EGFR Exon 21 L858R Mutation

This is a multicenter, phase II exploratory clinical trial in untreated patients with EGFR-mutant non-small cell lung cancer (stages IIIB-IV) .All participants will receive oral befotertinib monotherapy for 3 weeks first, then serial minimal residual disease (MRD/MRD) testing is performed to adjust subsequent treatment. Patients with positive MRD will receive 4 cycles of pemetrexed plus platinum chemotherapy; patients with negative MRD will continue single-agent befotertinib. After induction, maintenance therapy will be given according to follow-up MRD results. The primary goal is to evaluate progression-free survival guided by dynamic MRD monitoring, and secondary endpoints include objective response rate, disease control rate, safety and MRD clearance rate.