S1278
Clinical - Urology
ESTRO 2026
Digital Poster 3917
mean rOEF slopes of the GTV and prostate were assessed using Spearman tests (statistical significance: p < 0.05).
Evolution of oxygenation based on MRI in renal cell carcinoma patients treated with stereotactic body radiotherapy on an MR-Linac Hayder Aloraibi 1,2 , Thomas Berger 1 , Corina Udrescu 1 , Mazen Moussallem 3,4 , Olivier Beuf 5 , Frederic Cervenansky 5 , Benjamin Leporq 5 , Olivier Chapet 1,2 1 Radiation Oncology Department, Center Hospitalier Lyon Sud, lyon, France. 2 Canbios, Université Claude Bernard Lyon 1, lyon, France. 3 Medical Imaging Department, Holy Family University, Batroun, Lebanon. 4 Research and Development Department, Healthy Innovations, Rachiine, Zgharta, Lebanon. 5 INSA-Lyon, Université Claude Bernard Lyon 1, lyon, France Purpose/Objective: Hypoxia is a factor of radio-resistance of tumours and is associated with treatment failure. The purpose of this work was to determine the evolution of oxygenation in renal cell carcinoma (RCC) tumors, by using MRI acquisition during stereotactic body radiation therapy (SBRT) on a 1.5T MR-Linac Unity®. Material/Methods: Five patients, treated for a RCC with 48 Gy in 4 fractions, had T2 mapping, chemical-shift encoded T2* mapping, and IVIM diffusion quantitative MRI sequences acquired at the treatment planning stage and the last fraction of treatment. The GTV and the ipsi-lateral kidney were delineated on T2 images and registered with the MRI sequences that were acquired for hypoxia mapping. The relative Oxygen Extraction Fraction (rOEF) map was calculated by combining parameters extracted from the MR sequences and was used as a surrogate marker for hypoxia. A rOEF value greater than 70% was considered as hypoxic. The average rOEF for the GTV and the healthy kidney was calculated, and a slope generated from the two acquisition times. The variation in hypoxic fraction (rOEF>70%) was also calculated. Results: The median [Q1-Q3] slope value was -0.3 [-2.17;0.004] for the healthy kidney and -2.31[-2.78;0.91] for GTV (Figure 1). The hypoxic fraction of the GTV decreased by 13 to 19% in three patients who had a high rOEF at the planning stage, and by less than 2% in the two patients who had a low rOEF (very small hypoxic volume) at the planning stage so a negligible decrease in hypoxia was expected, as seen in Table 1.
Results: Median PSA follow-up was 4.6 months (range: 1.2–6.7) post-SBRT. One patient (patient 1) showed a nearly null PSA density slope (0.0018 (ng/mL/cc)/month), while the median (range) slope for others was -0.0283 (-0.3208; -0.0106) (ng/mL/cc)/month. At planning, median mean prostate rOEF was 24.8% (13.7; 35.2). rOEF slopes were predominantly negative (median - 2.3%/fraction, range -3.8; -0.4), except for one patient (patient 6) with a positive slope (+1.0%/fraction). Higher baseline rOEF in the prostate was statistically significantly associated with steeper PSA density decline post-SBRT (Figure 2 – left ; rho = -0.54; p = 0.026). Although steeper PSA density decline appeared to be associated with steeper rOEF decline, the correlation was weak and not statistically significant (Figure 2 - right; rho = 0.22; p = 0.34). No statistically significant trend was identified for the GTV.
Conclusion: Higher mean prostate rOEF at baseline was associated with better early treatment response, reflected by PSA density decline within seven months post-SBRT. These preliminary findings warrant validation in larger
cohorts with extended PSA follow-up. Keywords: MR-guided SBRT, rOEF, PSA
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