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Objectives: Tumor spread through air spaces (STAS) is a newly identified invasion pattern in lung adenocarcinoma. This study aimed to analyze and validate the clinical impact of tumor STAS in surgically resected lung squamous cell carcinoma (SQCC)., Materials and Methods: We retrospectively reviewed 220 patients with lung SQCC. Tumor STAS was defined as detached tumor cells within the air spaces in the lung parenchyma beyond the edge of the main tumor. Statistical analyses were conducted to investigate the proportion of STAS and the relationship between the presence of STAS and clinicopathological factors, including clinical outcome., Results: STAS was identified in 42 of 220 patients (19.1%). The patients with STAS had a significantly worse 5-year recurrence-free survival (RFS) and 5-year overall survival (OS) than those without STAS (5-year RFS: 37.4% vs. 68.4%; p = 0.0006; 5-year OS: 50.2% vs. 71.4%, p = 0.0078) in stage I, but not in stage II and III. A multivariate analysis showed that the presence of STAS was an independent predictive factor of recurrence (hazard ratio = 3.27; 95% confidence interval, 1.7-6.29; p = 0.0004) and an independent prognostic factor (hazard ratio = 3.01; 95% confidence interval, 1.54-5.89; p = 0.0013) in stage I, but not in stage II and III., Conclusion: We found that STAS was detected in 19.1% of surgical resected SQCC, and it was associated with recurrence and worse survival in stage I SQCC, but not in stage II and III SQCC. Therefore, we suggest that STAS is a useful predictor of recurrence and prognosis in stage I lung SQCC., (Copyright © 2018 Elsevier B.V. All rights reserved.)