Local ablative radiotherapy: A means to revert low volume castration-resistant prostate cancer into a hormone-sensitive status?

188 Background: (Stereotactic) Ablative radiotherapy (aRT) in patients with oligometastatic prostate cancer is safe, achieves high local control rates and its clinical benefit is currently being addressed in clinical trials. However, the selection of patients benefitting most from aRT remains unclear. Moreover, the highly specific and sensitive multimodality functional imaging modality, 68Ga-PSMA-PET-CT/-MR, is widely available. Therefore, we retrospectively evaluated the efficacy of PSMA-PET guided local aRT in low volume oligometastatic castration resistant prostate cancer (CRPC) patients. Methods: Fifteen patients with metachronous oligometastatic CRPC as diagnosed on PSMA-PET were treated with local aRT. Nine patients were treated with a conventionally fractionated regime (25 x 2 Gy) and six with stereotactic hypofractionated RT (3 x 10 Gy), while in all androgen deprivation was continued. The time to PSA-progression, i.e. PSA-nadir + 2 ng/ml (PSA+2) was estimated with the Kaplan-Meier method and compared to an individually estimated time to PSA+2 according to the individually calculated pre-RT PSA doubling time (PSA-DT). Results: At staging PSMA-PET, the median PSA was 3.4 ng/ml (range 1.3 -14.5) and the median PSA-DT was 3.2 months (range 0.6 - 15.3). In four patients no PSA-response after aRT was observed. The responding eleven patients (73%) had a mean decrease in PSA-level of 80% from baseline. The mean time to PSA-nadir was 10.7 months (range 4.5 - 11.5). The mean time to PSA+2 or last follow-up was 15.6 months [95%CI: 9.7 - 21.4] compared to 3.3 months [95%CI: 1.5 - 5.1] estimated PSA-DT without local aRT (p < 0.001). Conclusions: A relevant subset of patients with 68Ga PSMA-PET-detected oligometastatic low volume CRPC had a meaningful PSA-response with aRT. They were reverted into an earlier stage of their disease again. A prospective clinical trial on this clinically highly relevant question is being prepared.