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.

Acquired resistance to first- and second-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) in advanced EGFR-mutated non-small cell lung cancer (NSCLC) is most commonly mediated by the secondary EGFR T790M mutation. Identification of this resistance mechanism is clinically relevant, as it may guide subsequent treatment strategies. However, repeat tissue biopsy at disease progression is often limited by tumor inaccessibility, patient condition, or procedural risk. Plasma-based analysis of circulating tumor DNA (ctDNA) offers a minimally invasive alternative for molecular reassessment.

This retrospective real-world study evaluates the implementation of plasma EGFR testing for the detection of acquired T790M mutation in routine clinical practice in Tunisia. The study focuses on the detection rate of T790M at progression, the added value of repeated liquid biopsy sampling in initially negative cases, and the relationship between T790M emergence and baseline clinical or molecular characteristics.

In addition, longitudinal patterns of T790M appearance, persistence, or loss across serial plasma samples are explored to better understand resistance dynamics under TKI selective pressure. The association between T790M status and progression-free survival is also assessed.

By integrating molecular results with longitudinal clinical follow-up, this study seeks to characterize real-world patterns of acquired resistance and to evaluate the practical contribution of plasma-based EGFR testing to therapeutic decision-making.

Inclusion Criteria:

• Confirmed advanced (stage IIIB/IV) non-small cell lung cancer
• Documented activating EGFR mutations
• Disease progression on first- or second-generation EGFR TKIs

Exclusion Criteria:

• Prior treatment with third-generation EGFR TKIs before plasma sampling
• Lack of follow-up clinical data

This study included patients with advanced non-small cell lung cancer consulting in different hospitals in Tunisia (The Military Hospital of Tunis, Salah Azaiez Institute, Abderhamen Mami Hospital, Jendouba Hospital, Charles Nicolle Hospital of Tunis...) and referred to the Pathology Department of Charles Nicolle Hospital of Tunis for EGFR T790M monitoring using liquid biopsy.