EGFR and small cell lung carcinoma: Understanding the mechanisms underlying this transformation is crucial for effective differential diagnosis and the formulation of treatment strategies.<h4>Methods</h4>In this study, we collected tissue from 5 primary LUAD before SCLC transformation, 12 transformed SCLC after EGFR tyrosine kinase inhibitor (TKI) treatment, and 18 de novo SCLC from lung cancer patients treated at Beijing Chest Hospital, Capital Medical University from January 2015 to December 2021.