The goal of this retrospective observational study is to assess the clinical utility of plasma-based EGFR testing for detection and longitudinal monitoring of the acquired T790M resistance mutation in patients with advanced EGFR-mutated non-small cell lung cancer (NSCLC) treated with first- or second-generation EGFR tyrosine kinase inhibitors in routine clinical practice in Tunisia. The main questions it aims to answer are: * What is the detection rate of EGFR T790M mutation in plasma at the time of disease progression? * Does repeated liquid biopsy increase the cumulative detection of T790M? * Is T790M emergence associated with baseline clinical and molecular characteristics? * Is T790M status associated with progression-free survival? Participants underwent plasma sampling for circulating tumor DNA analysis during follow-up, and clinical and molecular data were retrospectively collected from medical records to evaluate mutation dynamics and outcomes.
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Detection rate of EGFR T790M mutation in plasma at disease progression
Timeframe: From initiation of first- or second-generation EGFR-TKI therapy until the first documented radiological disease progression at which plasma EGFR T790M testing is performed, assessed up to 36 months.
Proportion of participants with newly detected EGFR T790M on repeat liquid biopsy after an initial negative plasma result
Timeframe: From the initial negative plasma EGFR T790M result at radiological disease progression until detection of T790M on repeat liquid biopsy testing during follow-up, assessed up to 36 months.