Secondary: 1. To determine the diagnostic accuracy of intraoperative histologic subtyping by frozen section. 2. To determine the mediastinal lymph node metastasis rate in clinical T1N0M0 lung cancer with different histologic subtypes. 3. To evaluate the pattern of mediastinal involvement of tumors in different lung segments. 4. To determine the mediastinal nodal status of tumors with different radiological features (pure ground glass opacity, mixed ground glass opacity and solid nodules).