Immunotherapy is probably, since the development of therapies targeting EGFR mutations or ALK rearrangement, the most attractive therapeutic perspective in the management of metastatic lung cancer. Among the compounds tested, the inhibitors of the immune checkpoint PROGRAMME DEATH 1 / PROGRAMME DEATH LIGAND 1 (PD-1/PD-L1) have been tested in numerous clinical trials with recently published positive results leading to the approval of one drug in the USA and an expanded access program for two drugs in France. PROGRAMME DEATH LIGAND 1 (PD-L1) expression by tumor cells is strongly associated with the response to such molecules so that the participation in various clinical trials is currently reserved for patients expressing this biomarker and therefore justifies a new invasive biopsy (bronchoscopic or CT-guided) representing a considerable drag on the access to these treatments. Circulating tumor cells (CTCs) isolated by Isolation by Size of Tumor Cells (ISET) offer a direct and non-invasive access to the tumor. It has already been demonstrated that molecular characterization (EGFR, ALK) on these blood samples is possible. We propose to demonstrate the feasibility of the analysis PDL-1 expression in these cells by immunocytochemistry.