S746
Clinical – Lower GI
ESTRO 2026
cm ³ ) as most prognostic; patients in high-dose strata had median OS around 35–40 months, whereas low- dose strata had unreached medians.
Digital Poster 5197 Prognostic role of early lymphocyte kinetics in locally advanced rectal cancer Konrad Stawiski 1,2 , Jacek Burzy ń ski 1,2 , Nina J ę drzejczak 2 , Adam Zi ę ba 2 , Lucjan S ł awi ń ski 2 , Magdalena Peszy ń ska-Piorun 2 , Micha ł Mas ł owski 2 1 Department of Biostatistics and Translational Medicine, Medical University of Lodz, Lodz, Poland. 2 Department of Radiotherapy, Copernicus Memorial Hospital, Lodz, Poland Purpose/Objective: To evaluate an early, clinically applicable marker based on lymphocyte kinetics during neoadjuvant chemoradiotherapy (nCRT), and to reassess the relative impact of systemic and pelvic bone marrow dose on overall survival (OS) in locally advanced rectal cancer (LARC). Material/Methods: We retrospectively analysed 205 patients with cT2–T4 LARC treated with long-course nCRT between 2018 and 2024. Absolute lymphocyte counts (ALC) were collected at baseline (closest value within − 30 to +7 days from the start of radiotherapy) and from the earliest on-treatment complete blood count obtained between days 7 and 14. The relative week-2 ALC decline was calculated as (baseline − week-2)/baseline. A maximally selected log-rank statistic (scan 35–55%) identified a 45.5% decline as the optimal cut-off, defining two groups: week-2 ALC decline ≥ 45.5% versus <45.5%. Dose–volume histograms (DVHs) for the planning body contour and automatically segmented pelvic bones were analysed in absolute volumes (cm ³ ). Univariable Cox models evaluated associations with OS, with false-discovery rate control. A systematic sweep across all body and pelvic VxGy cm ³ bins identified the most prognostic dose levels. Principal component analysis (PCA) of body DVHs was used as a quality-assurance tool to detect acquisition
Figure 1. Panel plot of results. Conclusion:
An early (week-2) ALC decline of at least 45.5% is associated with improved OS after nCRT for LARC, whereas smaller early declines identify a higher-risk group that may merit closer monitoring. Absolute DVH analysis and systematic bin sweeping suggest that extensive high-dose systemic and pelvic coverage remains undesirable. Keywords: rectal cancer, lymphopenia, bone marrow
outliers. Results:
Stage III disease was present in 81.5% of patients; 186/205 (90.7%) had evaluable week-2 ALC. Median baseline ALC was 1.96 × 10 ⁹ /L and median week-2 ALC 1.17 × 10 ⁹ /L, corresponding to a median relative decline of 42.5%. Patients with a week-2 ALC decline ≥ 45.5% (n = 79) had superior OS compared with those with a smaller decline (n = 107), with 12-/24-/60-month OS of 96.0/89.4/78.7% versus 89.9/81.1/51.7%, respectively (hazard ratio [HR] 0.44; 95% CI 0.23–0.85; p = 0.015). A longer time to lymphocyte nadir was associated with worse OS (HR 1.015 per day; p = 0.004). Absolute DVH metrics showed only modest trends: body volume receiving ≥ 47 Gy HR 1.05 per 100 cm ³ (p = 0.069). Systematic bin sweeping highlighted body V46Gy (cut- off 1638 cm ³ ) and pelvic bones V48Gy (cut-off 65.4
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