Outcomes & predictors of progression: SBRT for lymph node oligorecurrent prostate cancer on PSMA-PET

Abstract

Purpose or Objective To describe outcomes and identify predictors of progression for patients with lymph node oligorecurrent prostate cancer diagnosed on PSMA-PET and treated with stereotactic body radiotherapy (SBRT). Materials and Methods Patients from a prospective cohort study were included if they had 1-3 lymph node metastases diagnosed with PSMA-PET after initial treatment for primary prostate cancer and had not received hormonal therapy in the preceding 24 months. Patients were treated with 5 x 7 Gy or 3 x 10 Gy using MRI- or CBCT-guided SBRT. Acute toxicity was assessed until 3 months post-SBRT. PSA was measured pre-SBRT, after 3 months and then at the discretion of the referring physician. QoL questionnaires were sent pre-SBRT and at 1 and 4 weeks, 3 and 6 months after SBRT and then every 6 months. Clinical and treatment-related variables were assessed for their influence on progression free survival (PFS) using Kaplan-Meier analyses and Cox-regression. Multiple imputation was used for missing data. Progression was defined as a local recurrence (>20% increase in short axis diameter of a target lesion), a newly diagnosed metastatic site, biochemical progression (PSA nadir + 2 ng/mL and >25% increase) or start of ADT. Secondary outcomes were ADT-free survival, biochemical PFS (BPFS), toxicity and QoL. A preliminary risk score was created based on quantiles of the linear predictor from multivariable analysis for PFS. Results 92 patients were included, median follow-up was 26 months. 54 (59%) patients developed at least one progression: diagnosis of a new metastasis in 48 (52%) patients, biochemical progression in 40 (43%) patients and start of ADT in 17 (18%) patients. No local recurrences were observed. Median PFS and BPFS were 15.5 and 20.9 months, respectively. Median ADT-FS was not reached, ADT-FS at 24 months was 73% (95%-CI 62-86%). After correcting for age in multivariable analysis, higher PSA prior to SBRT was significantly associated with worse PFS (Table 1). The risk score, based on multivariable analysis, is depicted in Figure 1. It subdivides patients in low, intermediate and high risk groups, for which significant differences in PFS were observed. Two (1.7%) and five (5.4%) patients experienced acute and late grade 2 genitourinary toxicity, respectively. Grade 1 fatigue was the most predominant acute toxicity (n=40, 34%). No ≥ grade 3 toxicities were observed. QoL analysis only showed a mild increase in fatigue at 1 and 4 weeks after SBRT (median EORTC-C30 fatigue increased from 11 to 22 at 4 weeks compared with baseline, on a 100-point scale). This was normalised again at 6 months and other QoL aspects were unaffected. Conclusion Median PFS of >12 months was attained in patients with lymph node oligometastatic prostate cancer who have been diagnosed on PSMA-PET and treated with image-guided SBRT. Higher pre-SBRT PSA and younger age were found to be predictors of shorter PFS. Toxicity was minor and only transient mild fatigue was observed in quality of life analysis.