ESTRO 2021 Abstract Book

S574

ESTRO 2021

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.