Background: Genomic DNA profiles are mandatory in advanced, treatment naive non-small cell lung cancer (NSCLC) patients (pts) and strongly recommended at progression (PD) on personalized treatment. In pts with PD limited to central nervous system (CNS), tissue biopsy is difficult and the performance of ctDNA is unknown. Methods: Clinical, molecular, imaging data of NSCLC pts included in 2 prospective studies from 01.2016 to 11.2018 at Gustave Roussy were collected. Inclusion criteria were: stage IV disease and any known tissue genomic alteration (GA) EGFR, ALK, BRAF, KRAS, HER2, ROS1, MET, PIK3CA, TP53. Plasma ctDNA collected at baseline/PD were analyzed by next-generation sequencing (NGS-InVisionFirst™-Lung) in 3 groups: pts with isolated CNS (iCNS), extra-CNS only (noCNS) or both combined (cCNS) disease. iCNS was defined as any PD to CNS, while stable/absent extra-CNS metastases (mts). ctDNA was considered positive if ≥1 GA was found. ctDNA in cerebrospinal fluid (CSF) were also collected. Results: Out of 245 pts with ≥1 ctDNA: 56 had iCNS (66 samples), 97 noCNS (127 samples) and 92 cCNS (107 samples). In this cohort, 60% were female, median age 60 years, 47% smokers; 92% had adenocarcinoma. The median number of mts sites was 3 in noCNS/cCNS groups. Proportions of tissue GA at baseline were (iCNS vs noCNS/cCNS): EGFR (50% vs 44%), ALK (30% vs 11%), BRAF (4% vs 12%), KRAS (5% vs 15%), HER2 (2% vs 5%), ROS1 (5% vs 4%). Tyrosine kinase inhibitors were used in 73% iCNS vs 61% noCNS/cCNS. Local brain treatments were performed in 43% (n = 24) vs 32% (n = 29) and leptomeningeal mts (LM) detected in 34% (n = 19) vs 8% (n = 9), in iCNS and cCNS, respectively. CtDNA was positive (+) in 52% in iCNS vs 84% in noCNS and 92% in cCNS (p < 0.0001). In iCNS, there was a non-significantly higher proportion of + ctDNA in pts with LM vs only brain disease (59 vs 48%, P = 0.44). 12/56 pts of iCNS group had serial ctDNA, being collected also at time of cCNS. In 25% of cases, a negative ctDNA at time of iCNS shifted to + at time of cCNS. In 12 iCNS pts, ctDNA was + in 6 (50%) plasma and in 10 (83%) paired CSF (p = 0.193). Conclusions: Detection of GA by plasma ctDNA is lower in NSCLC pts with isolated CNS PD. Alternative strategies (as CSF analysis) should be explored.