S2539
Radiobiology – Immuno-radiobiology
ESTRO 2026
initiation r = 0.043; VMAT at initiation r = –0.294 (weak correlation); at the end, 3D-CRT r = –0.016; VMAT r = – 0.48. The negative correlation indicates that longer lymphocyte exposure times are associated with lower lymphocyte counts and greater lymphopenia. Total irradiation time was twice with 3D-CRT (45 vs. 22.5 min over 30 fractions). Concurrent chemotherapy markedly increased lymphopenia prevalence (90.9% vs. 55.6% for sequential; OR 8 [0.74–86.45], borderline significance). Larger PTV volumes ( ≥ 268 cc) were associated with 90% lymphopenia vs. 60% for smaller volumes (p < 0.10; r = –0.38, negative correlation at the end of RT). Mean EDIC was 1.67 ± 0.60 Gy 3D-CRT and 1.87 ± 0.94 Gy VMAT (NS), with slightly higher severe lymphopenia in VMAT (70% vs. 60%,NS). Conclusion:
Effective Dose to Immune Cells (EDIC) quantifies incidental blood irradiation. We analyzed the incidence of severe lymphopenia and the associated risk factors by comparing three-dimensional conformal radiotherapy (3D-CRT) and volumetric modulated arc therapy (VMAT), with the aim of establishing an
institutional protocol. Material/Methods:
Retrospective pilot study with 20 patients with lung cancer (10 with 3D-CRT and 10 with VMAT) who received definitive thoracic radiotherapy in 2025. 100% received platinum-based chemotherapy, and 25% received durvalumab. Dosimetric parameters (MLD, MHD, V5/V10/V20, EDIC), serial lymphocyte counts, and toxicities were analyzed. Severe lymphopenia was defined as <500 lymphocytes/ μ L. Results:
3D-CRT caused more lymphopenia and had longer treatment times (p < 0.05).Treatment time is a factor to lymphopenia in 3D-CRT technique. VMAT has shorter irradiation time and reduced hematologic toxicity. Concurrent chemotherapy was the strongest predictor (OR = 8). Larger PTV volumes increased lymphopenia risk. Limitations: small sample size. A multicenter database is proposed to validate predictors and guide preventive strategies. References: (Stathakis et al., s. f.)(Antonia et al., 2017)(Kim et al., 2021)(Bradley et al., 2015)(Friedes et al., 2024) Keywords: lymphopenia, EDIC, lung cancer
In the bivariate analysis, severe lymphopenia occurred in 65% of cases, with a median lymphocyte reduction of 72% (80% in 3D-CRT particularly at the end of treatment, 70% in VMAT likely due to a smaller volume of healthy tissue irradiated and improved dose conformity) (OR 1.71; 95% CI: 0.23–12.71). Mean treatment time was significantly longer for 3D-CRT (1.43 min; range 1–2.06) than for VMAT (0.84 min; range 0.58–1.24) (p < 0.05). Correlation analyses between treatment time and lymphopenia, as well as between lymphocyte count at the beginning and end of treatment and technique, showed: for 3D-CRT at
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