S1277
Clinical - Urology
ESTRO 2026
Radiotherapy for Prostate Cancer: The MIRAGE Randomized Clinical Trial. JAMA oncology, 9(3), 365– 373. https://doi.org/10.1001/jamaoncol.2022.65583-. Mohamad, (2025). Salvage Radiotherapy Following Nonradiotherapy Ablative Techniques for Primary Prostate Cancer: A Systematic Review and Meta- analysis. European urology focus, S2405- 4569(25)00022-7. https://doi.org/10.1016/j.euf.2025.02.001 Keywords: HIFU, Relapse, SBRT Correlation of oxygenation during MR-guided SBRT of prostate cancer patients with early treatment response through Prostate-Specific Antigen kinetics Thomas Berger 1 , Ariane Lapierre 1 , Perla Zidane 1 , Hayder Aloraibi 1 , Samy Horn 1 , Frederic Cervenansky 2 , Benjamin Leporq 2 , Olivier Beuf 2 , William Henry Nailon 3,4 , Olivier Chapet 1 Digital Poster 3819 1 Radiation Oncology Department, Center Hospitalier Lyon Sud, Lyon, France. 2 INSA-Lyon, Université Claude Bernard Lyon 1, CNRS, Inserm, CREATIS UMR 5220, U1294, Lyon, France. 3 Department of Oncology Physics, Edinburgh Cancer Centre, Western General Hospital, Crewe Road South, Edinburgh, United Kingdom. 4 Institute for Imaging, Data and Communications (IDCOM),, School of Engineering, University of Edinburgh, Kings Buildings,, Edinburgh, United Kingdom Purpose/Objective: MR-guided radiotherapy allows the monitoring of physiological changes in organs in response to irradiation. This study investigates whether prostate relative Oxygen Extraction Fraction (rOEF) - reflecting pre-treatment oxygenation and its evolution during radiotherapy- is associated with Prostate-Specific Antigen (PSA) response after radiotherapy. Material/Methods: We analyzed 20 prostate cancer patients treated in five fractions with MR-guided Stereotactic Body Radiotherapy (SBRT; 36.25 Gy to the prostate, 40 Gy to the tumor). PSA was measured before SBRT and within seven months post-treatment (Figure 1 – left), and PSA density was calculated as PSA divided by prostate volume. rOEF, quantifying the fraction of delivered oxygen extracted by tissue, was calculated in 3D from MR sequences acquired at planning and across fractions. Mean prostate rOEF and mean GTV rOEF were computed at each session (Figure 1 – right). First- order linear models were fitted to PSA density and rOEF over time, and model slopes were extracted to quantify their variations. Correlations between PSA density slopes and (1) pre-SBRT mean rOEF and (2)
adaptation, intra-fraction motion monitoring, and improved organ-at-risk (OAR) sparing. We report early results from an ongoing prospective study evaluating feasibility and acute toxicity of MR-guided salvage SBRT after focal ablation. Material/Methods: Materials and MethodsThis ongoing prospective, observational, basket-type study (Ethics Committee approval #5289, Hospital Universitario de La Princesa) includes patients with biopsy-proven prostate adenocarcinoma previously treated with focal HIFU or cryoablation (August 2023–August 2025).Local recurrence was defined by PSA kinetics (nadir +1 ng/mL at 12 months or nadir +1.5 ng/mL at 24–36 months), MRI findings, and biopsy confirmation when indicated. Informed consent was obtained from all patients.MR-guided SBRT was delivered on a ViewRay MRIdian Linac using ultra-hypofractionation: 36.25 Gy/5 fractions for low-risk and 40 Gy/5 fractions for intermediate/high-risk disease. When pelvic nodes were treated, 25 Gy/5 fractions were prescribed following MIRAGE trial–based OAR constraints.Daily adaptive replanning employed Monte Carlo dose calculation and step-and-shoot IMRT optimization. Preparation included bladder filling, rectal emptying, dietary advice, and urethral catheter placement. Baseline characteristics, dosimetric data, and acute toxicity (CTCAE v5.0) were collected. Results: Ten patients were analyzed. Median age was 77 years (mean 78.2 ± 7.1) and median prostate volume 47 cc (mean 80.2). Gleason distribution: G7(3+4) 40%, G6(3+3) 30%, G7(4+3), G8, and G10 10% each. Median PSA at diagnosis was 5.81 ng/mL (mean 10.26) and pre-SBRT PSA 5.41 ng/mL (mean 9.06). Median follow- up was 11.0 months (mean 10.9; range 2.6–21.0). Two patients (20%) developed grade 2 cystitis at 3 months, both resolved by 6 months. One patient (10%) had grade 2 urinary incontinence persisting up to 12 months. No ≥ grade 3 events occurred. Conclusion: MRI-guided salvage SBRT after focal HIFU or cryoablation is feasible, safe, and well tolerated, showing low acute genitourinary toxicity and favorable early outcomes. MRgRT allows accurate dose escalation with optimal OAR protection through daily adaptive planning. Larger cohorts and longer follow- up are needed to assess long-term control and late effects. References: 1-.Gonsalves, D.,etal(2024). Feasibility and Acute Toxicity of Hypo-Fractionated Radiotherapy on 0.35T MR-LINAC: The First Prospective Study in Spain. Cancers, 16(9), 1685. https://doi.org/10.3390/cancers160916852- Kishan, A. U.et al (2023). Magnetic Resonance Imaging-Guided vs Computed Tomography-Guided Stereotactic Body
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