S1140
Clinical - Urology
ESTRO 2026
Hatau, Turkey. 5 Department of Radiation Oncology, Baskent University, Adana Dr. Turgut Noyan Research and Treatment Center, New York, USA. 6 Department of Radiation Oncology, Rutgers Cancer Institute of New Jersey, New Jersey, USA. 7 Department of Genitourinary Radiation Oncology, The University of Texas MD Anderson Cancer Center, Texas, USA Purpose/Objective: Quantitative MRI parameters such as apparent diffusion coefficient (ADC) have emerged as promising imaging biomarkers for assessing tumor cellularity and response after radiotherapy (RT). However, the prognostic significance of post-treatment ADC and the influence of simultaneous integrated boost (SIB) on ADC dynamics remain uncertain. This study aimed to determine whether focal dose escalation modifies ADC response and to evaluate the prognostic value of post- treatment ADC in men with localized prostate cancer (PCa) receiving definitive RT and androgen deprivation therapy (ADT). Material/Methods: We retrospectively analyzed 337 intermediate- and high-risk PCa patients treated between 2011 and 2023. All underwent pre- and post-treatment diffusion- weighted MRI on 1.5T or 3T scanners for quantitative ADC evaluation. Patients were stratified by treatment technique: SIB+ (n=151) who received a focal intraprostatic boost (median 86 Gy) and SIB– (n=186) who received standard-dose RT (median 78 Gy). ADC values were extracted from dominant intraprostatic lesions on identical slice positions, and percentage change ( Δ ADC) was calculated. Primary endpoints were progression-free survival (PFS) and prostate cancer–specific survival (PCSS). Cox and Fine–Gray regression, ROC analysis, calibration, and decision- curve analysis (DCA) were performed to assess prognostic performance and clinical utility. Results:
clinical benefit, particularly for 3–5-year PFS prediction (Figure). Subgroup analysis showed SIB mitigated the adverse effect of low post-treatment ADC (interaction p=0.04).
Conclusion: Post-treatment ADC from mpMRI is reproducible, noninvasive biomarker of response and long-term prognosis after prostate RT. Low post-treatment ADC identifies patients at increased risk of progression and disease-specific mortality, while focal SIB improves tumor control and mitigates this adverse effect. Integrating quantitative ADC metrics into clinical models enhances risk stratification and supports their
use in adaptive, precision-guided RT. Keywords: Prostate cancer, SIB, ADC.
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Definitive Intensity-Modulated Radiotherapy for Upper Urothelial Carcinoma: A Single-Institution Retrospective Study Yuki Mukai 1 , Motoko Oomura 2 1 Radiation Oncology, Yokohama City University Medical Center, Yokohama, Japan. 2 Radiation Oncology, Syounann Kamakura General Hospital, Yokohama, Japan Purpose/Objective: The upper urinary tract urothelial carcinoma (UTUC) is a rare malignancy, and the role of definitive radiotherapy remains unclear. This study aimed to evaluate the clinical outcomes and side effects of definitive intensity-modulated radiotherapy (IMRT) in patients with UTUC who were medically inoperable or refused surgery. Material/Methods: This retrospective single-institution study included 23
Median follow-up was 11.7 years. Tumor ADC increased significantly post-RT (median Δ ADC
+43.1%,p<0.001), with a greater rise in SIB+ than SIB– patients (49.2% vs 36.6%, p=0.01).Overall, 63 patients (18.7%) experienced disease progression and 22 (6.5%) died from PCa. Progression was less frequent in SIB+ (11.9%) versus SIB– (24.2%) cohorts. In multivariable analysis, older age, PSA ≥ 20 ng/mL, Gleason ≥ 8, omission of SIB, and low post-treatment ADC independently predicted inferior PFS (p<0.05). At 8 years, PFS and PCSS were significantly higher among both SIB+ (72.7% vs 58.9%; 96.1% vs 87.1%) and high- ADC (70.3% vs 61.0%; 94.0% vs 89.4%) groups. Post- treatment ADC was an independent predictor of outcomes (AUC=0.63,p<0.001), whereas baseline ADC and Δ ADC were not. Incorporating post-treatment ADC into multivariable models improved calibration, discrimination (C-index +0.05), and DCA-derived net
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