Omitting CTV for Primary Tumor in LS-SCLC

• Radiation Therapy (Radiation): Twice-daily (45 Gy/30 fractions) or once-daily (45 Gy/15 fractions) thoracic radiotherapy after 2-4 cycles of chemotherapy
• Etoposide + carboplatin; Etoposide + cisplatin (Drug): Carboplatin IV (AUC=5) on day 1 combined with etoposide IV (100mg/m2) on days 1-3, or cisplatin IV (25mg/m2) on days 1-3 combined with etoposide IV (100mg/m2) on days 1-3. Treatment is repeated every 21 days for 4-6 cycles.
• Creating CTV for primary tumor (Radiation): A margin of 0.8 cm beyond the gross target volume of primary tumor.
• prophylactic cranial irradiation (PCI) (Radiation): Beginning 4-6 weeks after chemoradiotherapy completion, patients in both arms who achieve a complete or partial response without brain metastasis receive PCI at 25 Gy/10 fractions or 26 Gy/13 fractions, delivered once daily (5 days per week).

Primary outcomes

• Local progression free survival [From the date chemotherapy commenced to the first clinical or radiological evidence of progressive disease at the primary site or death, whichever occurs first, to be assessed up to 3 years]
• Severe toxicity free survival [From the date chemotherapy commenced to the first record of serious adverse events related to radiotherapy, to be assessed up to 3 years]

Secondary outcomes

• Overall survival [From the date chemotherapy commenced to the date of death from any cause or the end of follow-up at 3 years]
• Progression free survival [From the date chemotherapy commenced to disease progression or death, whichever occurs first, to be assessed up to 3 years]
• Number of participants with recurrence or metastasis as evaluated by RECIST 1.1 [From the date chemotherapy commenced to the end of follow-up at 3 years]
• Number of participants with treatment-related adverse events as assessed by CTCAE v4.0 [From the date chemotherapy commenced to the end of follow-up at 3 years]

Sex: All, ages: 18 Years to 75 Years

Inclusion Criteria:

1. Small cell lung cancer confirmed by histology or cytology; TNM stage IA-IIIC (AJCC 8th edition, 2017; limited-stage) without intrapulmonary metastasis;
2. Aged 18-75 years, KPS score ≥ 80, ≤ 10% weight loss within the past 6 months;
3. Have measurable lesions per RECIST 1.1 criteria;
4. Have no disease progression after 2-4 cycles of etoposide/carboplatin or etoposide/cisplatin;
5. Lung function test: FEV1 ≥ 1 L (Optional);
6. Complete blood count: neutrophil count ≥ 1.5 x 10\^9/L, hemoglobin ≥ 100 g/L, platelet count ≥ 100 x 10\^9/L;
7. Renal function: serum creatinine ≤ 1.5 x upper limit of normal (ULN);
8. Liver function: AST and ALT ≤ 2.5 x ULN, bilirubin ≤ 1.5 x ULN;
9. Fully understand the study, able to complete treatment and follow-up, and voluntarily sign the informed consent.

Exclusion Criteria:

1. Other malignant tumors (prior or concurrent), except those that have been curatively treated with disease-free survival ≥ 5 years, such as non-melanoma skin cancer, cervical carcinoma in situ, or early-stage papillary thyroid cancer;
2. Uncontrolled heart disease or myocardial infarction within the past 6 months;
3. Patients with a history of mental illness;
4. Pregnant and lactating patients;
5. Poorly controlled diabetes and hypertension;
6. Interstitial pneumonia or active pulmonary fibrosis;
7. Active infection;
8. Other conditions unsuitable for enrollment (per investigator judgment).