This is a pilot prospective biomedical study of interventional type which includes 17 patients on 24 months (12 months of inclusion and 12 months of follow-up).

The objective of this study is to verify that the focal therapy technique used (with the help of Koelis® system) allows to obtain optimal dosimetric coverage of the prostate target (ie dose of 160 Gy +-5% delivered on the envelope isodose) evaluated by CT scan performed 30 days after implantation.

Inclusion Criteria:

1. Men of more than 18 years old

2. Patient with localized prostate cancer with a good prognosis (AMICO classification), ie satisfying the following conditions according to the urologist medical record transmitted:

   • Lesion classified T1c or T2a based on digital rectal exam
   • histologically proven diagnosis of prostatic adenocarcinoma with a Gleason score ≤ 6
   • Serum Prostatic Specific Antigen (PSA) <10ng/ml

3. Patient for whom the Multidisciplinary Consultative Meeting (RCP) has adopted the following treatment options: active surveillance, brachytherapy, radical prostatectomy or external radiation

4. Patient with good micturating function at inclusion, defined by IPSS score <10 (IPSS Questionnaire)

5. Patient for whom the result of centralized PSA assay confirms a serum level < 10ng/ml

6. Patient for whom the unique and intracapsular character of the target area (area to be treated) was confirmed on pre implantation multimodal MRI imaging (ESUR Score ≥ 9) and whose major axis has a size ≤ 20 mm

7. Patient with a Gleason score on the target ≤ 6 (3 +3) confirmed on biopsies performed using KOELIS ® system and with no other derogatory criteria such as the invasion of the entire core or the presence of a grade 4 or perineural emboli

8. Patients without history of transurethral resection that could have significantly modified the anatomy of the gland nor obstructive adenoma

9. Patient who accepts, at the end of the study, the principle of active surveillance for the rest of the gland and the treated area according to the current standard protocol

10. WHO ≤ 2

11. Patient with life expectancy > 10 years

12. Informed consent obtained and signed before any specific procedure in the study

13. Patient affiliated to social security regimen

Exclusion Criteria:

1. Image in favour of the crossing of the capsule, based on pre-implantation multimodal MRI (stage T2 MRI only)
2. Image in favor of the invasion of the seminal vesicle, based on pre-implantation multimodal MRI (stage T2 MRI only)
3. Multifocal lesions (ESUR ≥ 9/15) to the pre-implantation multimodal MRI and for which biopsies have shown the cancerous nature
4. Lesion with larger diameter ≥ 20mm, to the pre-implantation multimodal MRI
5. Patient who requires pre-implantation hormonal treatment in order to reduce prostatic volume
6. Patient with current indication against prostate brachytherapy, including a significant limitation of the mobility of the hips, a prostate volume greater than 60 cm3 (measured by planimetry MRI) or a significant dysuria (IPSS ≥ 10)
7. Patient unable to follow procedures, visits, examinations described in the the study
8. Patient with absolute indication against imaging tests (significant claustrophobia, wearing a heart valve, pacemaker, ..)
9. Man of childbearing age who do not want follow the instructions about sexual activities and condom use during the days following the treatment of brachytherapy and / or unwilling to hold (him or her partner) effective contraception for the duration of the study
10. Any concomitant or previous malignant disease in the past five years with the exception of superficial basal cell carcinoma or non-metastatic of the skin
11. Any prior systemic chemotherapy within 5 years prior to inclusion for malignant disease in the medical history
12. Any coexisting medical condition that in the opinion of the investigator could be a risk in this study
13. Patient protected by law