Can a tiny beacon make prostate radiation safer?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Localized prostate cancer, histologically proven. * No evidence of metastases (M0). No evidence of lymph nodes involvement (N0) (bilateral lymph node dissection is not mandatory if lymph node involvement risk is low according to the Partin tables). * Low-risk or intermediate clinical stage according to the D'Amico classification (T1-T2 and Gleason \< 8 and PSA \< 20 ng/ml) (appendix 3). * No grade \> 2 urinary or rectal clinical sign or symptom according to the CTCAE V4.03 scale. * Performance status ECOG ≤ 1. * No hip prosthesis or metallic vascular graft near the prostate. * No endopenian stent. * No pace maker, implanted defibrillator or neurostimulator. * No allergy to local anesthetics. * No irreversible anticoagulation or antiplatelet treatment for the implantation period. * Pelvic and abdominal anatomy compatible with the use of the Calypso® system (predictive detector to fiducials distance less than 19 cm, evaluated on planning CT-scan) (distance from skin surface to prostate center less than 17 cm). * Patient aged ≥ 18 and less than 80 years old. * Dated and signed written informed consent available. * Patients must be affiliated to a French Social Security System. Exclusion Criteria: * Indication of pelvic nodes irradiation. * Prior pelvic irradiation. * Biopsy-proven seminal vesicle invasion. * Prior bilateral orchiectomy. * Prior radical prostatectomy. * Other malignancy except adequately-treated basal cell carcinoma of the skin or other malignancy from which the patient has been disease-free for at least 5 years. * Psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions should be discussed with the patient before registration in the trial. * Other uncontrolled systemic disease (cardiovascular, renal, liver, pulmonary embolism, etc.). * Known VIH positive patients (no specific test needed). * Known homozygote ATM Mutation (Ataxia telengiectasia).