Concurrent chemoradiotherapy without disease progression followed by consolidation durvalumab is standard of care for unresectable, stage III non-small-cell lung cancer (NSCLC) (the 'PACIFIC regimen'). However, many patients with poor performance status, older age or comorbidities may be ineligible for chemotherapy due to expected high toxicity. The present study aim to investigate the efficacy and toxicities of sequential chemo-immunotherapy plus thoracic radiotherapy for elderly and/or frail stage III NSCLC patients unfit for concurrent chemoradiotherapy, and to identify the optimal thoracic dose for this patient population.

Inclusion Criteria:

1. Age ≥18 years at time of study entry
2. Histologically documented diagnosis of unresectable stage III NSCLC;
3. Fully-informed written consent obtained from patients;
4. Unfit for concurrent chemoradiotherapy as determined by the multi-disciplinary team board due to one of the following reasons: (1) ECOG 2; (2)age≥70;(3) ECOG 1 and CCI≥1；
5. Adequate bone marrow, liver and kidney function
6. Life expectancy of at least 3 months
7. At least one measurable (RECIST 1.1), thoracic lesion that can be irradiated
8. Histologic or cytologic confirmation of small cell lung cancer
9. Adequate pulmonary function with FEV1 >1 L or >30 % of predicted value and DLCO >30 % of predicted value

Exclusion Criteria:

1. Previous chemo-, immuno- or radiotherapy for NSCLC
2. Major surgical procedure last 28 days
3. History of allogenic organ transplantation, autoimmune disease, immunodeficiency, hepatitis or HIV
4. Uncontrolled intercurrent illness
5. Other active malignancy
6. Leptomeningeal carcinomatosis
7. Immunosuppressive medication
8. Pregnant or breastfeeding women