Relapse in Previously Irradiated Prostate Bed : Stereotactic Ablative Reirradiation Potentiated by Metformin

Inclusion Criteria: * Written informed consent according to International Conference on Harmonisation (ICH)/ Good Clinical Practice (GCP) regulations before registration and prior to any trial specific procedures. * Biochemical recurrence occurring at least 2 years after external radiotherapy to the prostate lodge and the end of hormone therapy, for prostatic adenocarcinoma previously treated by radical prostatectomy. * Local recurrence in irradiated areas proven by biological (PSA \> 0.2 ng/ml and ascending confirmed by at least 2 successive assays) and radiological (lesion visible on MRI and/or Choline PET and/or Prostate-Specific Membrane Antigen (PSMA) PET). * Recurrence without rectal invasion * Pelvic and prostate MRI evaluation * Absence of pelvic lymph node or metastatic recurrence proven by choline PET or PSMA PET scan * World Health Organisation (WHO) performance status 0-1 * Low risk, intermediate risk and high risk with a single risk factor * PSA doubling time \> 6 months * No anti-cancer treatments planned for the current relapse, including hormone therapy. * Age \> 18 years old. * Life expectancy greater than or equal to 5 years. * Patient registered with a health insurance system. * Patients willing and able to comply with the planned visits, treatment plan, laboratory tests and other study procedures indicated in the protocol. Exclusion Criteria: * Metastatic disease (bone, lymph node or other) * Late radiotherapy urinary or gastrointestinal toxicity (grade ≥ 2) (after radiotherapy of prostate lodge) * History of cancer in the 5 years prior to trial entry other than cutaneous basal cell carcinoma * Inflammatory bowel disease * Contraindications for performing MRI * Rectal surgery history * Patient treated for Diabetes * Creatinine clearance \< 45 mL/min * Treatment with metformin in the last 3 months prior to inclusion * Severe comorbidity that may affect treatment, for example : * Acute bacterial or fungal infection requiring intravenous antibiotics at the time of inclusion. * Unstable angina, myocardial infarction and/or congestive heart failure requiring hospitalization within the last 6 months * Myocardial infarction in the last 6 months. * Exacerbation of Chronic Obstructive Pulmonary Disease (COPD) or other respiratory conditions requiring hospitalization or preventing metformin therapy at the time of inclusion. * Any condition associated with an increased risk of lactic acidosis (e.g., alcohol abuse, New York Heart Association (NYHA) III or IV congestive heart failure). * Clinically significant history of hepatopathy with Child-Pugh B or C score, including viral infection or hepatitis, alcohol abuse or cirrhosis. * Any acute or chronic condition that may result in tissue hypoxia (e.g. heart or respiratory failure, shock). * Bilateral hip prosthesis * Treatment with any investigational drug or participation in a clinical trial within 30 days prior to inclusion. * Known hypersensitivity to metformin or any of its components * Inability or reluctance to swallow oral medications * Persons deprived of liberty, under a measure of safeguard of justice, under guardianship or under the tutor authority * Inability to undergo medical monitoring of the trial for geographical, social or psychological reasons.