Table 1 Key inclusion and exclusion criteria of the TREASURE trial

• Fully-informed written consent

• Confirmed ED SCLC

• ECOG performance status score ≤ 1

• Any response after four cycles of induction chemo-immunotherapy defined as CR/PR or thoracic SD with CR/PR of extrathoracic lesions

• Thoracic treatment volume considered treatable using acceptable radiation fields as judged by a radiation oncologist

• 28 ± 7 days between last administration of chemo-immunotherapy and randomization.

• Patients with a history of treated CNS metastases are eligible, if there is no ongoing requirement for corticosteroids as therapy for CNS disease. Patients with asymptomatic brain metastases that do not require local therapy with irradiation (whole brain irradiation) can be included.

• No previous radiotherapy to lung and mediastinal lymph nodes within the past 5 years

• Availability of pre-treatment tumor tissue specimen

• FEV1 ≥ 40%

• Adequate bone marrow, renal function, and hepatic functions

• Prior treatment with immunotherapeutic drugs (with the exception of induction chemo-immunotherapy as defined in inclusion criteria)

• Prior therapy for limited-stage SCLC with curative intent

• History or current radiology suggestive of interstitial lung disease (ILD) (including but not limited to idiopathic pulmonary fibrosis (IPF), formerly described as usual interstitial pneumonia (UIP), and cryptogenic fibrosing alveolitis (CFA)), non-infectious pneumonitis, drug-induced pneumonitis, idiopathic pneumonitis.

• Any concurrent cancer treatment or major surgery (as defined by the Investigator)

• Active or prior documented autoimmune or inflammatory disorders or history of active primary immunodeficiency

• Current use of immunosuppressive medication

• Positive testing for hepatitis B virus surface antigen (HBV sAg), hepatitis C virus ribonucleic acid (HCV RNA), or human immunodeficiency virus (HIV)

• History of another primary malignancy except for malignancies treated with curative intent and no known active disease ≥3 years before first dose of study medication

• Any co-existing medical condition that in the investigator’s judgement will substantially increase the risk associated with the patient’s participation in the study.