*2.3. Histopathological Evaluation*

All surgical specimens were evaluated for pathologic staging. Histological typing was performed according to the World Health Organization classification. The recorded variables included tumor size, differentiation grade, visceral pleural involvement, lymphovascular permeation, perineural invasion, multiple N2 station, and N2 ratio. Multiple N2 station was defined as lymph node metastasis involving more than one N2 station. N2 ratio was calculated by dividing the total number of metastatic by the total number of N2 lymph nodes examined.
