When treatment with pADT is initiated in case of mHSPC, the sensitivity to castration will eventually disappear due to the out-selection of castration-refractory clones. At that moment, the stage of mCRPC is attained. In the setting of mCRPC, clinical and iconographic progression (and to a lesser extent biochemical progression) traditionally implies a switch to a next-line systemic treatment (NEST). However, within the group of these progressive patients, there is a subgroup showing oligoprogressive disease, which is defined as the progression of a limited number of metastatic spots, whereas the majority of metastases is controlled by the systemic therapy the patient is receiving at that time. This heterogeneous response to treatment reflects the heterogeneity of the clonogenic cells that give rise to mCRPC. The hypothesis of this trial is that treatment with MDT of these oligoprogressive lesions allow patients to remain on their current systemic therapy, thereby delaying the need for NEST.

A multicentric retrospective chart analysis on MDT for oligoprogressive CRPC has been conductedat previously the UZ Leuven together with UZ Gent (doi: 10.1016/j.euo.2019.08.012.). A total of 30 patients with oligoprogressive CRPC were selected for further analysis. In this population, after a median follow-up of 17 months (IQR 9;25), the median NEST-FS in the MDT group was 16 months (95% CI: 10-22 months). The median progression free survival (PFS) was 10 months (95% CI: 6-15 months). Subsequently, the investigators selected within the MDT group those cases who received SBRT or surgery (metastasectomy), as these patients will be the subjects in this phase 2 trial. There was a median NEST-free survival of 21 months (CI 95% 12-21 months) and a median PFS of 10 months (95% CI: 6-14 months). SBRT or surgery-related toxicity was minor, with limited grade 1 and 2 toxicity and only one patient experiencing late grade 3 GU/GI toxicity after treatment for local relapse in the prostate.

These findings of this retrospective analysis clearly show the need for further prospective investigation. Therefore, this prospective phase 2 trial was initiated in patients with oligoprogressive mCRPC who will receive MDT. If the primary tumor has not been treated yet, local treatment will be added. Ongoing systemic treatment will be continued until progression. The trial is explorative and still hypothesis generating. The results from this trial will serve as a guidance for a randomized phase 3 trial in the near future.