ESTRO 2021 Abstract Book

S1095

ESTRO 2021

without concurrent Androgen Deprivation Therapy (ADT) were retrospectively evaluated. Biochemical progression free survival (bPFS), distant progression free survival (DPFS), Overall survival (OS) and toxicity outcomes were investigated using Longrank-test. Results From November 2012 to December 2019, 74 pts (117 lesions in total) were included. Median follow up was 31 months (range 6-89). Median Planning Treatment Volume (PTV) size was 16.5 cm 3 (range 3.5-71.6). Concurrent ADT was administered in (58.1%) patients, median. Following the first SBRT, 38 (51%) pts developed progression of disease (PD): 15 (20.2%) bone PD, 11 (14.8%) nodal PD, 12 (16.23%) bone and nodal PD. Median time to progression was 48.8 months (range 0–180). Five pts showed blood Prostate Specific Antigen (PSA) rising immediately after SBRT, with evidence of metastatic PD at the subsequent imaging re-staging. 1y-, 2y-, and 3-y DPFS were 77%, 37%, 19%, respectively; 1y-, 2y-, and 3y-OS were 98%, 98%, 95%, respectively (Fig.1). The presence of a single target lesion, rather than 2 or 3 metastatic lymph nodes showed a statistically significant influence on OS (p=0.021) (Fig.2). Concomitant ADT did not seem to correlate with improved clinical outcomes, even after stratification of patients for single node oligorecurrence and more-than-one metastatic lesion. The mean time to PSA nadir after SBRT was 9 months. Patients who reached early nadir (<3 months after SBRT) had a lower 3-y survival than patients who gained nadir longer after 3 months, 73% and 100%, respectively (p = 0.004). Univariate analysis showed no other parameters, including doubling time PSA, D’Amico risk class, ISUP Grade Group, tumor stage presentation of disease as statistically significant. No acute or late events of any grade were reported. Conclusion Optimal radiotherapy regimen is not yet well defined in the setting of oligo-rPC, with wide variability among different Cancer Centers and no international consensus on treated volumes, radiation schedules and techniques. Although retrospective, our study confirmed nodal-involved SBRT to be safe and effective for nodal oligo-rPC. The absence of any significant impact of concurrent ADT on survival outcomes may suggest the opportunity to delay the start of ADT and its side effetcts. As reported in literature pts who reached early nadir had a lower 3-y survival. Our findings may also suggest to separately consider single lesion and ≥1 nodal oligo-recurrence, as this may have a significant impact on survival.

PO-1334 Early Salvage Radiotherapy In the post RADICALS-RT Era: The Gap From Trials to Reality M. sanchez 1 1 Fundación Arturo López pérez, Department of Radiation Oncology, Santiago, Chile