Systemic mediastinal lymphadenectomy is deemed indispensable in lung cancer surgery for accurate staging and complete resection. However, extensive lymphadenectomy in patients without nodal metastasis may not improve survival and would increase operative duration and cause damage to mediastinal structures.Therefore the precise selection of patients without mediastinal nodal metastasis is the key to avoid unnecessary lymphadenectomy.The investigator's previous retrospective study shows tumor location and histological subtypes are exclusive predictors of negative mediastinal nodal disease. The current prospective observational study is to further verify the staging accuracy of this selective mediastinal lymphadenectomy strategy.