Case reportHistologic transformation of EGFR mutant lung adenocarcinoma without exposure to EGFR inhibition

•Transformation of EGFR mutant lung adenocarcinoma to squamous cell carcinoma.•Histologic transformation without prior EGFR TKI exposure.•Histologic transformation is a potential resistance mechanism to EGFR TKIs.

Resistance to EGFR kinase inhibitors appears to be invariable in the treatment of non-small cell lung cancer. Several mechanisms have been described. Here, we report the first case of histologic transformation of EGFR mutant lung adenocarcinoma without prior exposure to EGFR inhibition.