This multicenter, prospective study aims to investigate the efficacy and safety of low-dose radiotherapy (15Gy/1.5 Gy bid × 10 fractions) combined with 4-6 cycles of systemic chemotherapy plus anti-PD-L1 antibody, followed by anti-PD-L1 antibody maintenance for up to two years, in participants with extensive-stage small cell lung cancer (ES-SCLC).

Additionally, dynamic monitoring using next-generation sequencing (NGS) technology will be performed to assess ctDNA levels, genetic information, and mutation abundance at the following time points: before and after low-dose radiotherapy, after 4-6 cycles of chemotherapy plus anti-PD-L1 antibody (chemotherapy-immunotherapy), and before the initiation of anti-PD-L1 maintenance therapy. This approach aims to guide efficacy and prognosis prediction, as well as to identify potential biomarkers associated with treatment response.

This prospective, multicenter study plans to enroll 43 ES-SCLC participants without malignant pleural effusion. All participants will first receive low-dose radiotherapy (15Gy/1.5 Gy bid × 10 fractions), followed by systemic chemotherapy (EP or EC regimen) combined with anti-PD-L1 antibody within one week after radiotherapy completion for 4-6 cycles. After 4-6 cycles of chemo-immunotherapy, anti-PD-L1 antibody maintenance therapy will be continued for two years. This treatment model is expected to achieve survival outcomes (PFS and OS) comparable to or even better than those of the MATCH study from West China Hospital (mPFS 6.9 months) and the NCT04562337 study from Shandong Cancer Hospital (mPFS 6.9-10.1 months), with manageable toxicity.

Furthermore, dynamic monitoring using next-generation sequencing (NGS) technology will be performed to assess ctDNA levels, genetic information, and mutation abundance at three time points: before and after low-dose radiotherapy, and after 4-6 cycles of chemo-immunotherapy (before the initiation of anti-PD-L1 maintenance therapy). This approach is expected to identify potential biomarkers that can guide efficacy and prognosis prediction, thereby better serving clinical practice.

Inclusion Criteria:

1. Age ≥18 years and ≤75 years.
2. Histologically or cytologically confirmed extensive-stage small cell lung cancer (SCLC).
3. No prior systemic anti-tumor therapy.
4. ECOG performance status 0-2 and life expectancy ≥12 weeks.
5. At least one measurable lesion per RECIST version 1.1.
6. No brainstem metastasis and no significant neurological symptoms from brain metastases.
7. All participants must undergo routine contrast-enhanced CT of the chest and upper abdomen, as well as brain MRI, before enrollment. Whole-body PET-CT plus brain MRI is preferred. For participants with limited financial resources who also have significant obstructive pneumonia or atelectasis, an additional chest MRI is recommended.
8. No malignant pleural effusion or pericardial effusion.
9. All participants must provide written informed consent.

Exclusion Criteria:

1. Presence of interstitial pneumonia or infectious fever prior to treatment.
2. Comorbid autoimmune diseases or long-term oral corticosteroid use (including those who received oral corticosteroids within 14 days prior to treatment).
3. Prior history of thoracic radiotherapy or thoracic surgery.
4. Presence of Grade ≥3 hematologic toxicity or hepatic/renal impairment.
5. Hypersensitivity to adebrelimab.
6. Significant respiratory symptoms that preclude tolerance to radiotherapy.
7. Active hepatitis B or hepatitis C infection with ongoing antiviral therapy. Subjects with a documented history of HBV infection who have achieved undetectable HBV levels after prior active treatment at the time of enrollment may be included.
8. Presence of pleural effusion or pericardial effusion.