9mm lung cancer generally belongs to stage I. As 9mm is not yet 1cm, the tumor is small and often does not invade other tissues and organs around it. However, small cell lung cancer or undifferentiated non-small cell lung cancer may have regional lymph node metastasis and distant organ metastasis, even if the primary tumor is small. Therefore, for lung cancer of 9 mm size, surgery requires mediastinal lymph node dissection in addition to tumor removal. If the postoperative pathology shows metastasis in regional lymph nodes, it is definitely not stage I. Adjuvant radiotherapy and chemotherapy are also needed after surgery.