ESTRO 2025 - Abstract Book

S3953

Radiobiology - Normal tissue radiobiology

ESTRO 2025

For urinary incontinence, significant predictors include prostatectomy (OR=3.2 95%CI=2.1-4.9), spread of treatment times (sd, OR=1.5 95%CI=1.2-1.9), treatment time (hours before or after 1pm), OR=1.5 95%CI=0.3-0.7) and PER3 rs172933 (OR=0.45 95%CI=0.25-0.82). A significant interaction exists between the SNP and treatment time (P<0.001) suggesting that circadian genotype modifies the time of peak radiotoxicity risk. Models have a ROC AUC of 0.67. Conclusion: Both rectal and urinary side effects could be reduced by selecting the optimal treatment time based on the genotypes of circadian genes. Predictions show 44% of patients could reduce their risk of rectal bleeding from 11% treated at 9am to 5% treated at 5pm, and a significant reduction in risk of urinary incontinence from 15% to 5% by avoiding treatment in the middle of the day. Future clinical trials could stratify patients treated at optimal times compared to those scheduled conventionally to show if chronoradiotherapy improves quality-of-life.

Keywords: prostate cancer, circadian rhythm

4232

Digital Poster Physicochemical Indication of the FLASH Effect from Shoot-through Proton Pencil Beam Scanning Parameters Delivered under Ultra-high Dose Rates Andrew M Friberg 1 , Hai Siong Tan 1 , Eric S Diffenderfer 1 , Ioannis Verginadis 2 , Michele M Kim 1 , Keith Cengel 1 , Costantinos Koumenis 2 , Rodney Weirsma 1 , Lei Dong 1 , BoonKeng K Teo 1 , Jennifer W Zou 1 1 Radiation Oncology, University of Pennsylvania, Philadelphia, USA. 2 Radiation Biology, University of Pennsylvania, Philadelphia, USA Purpose/Objective: Purpose: Ultra-High Dose Rate (UHDR) proton PBS delivery results in irregular temporal-varying dose accumulation. It is difficult to establish a dose rate standard for the indication of proton PBS FLASH effect. In this work, we adopted a published physicochemical approach and investigate the impact of proton PBS UHDR parameters on the formation and downstream reactions of Reactive Oxygen Species (ROS). Material/Methods: Methods: A physicochemical model of ten dynamical species was implemented using H·, OH·, H 2 , H 2 O, e R·, ROO·, and ROOH. Hydroperoxyl (ROOH) formation was validated against a published fluorescent data for liposomes. For proton PBS delivery with specified beam current, voxelized temporal dose accumulation was calculated at the plateau region to simulate a shoot-through FLASH delivery. Time-dependent modeling of the formation and decay of various reactive oxygen species were obtained mimicking hypoxic (10 um O2) skin irradiation. We examine the ROS-Volume Histogram (RVH) in relation to the proton PBS delivery parameters. aq , O 2 ·, HO 2 - ,