A Phase I Study to Evaluate the Safety and Preliminary Efficacy of [225Ac]Ac-DOTATATE Injection Combined With Tislelizumab in the Maintenance Treatment Period for Patients of Extensive-stage Small Cell Lung Cancer (ES-SCLC) With Somatostatin Receptors (SSTR)+ as First-line Treatment

Inclusion Criteria:

1. Patients must have the ability to understand and sign an approved informed consent form (ICF).
2. Patients must be \>= 18 and \<＝80 years of age.
3. Extensive-stage small cell lung cancer that requires histopathological or cytological confirmation.
4. Presence of at least 1 measurable site of disease (based on RECIST 1.1).
5. ECOG score of 0 or 1.
6. SSTR-PET positive.
7. Sufficient bone marrow capacity and organ function:

   Serum creatinine ≤1.5×ULN or creatinine clearance ≥50 ml/min (Cockcroft Gault formula).

   Hemoglobin≥90g/L, neutrophil count ≥1.5×10\^9/L, platelets≥100×10\^9/L. Serum total bilirubin ≤1.5×ULN. Serum albumin ≥30g/L. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤ 2.5×ULN，or ALT/AST≤5×ULN with liver metastases.

   Partially activated prothrombin time (APTT) ≤1.5 x ULN.
8. Subjects of childbearing potential voluntarily use an effective method of contraception, such as condoms, oral or injectable contraceptives, IUDs, etc., during treatment and within 6 months of the last use of the trial drug.

Exclusion Criteria:

1. Pregnant or lactating females.
2. Received systemic antitumor therapy such as targeted therapy, immunotherapy, antitumor herbal therapy, chemotherapy within 4 weeks prior to initiation of study treatment.
3. Uncontrolled congestive heart failure.
4. uncontrolled diabetes mellitus, including baseline fasting glucose \> 2 x ULN.
5. Known hypersensitivity to Lutetium\[177Lu\] Oxodotreotide Injection or \[225Ac\]Ac-DOTATATE Injection and their excipients.
6. Other treatment options (e.g., chemotherapy, targeted therapy) that, in the opinion of the investigator, are more appropriate for the patient than the treatment provided in the study based on the patient's disease characteristics.
7. Unsuitable for the study for any reason, in the opinion of the investigator.