ESTRO 2025 - Abstract Book

S1908

Clinical - Urology

ESTRO 2025

Conclusion: Experimental arm had a non significant trend in terms of early complete biochemical response rate (27% odds increase if compared to control arm). Increased benefit was evidenced in patients with <3 metastases. Accrual will be completed within the end of 2024, and early results about the 6 months follow up of the complete cohort will be available in the second half of 2025.

Keywords: Apalutamide, Stereotactic body radiotherapy

References: 1) Chi KN, Chowdhury S, Bjartell A, et al. Apalutamide in Patients With Metastatic Castration-Sensitive Prostate Cancer: Final Survival Analysis of the Randomized, Double-Blind, Phase III TITAN Study. J Clin Oncol . 2021;39(20):2294-2303. 2)Francolini G, Allegra AG, Detti B, et al. Stereotactic Body Radiation Therapy and Abiraterone Acetate for Patients Affected by Oligometastatic Castrate-Resistant Prostate Cancer: A Randomized Phase II Trial (ARTO). J Clin Oncol . 2023;41(36):5561-5568.

946

Digital Poster Creating a SPACE for Prostate SBRT- a Single Institution’s Pilot Study Su Xin Ghislaine Lee 1 , Li Kuan Ashley Ong 1 , Jeffrey Kit Loong Tuan 1,2 1 Radiation Oncology, National Cancer Centre, Singapore, Singapore. 2 Duke-NUS Graduate Medical School, Duke-NUS Graduate Medical School, Singapore, Singapore Purpose/Objective: Dominant Intraprostatic lesions (DILs) are frequent sites of local recurrence following prostate Stereotactic Body Radiation Therapy (SBRT), requiring an escalated biologically equivalent dose (BED) of at least 200Gy for improved 5 year disease-free survival 1-2 . However, rectal toxicities from DIL boosts limit its safety and efficacy due to proximity of DILs to the rectum. These toxicities are improved with SpaceOAR TM Hydrogel insertion, which separates the prostate from rectum. This study compares rectal doses pre and post SpaceOAR insertion of patients receiving prostate SBRT with DIL boost. We hypothesise, SpaceOAR significantly lowers, and keeps rectal doses safe during focal DIL dose escalation. Material/Methods: Between 2021-2022, a retrospective study of 16 prostate SBRT patients with SpaceOAR was performed. Selected patients had DILs within the posterior peripheral zone in the prostatic-rectal interface. They were planned with a prescription dose of 36.25Gy in 5 fractions and DIL boost to 40Gy in 5 fractions (BED >200Gy; α/β=1.5Gy) following our institution’s SBRT planning protocol. All plans achieved clinical goals of target coverage and all normal tissue constraints, which took priority. For each patient, the original CT rectal position in the absence of SpaceOAR was simulated by performing an image registration between the planning CT (pCT) and recent pre-RT MRI (prMR). Image fusion focused on the prostate SpaceOAR interface in the pCT and the prostate-rectum interface in the prMR. Subsequently, the rectum was contoured on prMR (prMR-rec) and transferred to the pCT, whereby dose to the simulated rectum (prMR-rec) pre

SpaceOAR was compared to actual rectum (CT-rec) doses post SpaceOAR insertion. Dose-volume (DV) metrics of rectum were analyzed using the Wilcoxon signed-rank test.

Results: Median (IQR) DV metrics of CT-rec (SpaceOAR) were significantly lower than prMR-rec (no-SpaceOAR). (Table 1). Median (IQR) D90% and mean dose to DILs achieved was 40.1Gy (39.9Gy-40.9Gy) and 40.8Gy (40.4Gy-43.1Gy) respectively (Figure 1). However, the boost dose achieved was limited by the DIL’s size and proximity to the urethra.