ESTRO 2021 Abstract Book

S644

ESTRO 2021

subgroup (SLND: 5.6%; p<0.001). Median ADT duration for the SLNRT patients was 9 months (IQR 3–21). 43 patients (65.2%) in the SLND cohort underwent ADT during FU. Biochemical recurrence rate was 40.3% (SLNRT) vs. 86.4% (SLND; p<0.001). In multivariable analysis, SLNRT could be confirmed as an independent predictor for increased biochemical recurrence-free survival (RFS; HR 0.10, 95%CI 0.05–0.16, p<0.001). Estimated mean metastasis-free survival (MFS) was 57.6mo (95%CI 51.4 – 63.8) for SLNRT patients and 39.5mo (33.4–45.6) for SLND patients (HR 0.64, 95%CI 0.32-1.31, p=0.222). We did not observe significant differences regarding general HRQOL (global health status), daily pad usage, and ICIQ-SF scores between both subgroups. Conclusion In a large contemporary series of patients with nodal-only recurrent PC based on PSMA-PET/CT staging, we observed significantly increased biochemical RFS in patients undergoing SLNRT while no significant differences could be observed in MFS, and functional outcomes including HRQOL. 1 Tampere University Hospital, Department of Oncology, Radiotherapy unit; Department of Medical Physics, Tampere, Finland; 2 Tampere University Hospital, Department of Oncology, Radiotherapy unit, Tampere, Finland; 3 Tampere University Hospital, Department of Medical Physics, Tampere, Finland Purpose or Objective Stereotactic body radiotherapy (SBRT) and MR-only based planning are increasingly used for the treatment of prostate cancer. Due to high fraction doses of SBRT, reduced CTV-to-PTV margins are often used. MR-only based planning reduces the PTV volumes even further when compared to CT-based planning. However, too narrow margins may compromise the target coverage due to radiation-induced volume changes of the prostate during the treatment. The aim of this study was to examine the extent of prostate volume changes and their effect on treatment margins during SBRT of the prostate Materials and Methods Twenty prostate patients treated between 04/2016-10/2017 with 5 x 7.25 Gy fractionation using two-arc VMAT technique and 10 MV FFF beams were included in the study, which is part of a clinical trial (NCT02319239). For the volume analysis, three MRI scans including planning MRI (baseline), and scans after the third (mid- treatment) and fifth (end of the treatment) fractions, were acquired for each patient using 3T Siemens Trio- Tim MRI system. Prostate was contoured on each of the MRI scans (axial T2 tse, FOV 200x200mm, resolution 0.625x0.625mm, 2 mm slices) for each patient by the same radiation oncologist. From the contoured images, volume and maximum diameter of the prostate in anterior-posterior (AP), superior-inferior (SI) and left-right (LR) directions were recorded. The significance of the differences in volume relative to the baseline was tested with a paired t-test (p<0.05 was considered significant). Total number of analyzed images was 60. Results Mean (±SD) volumes of the prostate were 37.6±11.7, 41.9±14.1 and 40.7±13.3 cm 3 at the baseline, third, and fifth fraction, respectively (fig.1). Mean change in prostate volume relative to the baseline was 11% (p=0.003) and 8% (p=0.016) after the third and fifth fractions, respectively. Prostate volume was larger than the baseline for 16 and 15 patients after the third and fifth fractions, respectively (fig.2). Compared to baseline, max LR, SI and AP dimensions were on average 0.7 (-0.2, 1.6), 1.7 (0.3, 3.1) and 1.7 (0.7, 2.8) mm larger after the third fraction, and 0.2 (-0.8, 1.2), 1.9 (0.5, 3.3) and 1.5 (0.6, 2.4) mm larger after the fifth fraction, respectively (95% CL in brackets). PD-0811 Radiation-induced prostate swelling during SBRT of the prostate A. Vanhanen 1 , P. Reinikainen 2 , M. Kapanen 3