• Ruofan Huang, Xiao Xu, Dong Li, Kun Chen, Qiong Zhan, Mengxi Ge, Xinli Zhou, Xiaohua Liang, and Ming Guan.
• Department of Oncology, Huashan Hospital, Shanghai Medical College, Fudan University, Shanghai, 200040, China.
• Target Oncol. 2019 Jun 1; 14 (3): 343-350.

BackgroundThe presence of specific mutations in the EGFR gene informs the clinical pathway of therapy for patients with lung adenocarcinoma (LAC), including those with central nervous system (CNS) metastases. Plasma circulating cell-free DNA (cfDNA) has been demonstrated to carry the mutational information of LACs, which serves as a biomarker to guide treatment. However, whether the cerebrospinal fluid (CSF) enriches circulating tumor DNA (ctDNA) released from CNS metastatic lesions of LAC, and whether the CSF ctDNA can be used to characterize these lesions remains unknown.ObjectiveTo explore the EGFR status in CNS metastases of LAC patients, and to guide the treatment of intra- and extracranial tumors in these patients.Patients And MethodsThe EGFR mutational status in the cfDNA from paired CSF and plasma samples from LAC patients with CNS metastases, including 20 brain metastases (BM) and 15 leptomeningeal metastases (LM), was assessed by droplet digital polymerase chain reaction (ddPCR). The clinical outcomes of the EGFR status-based intervention were investigated.ResultsEGFR mutations were detected in 23/35 LAC patients (65.7%). EGFR mutations in the plasma or CSF were detected in 6/11 (54.5%) and 5/10 (50%) BM patients, and in 4/11 (36.4%) and 9/12(75%) LM patients, respectively. The prevalence of the T790M mutation was significantly higher in plasma (9/23) than in CSF (3/23) samples. The sensitivity and specificity of the ddPCR-based EGFR mutation test in CSF or plasma samples versus the primary tumor samples were 56% and 89% versus 46% and 100%, respectively. Twelve patients received a first-generation EGFR TKI (tyrosine kinase inhibitor) after the detection of sensitive EGFR mutations in their CSF or plasma, and five patients were switched from a first-generation EGFR TKI to osimertinib after the detection of the T790M mutation.ConclusionsThe EGFR T790M mutation in plasma cfDNA is a sensitive marker for EGFR TKI resistance when CNS metastases progressed. CSF ctDNA increases the diagnostic validity for EGFR genotyping of lung cancer brain metastasis. ddPCR in CSF and plasma samples could provide less invasive and close monitoring of the EGFR status of LAC patients with CNS metastases.

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