Inclusion Criteria:  Provision of informed consent； Newly diagnosed and histological or cytological confirmed extensive stage small cell lung cancer; All indicators has confirmed and allow to receive radiotherapy and chemotherapy; Follow-up can be carried out and the peripheral blood samples can be collected; Cooperate with the provision of clinical pathological data required by the research.  Exclusion Criteria:  Patients have other primary cancers; Patients unable to cooperate with the study for follow-up according to the determined clinical follow-up cycle; Unable to accept the judgment of curative effect evaluation by the designated methods such as CT.