ESTRO 2025 - Abstract Book

S1316

Clinical - Lung

ESTRO 2025

1220

Poster Discussion Isolated nodal failure in stage III NSCLC after proton therapy in durvalumab era

Yuanyuan Lin 1 , Kyra van Keeken 2 , Judith van Loon 3 , Stéphanie Peeters 3 , Bart Reymen 3 , Angela A.W. van Baardwijk 3 , Karolien Verhoeven 3 , Lizza Hendriks 4 , Juliette Degens 5 , Esther Kneepkens 3 , Mirko Unipan 3 , Dirk De Ruysscher 6 1 Radiation Oncology, Hospital Duran i Reynals, Institut Català d’Oncologia (ICO)-L’Hospitalet, Barcelona, Spain. 2 Clinical Research, Maastro, Maastricht, Netherlands. 3 Radiation Oncology, Maastro, Maastricht, Netherlands. 4 Pulmonary Diseases, Maastricht UMC+, GROW - Research Institute for Oncology and Reproduction, Maastricht, Netherlands. 5 Department of respiratory medicine, Zuyderland medical center, Heerlen, Netherlands. 6 Radiation Oncology (Maastro), Maastricht UMC+, GROW - Research Institute for Oncology and Reproduction, Maastricht, Netherlands Purpose/Objective: Isolated nodal failure (INF) after selective nodal irradiation (SNI) is a rare event in stage III non-small cell lung cancer (NSCLC), with reported rates of 2.2-2.3% in IMRT and 3DCRT cohorts [1,2]. The potentially reduced incidental dose with proton therapy (PT) may increase the risk of INF. We aimed to evaluate the incidence of INF in stage III NSCLC patients after PT. Material/Methods: We retrospectively reviewed consecutive stage III NSCLC patients treated with SNI involving PT between 2019-2022. PT selection followed the Dutch model-based approach [3]. All treatments were delivered with intensity-modulated PT (IMPT). Plans were robustly optimized and evaluated. If PT was not possible (due to machine downtime or clinically unacceptable plan as result of anatomical changes), a backup photon fraction with volumetric modulated arc therapy (VMAT) was administered. The primary endpoint was the incidence of INF, defined as recurrence in the regional nodes outside of the clinical target volume without a synchronous local, regional recurrence or distant metastases. Secondary endpoints were incidental doses to INF, patterns of recurrence, overall survival (OS) and progression-free survival (PFS). Based on the dose at planning CT, we estimated the hypothetical incidental dose to INF (assuming single-modality administration with IMPT or VMAT,) and actual incidental dose (accounting for fractions from both modalities). Results: 96 stage III NSCLC patients were included. Median age: 66 years (range 35-86). Stage (TNM8): IIIA (55.2%), IIIB (37.5%) and IIIC (7.3%). For all patients, 89% of treatment fractions were delivered with IMPT. In 82 patients (85.4%), 17% of fractions were delivered with VMAT. Treatment was concurrent chemoradiotherapy in 80% of patients (70% initiated durvalumab), sequential chemoradiotherapy in 17% (19% initiated durvalumab), and radiotherapy alone in 3%. The median total dose was 60Gy, over a median of 30 fractions. With a median follow-up of 27 months (range 1-58), only two patients (2.1%) experienced INF with a 2-year cumulative risk of 3.3% (95% CI: 0–7.8%). The hypothetical incidental doses at INF are shown in Figure 1. The actual incidental dose: 30.7Gy and 0.7Gy. Mean OS was 39 months (95% CI: 34-43) overall, 44 months (95% CI: 38-49) with durvalumab, and 36 months (95% CI: 15-57) without durvalumab. Median PFS was 25 months (95% CI: 16-34) overall. Patterns of recurrence are presented in Figure 2.