The aim of this study is to describe the early dynamics of 18-FDG uptake in non smal cell lung cancer during first line treatment with pembrolizumab, and to evaluate whether or not they differ according to treatment response at 3 months.

Immunotherapy with anti-PD1 antibody, such as Pembrolizumab, is widely used in the treatment of stage III/IV non small cell lung cancer. In order to refine treatments and patients' prognosis, early assessment of tumour response is needed.

The aim of this study is to describe the early tumour metabolic changes in non smal cell lung cancer during first line treatment with pembrolizumab, and to evaluate whether or not they differ according to treatment response.

Eligible patients with stage III/IV, PDL1>50% NSCLC will receive pembrolizumab treatment as per standard of care.

Tumour metabolism will be assessed by 18-FDG PETscan before each of the 3 first pembrolizumab infusions. Treatment response will be determined by RECIST, irRC and mCHOI criteria at 3 months.

Prognostic and predictive value of baseline and dynamic metabolic parameters will be assessed.

Inclusion Criteria:

• Non operable non small cell lung cancer
• Indication of treatment by Pembrolizumab
• PD-L1 status superior to 50 %
• Age more than 18 years
• At least one measurable target
• Written inform consent

Exclusion Criteria:

• no treatment by pembrolizumab
• immunosuppressive treatment
• uncontrolled diabete
• Pregnancy or breast-feeding
• curatorship or guardianship
• not possibility to follow-up the procedures of the study due to geographic, social or psychic reasons