This study aimed to evaluate the effect of postoperative radiotherapy (PORT) in patients with resected pathologic N2 (pN2) non–small cell lung cancer (NSCLC) with different locoregional recurrence (LRR) risks. The primary cohort and validation cohort were retrieved from two independent medical centres. Data for all consecutive patients with completely resected pathologic stage T1-3N2M0 NSCLC were analysed. Patients without PORT in the primary cohort were identified as a training set. Significant prognostic factors for LRR were identified by the Fine-Gray model to develop a prognostic index (PI) in the training set.

The primary cohort consisted of 357 patients who met the eligibility criteria (training set, 287 patients without PORT). The external validation cohort consisted of 1044 patients who met the eligibility criteria (validation set, 711 patients without PORT). Heavy cigarette smoking history, clinical N2 status (cN2), and the number of positive lymph nodes >4 were identified as independent risk factors. In the low-risk group (PI score< = 3), PORT showed a trend towards decreased LRR rates but not significantly improved overall survival (OS). In the high-risk group (PI score>3), PORT significantly reduced the risk of LRR and improved OS.

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