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

The purpose of this escalation study is, first to select the recommended dose of re-irradiation SBRT in combination with Metformin (based on treatment toxicity monitoring) and then to estimate the efficacy of re-irradiation SBRT in combination with Metformin. Five or six fractions, at a level of 5 or 6 Gray (Gy) per session (either 5 x 6 Gy, 6 x 6 Gy, or 5 x 5 Gy), will be delivered over a maximum of 12 days (from day 1 to day 10 or 12) to provide a total dose of 25 to 36 Gy. Patient receive oral Metformin treatment from Day -15 and Day 75. Patient will be followed for 5 years: patients visits will be planned at week 2; 4; 8; 12; and month M6; M9; M12; M18; M24; M36; 4 years and 5 years.