•Boost to DIL with permanent seed prostate implant shows a trend toward improvement of biochemical control in intermediate risk cancer patient.•Our data support previous studies which demonstrated no increased toxicity with DIL boost technique in LDR brachytherapy.•This study presents the largest population with intermediate risk prostate cancer patient treated with DIL technique and low dose brachytherapy in the literature. To compare bDFS and toxicity outcomes in a population of intermediate risk prostate cancer patients treated using I-125 LDR brachytherapy with or without DIL boost based on multiple core biopsy maps. Between January 2005 and December 2013, all our intermediate risk prostate cancer patients treated with LDR I-125 brachytherapy were reviewed. All patients were given 144 Gy to the prostate. A pathologic DIL distribution (defined by sextant biopsy) was contoured prospectively prior to planning, to be covered by the 150% isodose line. Of the 165 patients treated, 55 received a DIL boost. Patients completed prospectively the IPSS questionnaire, a sexual and bowel function questionnaire. Gastro-intestinal toxicities were graded according to CTCAE v4.03. A patient was considered to have erectile dysfunction if he was unable to achieve erection to perform intercourse. BDFS was determined according to the Phoenix consensus definitions. The median follow-up was 78 months. The estimated 7-year bDFS rate was 96% (95% CI, 74–99%) in the DIL group versus 89% (95% CI, 79–94%) in the control group (p = 0.188). There was no difference between groups in urinary, gastro-intestinal or sexual toxicities up to 5 years of follow-up. There was no difference in urinary obstruction with catheterization between DIL versus control groups (3,6 vs 2,8 %, p = 1.00). Only 1 patient in the DIL group had ≥grade 3 toxicity (TURP) and none in the control group. Boost to DIL defined by sextant biopsy with permanent seed prostate implant shows a trend toward improvement of biochemical control in intermediate risk prostate cancer patient without increasing toxicity.