Transformation to small-cell lung cancer following treatment with EGFR tyrosine kinase inhibitors in a patient with lung adenocarcinoma

We report the case of a 52-year-old woman with lung adenocarcinoma treated with EGFR tyrosine kinase inhibitor (TKI) therapy. After disease progression, histological examination of a secondary biopsy specimen revealed small-cell lung cancer (SCLC) that was sensitive to standard SCLC treatment. Tumor markers, including ProGRP and NSE, were elevated. Transformation to SCLC is a mechanism for acquired resistance to EGFR-TKI therapy. Secondary biopsy is important for evaluation of genetic and histological changes and selection of appropriate treatment. Furthermore, ProGRP and NSE may be useful for early detection of SCLC transformation in cases resistant to EGFR-TKI therapy.