Squamous-cell transformation in a patient with lung adenocarcinoma receiving erlotinib: Co-occurrence with T790M mutation

We report on a patient with metastatic lung adenocarcinoma who underwent testing for EGFR mutations in a pleural effusion that failed to show any alterations and received standard first and second-line chemotherapy. She received erlotinib as third-line therapy with a prolonged partial response. At time of progression, re-biopsy showed squamous cell carcinoma, supported by histology and immunohistochemistry. Molecular profiling confirmed EGFR exon 21 L858R and exon 20 T790M mutations. Squamous cell transformation should not defer EGFR re-sequencing.