The goal of this retrospective study is to identify predictive biomarkers of efficacy in tumor samples of small cell lung cancer patients treated with chemotherapy-immunotherapy combination using imaging mass cytometry. The main question it aims to answer is: is there any feature in the tumour microenvironment that can discriminate responders from non responders before treatment? Participants will provide consent for the collection of their clinical data and the study of their tumor samples

Small cell lung cancer patients treated with first-line atezolizumab plus platinum-etoposide regimen with available formalin fixed paraffin embedded (FFPE) biopsies at baseline will be identified from Brest University Hospital medical records and their samples will be retrieved from Brest University Hospital Pathology Laboratory for imaging mass cytometry analyses.

Clinical data will be collected for each patient, including progression-free survival (defined as the time between treatment start and date of progression according to investigator, or death from any cause) and overall survival (defined as the time between treatment start and date of death from any cause).

For imaging mass cytometry analyses, a panel of 39 metal-tagged antibodies will be used to characterize the cell composition of lung tumors at baseline (inflammatory, tumor and stromal cells).

Statistical analyses will be performed to correlate the composition of the tumor microenvironment at baseline and the survival data.

Inclusion Criteria:

• Patients with extensive or metastatic small cell lung cancer
• Patients who started a first line treatment with atezolizumab plus carboplatin and etoposide in routine practice before February 2021
• Patients with available tumor sample before treatment start
• No objection from living patients

Exclusion Criteria:

• Patients who started first line treatment with atezolizumab plus carboplatin and etoposide in routine practice after February 2021
• Patients having received immunotherapy in a clinical trial
• Patients with auto-immune disorder
• Patients under legal protection (guardianship, curatorship)
• Patients refusing to participate to the study

All patients who started a first line treatment with atezolizumab plus carboplatin and etoposide at Brest University Hospital before February 2021 with available tumor sample before treatment start

All collected data that underlie results in a publication

Data will be available after the publication of result and ending five years maximum following the last visit of the last patient

Data access requests will be reviewed by the internal committee of Brest University Hospital. requestors will be required to sign and complete a data access agreement