The present study explores the incidence of treatment-related pneumonitis (TRP) in patients with locally advanced non-small cell lung cancer (NSCLC) undergoing radiotherapy combined with immunotherapy, and its association with systemic immune-inflammation index (SII).

A retrospective analysis of the clinical data of patients with unresectable locally advanced NSCLC patients who were treated at the hospital from January 2021 to December 2024 was conducted. The patients were divided into two groups according to whether they received immunotherapy: a combined group (radiotherapy + immunotherapy) and a radiotherapy group (radiotherapy only). The primary research objective was to make a comparison between the incidence and severity of TRP in the two groups, and the secondary research objective was to explore the relationship between pre-treatment SII and TRP in the combined group.

The study population comprised 89 patients, predominantly male 82 patients(92.13%), with 41 patients (46.07%) aged ≥65 years. The incidence of TRP in the combination group was 27 cases (61.36%), which was higher than that in the radiotherapy group 16 cases (35.56%)(P<0.05). The incidence of grade ≥3 TRP in the combination group was 7 cases (15.91%), higher than the radiotherapy group 2 cases(4.44%)(P>0.05). In the combined group, among the 27 patients with TRP, ground-glass opacities occurred in 23 cases (85.18%), consolidation in 15 cases (55.56%), reticular opacities in 10 cases (37.04%), and bilateral lung involvement in 10 cases (37.04%). In the radiotherapy group, among the 16 patients with TRP, ground-glass opacities occurred in 12 cases (75.00%), consolidation in 7 cases (43.75%), reticular opacities in 5 cases (31.25%), and bilateral lung involvement in 4 cases (25.00%). The incidence rates in the combination group were higher than those in the radiotherapy group, but the differences were not statistically significant (P>0.05). Higher pre-treatment SII values[1 035.1 (568.8, 1 727.5)] than those without TRP[472.0(336.4, 851.3)] (χ²=7.35, P=0.01). Multivariate logistic regression analysis confirmed that elevated SII was an independent risk factor for TRP in the combination group(OR=1.01, 95%CI: 1.01~1.01, P<0.05), with consistent results across different adjustment models.

In comparison with radiotherapy administered in isolation, the combination of radiotherapy and immunotherapy has been observed to result in an elevated probability of TRP. Consequently, it is imperative that caution is exercised when formulating combined treatment plans in clinical practice. The increase in SII prior to treatment may be associated with the occurrence of TRP, suggesting that SII may serve as a potential biomarker.