Prevalence and Clinical Implications of Somatic and Germline EGFR Mutations in Patients with Non-Small-Cell Lung Cancer

Epidermal growth factor receptor (EGFR)-targeted therapy represents one of the earliest and most established examples of precision oncology in non-small-cell lung cancer (NSCLC), with more than 10 approved agents, including tyrosine kinase inhibitors, bispecific antibodies and antibody–drug conjugates. Over the past two decades, the diagnostic and therapeutic landscape of EGFR-mutant NSCLC has evolved from empiric treatment to mutation subtype-guided strategies, from advanced disease to earlier-stage interventions, and from monotherapy to rational combination regimens. Somatic EGFR mutations remain key predictive biomarkers guiding treatment selection, therapeutic intensification, resistance mechanism-directed treatment, and disease monitoring through plasma circulating tumor DNA burden. In parallel, germline EGFR alterations are increasingly recognized as contributors to inherited lung cancer susceptibility, particularly among never-smokers and familial clusters. Germline EGFR T790M is the best-characterized pathogenic variant, creating a permissive background for multifocal lung nodules and lung adenocarcinoma development, often following acquisition of a second somatic EGFR driver mutation. Recent familial, regional, and paired tumor–normal sequencing studies have expanded the evidence beyond isolated case reports and support an emerging hereditary lung cancer predisposition phenotype. Clinically, germline EGFR should be suspected when EGFR T790M is detected prior to TKI exposure, particularly at variant allele fractions near 50%, or in patients with multifocal ground-glass nodules, multiple primary lung adenocarcinomas, early-onset disease, never/light smoking history, or family history of lung cancer. Confirmation requires germline testing and genetic counseling. This review highlights the current knowledge, recent advances, and future directions in somatic and germline EGFR-mutant NSCLC, emphasizing translational relevance for clinicians and researchers.