ESTRO 2025 - Abstract Book

S407

Clinical - Biomarkers

ESTRO 2025

cT1-2N0M0 NSCLC patients, including those with pathologically unproven, who have undergone ablative radiotherapy.

Keywords: NSCLC, ctDNA, radiotherapy

References: [1] Nakamura M, et al. Detection of Pretreatment Circulating Tumor DNA Predicts Recurrence after High-Dose Proton Beam Therapy for Early-Stage Non-Small Cell Lung Cancer. Int J Radiat Oncol Biol Phys, 2023;116(5):1085 1090. DOI:10.1016/j.ijrobp.2023.02.021

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Digital Poster Correlation between pre-treatment PSMA-PET and early PSA response in MR-guided SBRT with intraprostatic lesion boost for prostate cancer Darren MC Poon 1 , Cindy Xue 2 , Jing Yuan 2 , Oi Lei Wong 2 , Bin Yang 3 , Sin Ting Chiu 4 , Ben Yu 3 1 Comprehensive Oncology Centre, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong. 2 Research Department, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong. 3 Medical Physics Department, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong. 4 Radiotherapy Department, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong Purpose/Objective: While PSMA-PET has emerged as a standard tool for staging localized prostate cancer (PC), the prognostic value of pre-treatment PSMA-PET metrics in relation to biochemical response to stereotactic body radiotherapy (SBRT) remains under-explored. The benefit of focal boost to intra-prostatic lesion (IPL) might further be enhanced with magnetic resonance (MR)-guided SBRT (MRgSBRT) and PSMA-PET. This study investigated the correlation between the baseline PSMA-PET maximum standardized uptake value (SUV max ) of the IPL and the prostate-specific antigen (PSA) kinetics in PC patients (pts) who underwent MRgSBRT. Material/Methods: We retrospectively included consecutive intermediate/high-risk localized PC pts who underwent MRgSBRT (40 Gy/5 fractions to the whole prostate and 42.5 Gy/5 fractions to MR-visible PI-RADS 4/5 IPLs). All pts had pre-treatment 18 F PSMA-PET and started androgen deprivation therapy before SBRT. PSA levels (ng/mL) were measured at baseline (pre-MRgSBRT) and 4–6 weeks post-MRgSBRT. The correlation between IPL SUV max and pre-/post-MRgSBRT PSA levels, PSA absolute change (PSA pre – PSA post ) and PSA change rate [(PSA pre – PSA post )/number of days] was assessed using Pearson correlation and logistic regression analysis. Results: Sixty-three pts were included (Table). The mean SUV max was 23.29 (standard deviation, 28.77; range, 4.2–226.5), with significant inter-pt variability. SUV max was significantly correlated with the pre-MRgSBRT PSA level (r=0.77; 95% confidence interval [CI], 0.65–0.86; p<0.001), early post-MRgSBRT PSA level (r=0.62; 95% CI, 0.44–0.75; p<0.001), PSA absolute change (r=0.69; 95% CI, 0.53–0.80; p<0.001), and PSA change rate (r=0.4; 95% CI, 0.16–0.59; p=0.001). The logistic regression analysis indicated that SUV max was an independent and significant predictor (p=0.025) of early complete biochemical response (PSA post <0.1 ng/mL), with an odds ratio of 0.95 (95% CI, 0.89–0.99). A higher SUV max was significantly correlated with a greater absolute PSA reduction post-treatment (i.e. a more favorable response to MRgSBRT), especially in high-risk pts (r=0.75; 95% CI, 0.59–0.85; p<0.001), and a higher PSA reduction rate in high risk pts (r=0.46; 95% CI, 0.20–0.66; p<0.002), but not in intermediate-risk pts (r=–0.11; 95% CI, –0.58–0.41; p=0.68).