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Purpose: To assess the outcomes of a cohort of bone oligometastatic prostate cancer patients treated with PSMA-PET guided stereotactic body radiotherapy (SBRT)

Methods: From April 2017 to January 2021, 40 patients with oligorecurrent prostate cancer detected by PSMA-PET were treated with SBRT for bone oligometastases. Concurrent androgen deprivation therapy was an exclusion criterion for the purpose of this study. All treatments performed with image guided volumetric modulated arc therapy (IGRT-VMAT). A total of 56 prostate cancer bone oligometastases were included in the present analysis. Median age was 69.5 years (range, 54-85 years). Oligometastatic disease occurred after a median interval of 39 months (2-244 months) from the primary treatment, with a median PSA doubling time of 6.7 months (1.1-40.8 months) and a baseline PSA=0.60 ng/ml (range, 0.16 – 15.2 ng/ml). In 28 patients (70%), oligometastatic disease presented as a single lesion, two lesions in 22.5%, three lesions in 5%, four lesions in 2.5%.

Results: Among the 56 lesions, 30.3% were spine-metastases, while 69.7% were non-spine metastases. SBRT was delivered for a median dose of 30 Gy (24-40Gy) in 3-5 fractions, with a median EQD₂=85 Gy₂ (64.3 - 138.9Gy₂). With a median follow-up of 22 months (range, 2-48 months), median local control (LC) was 18 months (2-48) with 1- and 2-years rates of 96.3% and 93.9%. The median nadir PSA after SBRT was 0.9 ng/ml (0.36-13.8 ng/ml), with twelve patients who did not report a PSA drop after SBRT. Our 1- and 2-years distant progression-free survival (DPFS) rates were 45.3% and 27% for a median time interval of 9 months (3-37 months). At univariate analysis, a longer time to oligometastases onset favorably impacts on DPFS (p-value=0.0003); similarly, for lower number of treated metastases (p=0.003), lower PSA pre-SBRT (p=0.0013) and PSA nadir values after SBRT (p<0.0001). Also, patients who kept LC of the treated lesions maintained an advantage in terms of DPFS (p=0.017). At multivariate analysis, the lower PSA nadir value after SBRT remained significantly related to better DPFS rates (p=0.03). In 7 patients, a second SBRT course was proposed with concurrent ADT, while 11 patients, due to the evidence of polymetastatic spread, received ADT alone, thus resulting in 1- and 2-years ADT-free survival rates of 67.5% and 61.8%, for a median ADT-free survival time of 13.5 months (2-45 months).
At univariate analysis, ADT-free survival was found to be significantly related to lower number of treated oligometastases (p=0.0001), a longer disease-free interval (p=0.0097) and lower PSA values before (p<0.0001) and after SBRT (p=0.004). At multivariate analysis, the number of treated oligometastases maintained a correlation with higher ADT-free survival rates (p=0.04).

Conclusions: In our experience, PSMA-PET guided SBRT resulted in excellent results in terms of clinical outcomes, representing a helpful tool with the aim to delay the start of ADT.