Synchronous occurrence of squamous-cell carcinoma “transformation” and EGFR exon 20 S768I mutation as a novel mechanism of resistance in EGFR-mutated lung adenocarcinoma

The occurrence of secondary EGFR mutation T790M in exon 20 and histologic "transformation" are common mechanisms underlying resistance to EGFR first- or second-generation tyrosine kinase inhibitors (TKI). We describe here on a hitherto unreported mechanism of EGFR TKI resistance synchronously combining squamous-cell carcinoma change and occurrence of the EGFR exon 20 S768I secondary mutation in a 43 year-old woman with stage IV adenocarcinoma harbouring EGFR exon 21 L858R mutation. After 8 months of response to gefitinib, the patient experienced EGFR TKI resistance and died of leptomeningeal neoplastic dissemination.