ESTRO 2021 Abstract Book

S1112

ESTRO 2021

accuracy comparable to that of PSADT calculated on the entirety of PSA available. In addition, as is clearly evident from the Figure, the prognostic thresholds of PSADT exhibited a strong time-dependence. The “alarm threshold” for the PSADT calculated at 3, 6, 9 or 12 months after the first PSA ≥0.10 were significantly different. Awareness of this PSADT dynamicity over time and of the fact that the PSADTs commonly evaluated are often a heterogeneous grouping of PSADTs measured at different time intervals is crucial in the era of early SRT. PO-1355 Finding safe dose-volume constraints for re-irradiation of intraprostatic prostate cancer relapse. M. Augugliaro 1 , M. Pepa 1 , D. Zerini 1 , G. Marvaso 1,2 , R. Cambria 3 , V. Bagnardi 4 , S. Frassoni 4 , F. Pansini 3 , D.P. Rojas 1 , F. Colombo 1 , C.I. Fodor 1 , F. Cattani 3 , B.A. Jereczek-Fossa 1,5 1 IEO European Institute of Oncology, IRCCS, Division of Radiotherapy, Milano, Italy; 2 University of Milan, Department of Oncology and Hemato-Oncology, Milano, Italy; 3 IEO European Institute of Oncology, IRCCS, Unit of Medical Physics, Milano, Italy; 4 University of Milan-Bicocca, Department of Statistics and Quantitative Methods, Milano, Italy; 5 University of Milan, Department of Oncology and Hemato-Oncology, Milano, Italy Purpose or Objective Consensus for the optimal management of isolated prostate cancer (PCa) recurrence following external beam radiotherapy (EBRT) is lacking. Salvage EBRT given with hypofractionated schedules and high precision modalities (salvage stereotactic body RT, SBRT) represents a choice, but its diffusion is limited also for the lacking of safe cumulative dose-volume limits. The aim of this study is to provide preliminary indications for safe constraints of rectum and bladder, to contain genitourinary (GU) and gastrointestinal (GI) toxicity in PCa salvage SBRT. Materials and Methods Data from patients treated for PCa and intraprostatic relapse, from 1998 to 2016, were retrospectively collected. Before 2008, first RT course was delivered with 3D conformal RT techniques; after 2011, instead, treatments were performed either using SBRT (Cyberknife / VERO) or with volumetric modulated arc therapy (VMAT) (Trilogy). All patients underwent re-irradiation with SBRT with heavy hypofractionated schedules. Patients were divided in high risk (HR) and low risk (LR) categories according to their comorbidities. The cumulative dose-values to the planning target volume (PTV) and to OARs were computed and compared with salvage SBRT toxicities. Results Twenty-six patients were included in the analysis. Median age at re-irradiation was 75 years, and mean interval between the two RT courses was 5.6 years and the median follow-up was 4 years. The median follow- up after salvage SBRT was 47.7 months (13.4 - 114.3 months). First and second RT courses characteristics are reported in Table 1. After re-irradiation, 18 (69%) and 12 (46%) patients were free from any grade (G) acute and chronic GU/GI toxicity, respectively. On the other hand, while only three patients (12%) experienced acute GU G2 events, 12 patients (46%) developed chronic GU/GI G2/G3 toxicity. Regarding late GU toxicities, if patients are divided into G0-1 and G>1 groups, Dbladder30% parameter will be much below the dose-volume limit in the former group but very close to this point in the latter (Figure 1A). After analysing GI events, no statically significant associations were found neither between the cumulative dose and chronic toxicities, nor between risk category and toxicities (Figure 1B). No statically significant associations were found neither between the median cumulative dose and toxicities nor between risk category and toxicities: but, of note, 56% of the HR pts developed GU/GI toxicity while only the 25% of the LR ones did (odds ratio 3.75, p value 0.16).

Conclusion This study may serve as a starting point for finding safe bladder constraints for salvage SBRT. On the other