A randomized, multicenter, prospective PHASE II trial to assess the effect of short- versus long-term adjuvant ADT with high dose salvage radiotherapy on distant metastasis free survival in case of biochemical relapse (BR) after radical prostatectomy.

Radical prostatectomy (RP) is one of the standard treatment options for localized and locally advanced prostate cancer. RP should be combined with an extended pelvic lymph node dissection (ePLND) when the risk of pelvic involvement becomes substantial, at least 7%. In case of node-negative disease (pN0), adverse pathological features at examination of the RP-specimen such as extra-capsular extension (ECE), seminal vesicle invasion (SVI) and positive surgical margins (PSM) increase the risk of biochemical relapse (BR) and/or isolated local relapse (LR). Both a BR and LR, if not adequately treated, can finally result in the development of distant metastasis. In case of BR and/or LR, salvage radiotherapy (SRT) is the only treatment option with curative intent. Several factors play a significant role in predicting the outcome after SRT: Gleason score, pre-SRT PSA, pre-SRT doubling time, SVI, SRT dose and duration of adjuvant androgen deprivation therapy (ADT) associated with SRT. The 2 latter variables have never been tested in a randomized controlled trial. The GETUG-AFU 16 trial randomized between no vs. 6 months ADT while patients received 66 Gy to the prostate bed and 46 Gy to the pelvis. Moreover, the pN0 status was not needed for inclusion. Also the RADICALS trial is currently running this comparison with a radiation dose of 66 Gy to the prostate bed and also no information on pN status is needed to be included in this study. In the LOBSTER study, the pN0 status is obligatory and the prescription dose is set at 70 Gy to the prostate bed and seminal vesicles. These conditions make this study unique compared to other already conducted and currently running trials. In previous work, it has been demonstrated that ADT, added for 6 months to SRT, significantly improved biochemical relapse free survival at 5 years. Added to this, there is recent evidence that using a longer schedule of adjuvant ADT might be beneficial when compared to a 6-months schedule. Unfortunately, none of these suggestions are based on evidence coming from a randomized controlled trial. Therefore, this randomized phase 2 trial 'LOBSTER' is conducted, comparing 6 versus 24 months of adjuvant ADT together with high-dose SRT in case of BR after RP in pN0 prostate cancer patients