Introduction: The purpose of this study is to analyze and validate the prognostic impact of the new lung adenocarcinoma (ADC) classification proposed by the International Association for the Study of Lung Cancer, American Thoracic Society, and European Respiratory Society and invasive tumor size in stage I lung ADC of Japanese patients.

Methods: We reclassified 191 stage I ADCs according to the new classification. The percentage of each histological subtype and the predominant type were determined. In addition, both total tumor size and invasive tumor size were examined. The relationship between these results and clinicopathological backgrounds was investigated statistically.

Results: The 5-year disease-free survival (DFS) of adenocarcinoma in situ and minimally invasive adenocarcinoma was 100%; lipidic-predominant ADCs, 94.9%; papillary-predominant ADCs, 85.4%; acinar-predominant ADCs, 89.7%; and solid-predominant ADCs, 54%. The predominant growth pattern was significantly correlated with DFS (p < 0.001, overall). With regard to tumor size, total tumor size was not correlated with DFS (p = 0.475, overall), however, invasive tumor size was significantly correlated with DFS (≤ 0.5 cm/ > 0.5 cm, ≤ 1 cm/ >1 cm, ≤ 2 cm/>2 cm, ≤ 3 cm/ >3 cm, 100%/91.5%/85.9%/80.8%/66.7%% in 5-year DFS) (p = 0.006, overall). A multivariate analysis showed solid-predominant and invasive tumor size were independent predictors of increased risk of recurrence (solid versus nonsolid: hazard ratio = 4.08, 95% confidence interval:1.59-10.5, p = 0.003; invasive tumor size: hazard ratio = 2.04, 95% confidence interval:1.14-3.63, p = 0.016).

Conclusion: : The new International Association for the Study of Lung Cancer, American Thoracic Society, and European Respiratory Society ADC classification and invasive tumor size are very useful predictors of recurrence of stage I ADCs in Japanese patients.