BACKGROUND: Patients with locally advanced non-small-cell lung cancer (NSCLC) who undergo concurrent chemotherapy and radiotherapy often experience synergistic toxicity, and local regional control rates remain poor. We assessed the activity and safety outcomes of primary tumour stereotactic body radiotherapy (SBRT) followed by conventional chemoradiotherapy to the lymph nodes and consolidation immunotherapy in patients with unresectable locally advanced NSCLC. METHODS: In this multicentre, single-arm, phase 2 trial, patients aged 18 years and older were enrolled at eight regional cancer centres in North Carolina and South Carolina, USA. Patients were eligible if they had stage II-III, unresectable, locally advanced NSCLC (any histology), with peripheral or central primary tumours that were 7 cm or smaller, excluding central tumours within 2 cm of involved nodal disease, and an Eastern Cooperative Oncology Group performance status of 0-2. Patients who had previously received systemic therapy or radiotherapy were excluded. Participants received SBRT to the primary tumour (50-54 Gy in three to five fractions) followed by standard radiotherapy (planned up to 60 Gy in 30 2 Gy fractions) to the involved lymph nodes with concurrent platinum doublet chemotherapy (either paclitaxel 50 mg/m intravenously plus carboplatin area under the curve 2 mg/mL per min every 7 days for a total of six 1-week cycles or etoposide 50 mg/m intravenously on days 1-5 and days 29-33 plus cisplatin 50 mg/m intravenously on days 1, 8, 29, and 36 for two cycles of 4 weeks). An amendment to the protocol (Dec 11, 2017) permitted the administration of consolidation durvalumab at the discretion of the treating investigator. An additional protocol amendment on Jan 13, 2021, directed patients without disease progression after chemoradiotherapy to receive consolidation durvalumab (10 mg/kg intravenously on day 1 and day 15 of a 4-week cycle for up to 12 cycles or 1500 mg intravenously on day 1 of a 4-week cycle for up to 12 cycles). The primary endpoint was 1-year progression-free survival (per Response Evaluation Criteria in Solid Tumours version 1.1), assessed in all participants who received at least one fraction of SBRT and had radiological follow-up data up to 1 year. A 1-year progression-free survival rate of greater than 60% was required to reject the null hypothesis and show significant improvement in 1-year progression-free survival. One-sided exact binomial tests were used to compare the primary endpoint versus the historical control 1-year progression-free survival rate used to determine the sample size. Safety was assessed in all patients who received at least one fraction of SBRT. This study is registered with ClinicalTrials.gov, NCT03141359, and is closed to accrual. FINDINGS: Between May 11, 2017, and June 27, 2022, 61 patients were enrolled and received at least one dose of fractionated SBRT, of whom 59 were evaluable for the primary endpoint. Median age was 67 years (IQR 61-72), 28 (46%) of 61 were female, 33 (54%) were male, 51 (84%) were White, seven (11%) were Black, and three (5%) were of other or unknown race. Of the 61 patients enrolled, 47 received at least one dose of consolidation durvalumab. As of data cutoff (July 12, 2023), median follow-up was 29·5 months (IQR 14·9-47·1). 1-year progression-free survival was 62·7% (90% CI 51·2-73·2; one-sided p=0·39, compared with the historical control rate), with 37 of 59 evaluable participants progression free and alive 1 year after enrolment (n=14 progressed, n=8 died). The most common grade 3-4 treatment-related adverse events were decreased neutrophil count (nine [15%] of 61 patients), decreased white blood cell count (five [8%]), and anaemia (four [7%]). Treatment-related serious adverse events occurred in 11 (18%) of 61 patients, which included lung infection (three [5%]), pneumonitis (two [3%]), decreased neutrophil count (two [3%]), febrile neutropenia (two [3%]), and dyspnoea, hypoxia, respiratory failure, sinus tachycardia, bronchial infection, and acute kidney injury (each in one [2%] patient). Treatment-related deaths occurred in four (7%) of 61 patients (one each of respiratory failure, respiratory failure and dyspnoea, lung infection, and pneumonitis). INTERPRETATION: Although this study did not meet the primary endpoint, activity and safety profiles of primary lung tumour SBRT followed by concurrent mediastinal chemoradiotherapy were favourable compared with other modern trials treating locally advanced NSCLC with chemoradiotherapy. These findings serve as the basis for the ongoing randomised phase 3 study NRG Oncology LU008 (NCT05624996). FUNDING: AstraZeneca and Atrium Health Levine Cancer Institute.