Abstract
Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) is crucial for patients with lung cancer harboring EGFR mutations. However, almost all patients experience disease progression, regardless of their response to the targeted therapy, necessitating the development of additional treatment options. Two patients with lung cancer harboring EGFR-L858R mutations in exon 21 were treated by surgical resection during successful osimertinib treatment. Because the pathological diagnosis was suspected to be pleural metastasis, osimertinib treatment was continued until disease progression. We analyzed the evolution of genomic alterations and the levels of AXL using tumor specimens obtained by repeated biopsies during the course of treatment: initial diagnosis, operation, and disease progression. Genetic alterations detected at the three time points were dramatically changed and showed reductions in numbers, while EGFR-L858R mutations were detected in all samples tested in both patients. Immunohistochemical expression of AXL remained positive from the beginning of analysis to disease progression. Clonal evolution under oncogenesis is related to gradual accumulation of genomic alterations during tumor growth. However, our case series revealed that volume reduction procedures may cause this phenomenon. Therefore, identification of intrinsic drug-resistant cells in tumors may be as important as detection of acquired genetic alterations.