S788
Clinical - Lung
ESTRO 2026
(100%) completed RT on schedule, with no unplanned interruptions (table 1). Side effects: No grade ≥ 3 toxicity occurred; grade 2 events in 6/9 (67%), most commonly dysphagia and cough. One death at 3 months was deemed unrelated. Volume dynamics: Median absolute GTVprimary change at end of treatment was -11 cc (range − 174 to +5 cc) and − 67 cc (range − 363 to +8 cc) 3 months later. This corresponded to a relative volume change of -16% (range − 55% to +5%) at the end of treatment and -59% (range − 83% to +23%) at 3 months. Figure 1 shows a waterfall of primary tumor volume changes. Conclusion: Primer-shot RT with a planned early break was feasible, with 100% treatment completion, no ≥ G3 toxicity, and encouraging early tumor shrinkage as 8/9 had continued shrinkage at 3 months. Based on these results, we will continue with a phase 2 trial, evaluating whether primer-shot with a 3-week break increases treatment effect at constant RT dose.
patients died, including two non-oncological causes, with 1- and 2-years rates of 88.5% and 84.1%. Univariate tests confirmed worse OS rates with stage IIIC (p=0.00027) and age ≥ 75 (p=0.032). Conclusion: Our study supports the feasibility of SBRT after chemotherapy for patients with unresectable stage III NSCLC ineligible for concurrent chemo-radiation. Dose–response signals and the prognostic impact of late toxicity warrant validation in larger cohorts with longer follow-up. Keywords: stereotactic radiotherapy; locally advanced; NSCLC Digital Poster Highlight 2342 Primer-shot radiotherapy for NSCLC: a prospective feasibility trial (NCT06528743) Zeno AR Gouw 1 , Jose SA Belderbos 1 , Nienke Losekoot 1 , Kevin Groot-Lipman 2 , Jeho Jeong 3 , Joseph O Deasy 3 , Jan-Jakob Sonke 1 1 Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, Netherlands. 2 Imaging, The Netherlands Cancer Institute, Amsterdam, Netherlands. 3 Medical Physics, Memorial Sloan Kettering Cancer Center, New York, USA Purpose/Objective: Radiotherapy (RT) is typically delivered in equispaced fractions, despite dynamic tumor reoxygenation. Modeling(1) and preclinical work(2) suggest the hypothesis that a single “primer” fraction followed by an early break can reduce hypoxia and enhance subsequent fraction effectiveness (“primer-shot”). We tested feasibility (primary endpoint), safety, and early volumetric response in a prospective break-length escalation trial. Material/Methods: This is a prospective, non-randomized feasibility trial (NCT06528743) including patients with stage II–IV NSCLC, referred for palliative thoracic RT of at least the primary tumor. All gross tumor volumes (GTVs) received 5 × 6 Gy. The interval between fractions 1 and 2 was escalated by cohort (1, 2, then 3 weeks) using a 3+4 design to identify the longest tolerated break for expansion. Primary endpoint: completion of all five fractions as planned ( ≤ 2 unplanned days delay), to make sure the break does not compromise patients to finish their treatment. Concomitant systemic therapy was allowed, except for VEGFR-inhibitors. Secondary endpoints: treatment-related side effects (CTCAE v5.0) and GTV changes of the irradiated lesions (baseline vs end-of-treatment and +3 months). Results: Primary endpoint: Nine patients were enrolled, 3 per break-length (1-2-3 weeks). Feasibility was met: 9/9
Table 1. Patient and treatment characteristics
Figure 1. Relative primary tumor GTV change from baseline through end-of-treatment (left bar) and at 3 months after treatment (right bar). No 3-months CT was available for Patient 7 due to death from an unrelated cause. References:
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