S1266
Clinical - Urology
ESTRO 2026
Patients with low and favorable intermediate-risk prostate cancer were allocated with 1:1 central randomization, to 26 Gy; simultaneous integrated boost (SIB) DIL to 32 Gy in 2 fractions over 8 days or 36.25 Gy SIB DIL to 40 Gy in 5 fractions over 10-11 days. Every fraction was delivered using the MR- guided Adapt-to-Shape (ATS) workflow by Unity MR- Linac. Post-radiation WBC subpopulations change was analysed by a Linear mixed-effect model. Acute genitourinary (GU) and gastrointestinal (GI) AEs (CTCAE v5.0) were updated at prespecified 3-month posttreatment intervals after completion of enrollment. Results: Forty-four patients were randomized between December 2023 and August 2025 to 2-fraction SBRT (2F,n=21) or 5-fraction SBRT (5F,n=23). Median follow- up was 9 months (IQR 3-12). Baseline characteristics were similar between the two arms (Figure 1A). At Month1 follow-up, the mean WBC count was significantly decreased from baseline for both arms (2F: -8.3%, p=0.03 | 5F: -16.5%, p<0.001) and had not recovered to baseline levels afterward. However, no significant difference in WBC count when comparing between the 2 groups (p=0.94). Through the entire follow-up time, 5F demonstrated a lower absolute CD4+ counts ( Δ -74.1 cells/uL; 95%CI -188.5-40.3, p=0.21) compared to 2F without statistically significant differences with linear mix model analysis. Also, no significant differences in mean CD8+ (p=0.92) and NK cell counts (p=0.58), absolute lymphocyte count (p=0.39), or absolute neutrophil count (p=0.59) were observed when comparing between the two groups (Figure 2D). Cumulative incidence of maximum acute GU AEs grade ≥ 2 was observed in 19.0% in 2F versus 21.7% in 5F without a significant difference (p=0·83). Grade 2 GU was highest at month 1 (19.0%) in 2F and plateaued through month 3, while in 5F, the peak grade 2 GU was 17.4% at month 2 and trended to recovery in month 3 (Figure 2B). Neither acute grade ≥ 3 GU AEs nor grade ≥ 2 GI adverse events were observed. The 3-month mean PSA showed no significant difference in decline from baseline between the two groups (-56.69% vs -53.19%, p=0.567).
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