (UH) to a moderately hypofractionnated (MH) radiation therapy, with image guided HDR prostate brachytherapy. Using iso-equivalent doses, a non-inferiority analysis will be done in order to prove UH non-inferior to MH, toxicity wise. Acceptability, tolerability, acute and late toxicity will be reported. MRI visible dominant intra-prostatic lesion will be outlines and variability between radiation oncologists and radiologists will be reported. As secondary objective, biochemical and clinical failure free survival will be reported at 5 \& 10 years.