ESTRO 2021 Abstract Book

S393

ESTRO 2021

were acceptable. At median follow up of 30 months (range 18-42 months), late toxicity and QoL were acceptable. (Figures 1 and 2) Figure 1. Acute CTCAE prevalence and late RTOG cumulative incidence

Figure 2. Maximum acute and late drop in EPIC summary scores for individual patients

All adverse events were ≤ grade 2 with the exception of one late grade 3 GU toxicity occurring in the PPN arm. Serum citrulline, a marker of small bowel function, trended towards lower levels in the PPN arm, in keeping with a significantly higher dose delivered to the small bowel in this arm. DNA damage foci in circulating lymphocytes increased at 1 hour after radiation and was significantly higher in the PPN arm. DNA damage foci in ex-vivo irradiated lymphocytes correlated with acute urinary toxicity. Conclusion A randomised clinical trial comparing P-SABR to PPN-SABR is feasible. There was a suggestion of increased toxicity with PPN-SABR, but this remained acceptable. This data will inform a UK multi-centre phase 2 study. OC-0511 Urethra and bladder dose-effect relations for genitourinary toxicity after EBRT for prostate cancer V. Groen 1 , M. van Schie 2 , N.P. Zuithoff 3 , E.M. Monninkhof 3 , M. Kunze-Busch 4 , J.C. de Boer 1 , J. van der Voort van Zijp 1 , F.J. Pos 2 , R.J. Smeenk 4 , K. Haustermans 5 , S. Isebaert 5 , C. Draulans 5 , H.M. Verkooijen 6 , U.A. van der