• age ≥ 18 years;

• ECOG performance status score 0-2 points;

• pathologically confirmed small cell lung cancer;

• complete baseline imaging data (including brain enhanced MRI/CT, PET-CT or chest enhanced CT + bone scan + neck and abdominal B ultrasound/CT) before first-line treatment;

• stage extensive-stage SCLC at initial diagnosis, and first-line treatment received standard platinum-based doublet chemotherapy combined with immunotherapy (PD-1 or PD-L1) for at least 4 cycles after the efficacy assessment of SD or PR (residual lesions assessed by chest CT);

• no history of other malignancies;

• reproductive age male/female agreed to contraception during the trial (surgical ligation or oral contraceptives/intrauterine devices + condom contraception);

• life expectancy ≥ 3 months

• 1 week before enrollment, the investigator judged that the patient could continue immune maintenance therapy at the same time, And the organ function level meets the following criteria:

⁃ 1\) bone marrow function: hemoglobin ≥ 80 g/L, white blood cell count ≥ 4.0 \* 10 \^ 9/L or neutrophil count ≥ 1.5 \* 10 \^ 9/L, platelet count ≥ 100 \* 10 \^ 9/L; 2) liver: serum total bilirubin level ≤ 1.5 times the upper limit of normal, when serum total bilirubin level \> 1.5 times the upper limit of normal direct bilirubin level must be ≤ the upper limit of normal,Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 times of the upper limit of normal; serum albumin ≥ 27 g/L; 3) Kidney: serum creatinine level \< 1.5 times of the upper limit of normal or creatinine clearance ≥ 50 ml/min, urea nitrogen ≤ 200 mg/L; 10.Patients must have the ability to understand and voluntarily sign an informed consent form.