ESTRO 2025 - Abstract Book

S1940

Clinical - Urology

ESTRO 2025

Purpose/Objective: For localized prostate cancer, multiple randomized trials demonstrated improved (biochemical) disease-free survival with a focal tumor boost, either through brachytherapy or conventional external beam radiotherapy (35 fractions) 1,2,3 . This study investigates boosting versus no boost in moderately hypofractionated (20 fractions) external beam radiotherapy. Material/Methods: This study included all patients from the prospective Utrecht Prostate Cohort for Cancer Treatment Intervention Studies and Long-term Evaluation (NCT04228211) treated for localized prostate cancer with 20 fractions of external beam radiotherapy since 3-2020 4 . Until 12-2020, our standard radiotherapy scheme for intermediate or high risk prostate cancer patients was 62Gy in 20 fractions to the entire prostate (3.1Gy per fraction, total prostate EQD2 dose 83Gy, assuming a prostate α/β of 1.2Gy). Starting 1 -2021, our new standard radiotherapy scheme included an additional simultaneous integrated boost up to 74Gy delivered to the intraprostatic tumor (GTV boost up to 3.7Gy per fraction, EQD2 up to 113Gy). The boost lesion was determined on mpMRI and PSMA PET/CT. Organ at risk constraints were prioritized over the boost dose. The primary outcome was quality of life (EPIC-26 urinary and bowel domains) at 1, 3, 6, 9, 12, 18 and 24 months follow-up. Boost versus no boost between-group differences were assessed by multivariable linear mixed-effects models, correcting for baseline. Secondary outcomes were clinician reported genitourinary (GU) and gastrointestinal (GI) toxicity (CTCAE v5.0) and dosimetry. Results: Between 3-2020 and 1-2024, 216 patients were included (boost n=148, no boost n=68). Quality of life did not significantly differ between the boost and no boost group at any follow-up moment up to 24 months (figure1): urinary obstructive/irritative (median differences between -3.1 and +1.2), urinary incontinence (median differences between -6.4 and +0.4) and bowel (median differences between -2.3 and +1.6), all 95% confidence intervals include 0. Toxicity at 3 months did not differ between the boost versus no boost group: GU grade 2 or higher (27% versus 38%, p=0.2), no grade 3 GU, GI grade 2 or higher (0.8% versus 5.9%, p=0.1) and grade 3 GI (0.8% versus 0%). Median GTV boost dose was 73.1Gy [IQR 71.4-74.8]. Prostate and GTV volumes did not differ significantly between groups. Organ at risk doses were within constraints, but slightly higher in the boost group (table1).