Efficacy of osimertinib for lung squamous cell carcinoma with de novo EGFR T790M‐positive: Case report and literature review

Among epidermal growth factor receptor (EGFR) mutation‐positive non‐small cell lung cancers, squamous cell carcinoma is less common and shows lower responsiveness to first‐generation EGFR tyrosine kinase inhibitors (TKIs) compared to adenocarcinoma. However, the efficacy of osimertinib for squamous cell carcinoma with EGFR mutations is not well known. This study reports the case of a 57‐year‐old male diagnosed as having stage IIIC squamous cell lung cancer. Oncomine Dx Target Test identified EGFR exon19 deletion and de novo EGFR T790M mutation with variant allele frequencies (VAF) of 21.6% and 25.2%, respectively. The patient was treated with osimertinib after progression on chemoradiotherapy followed by durvalumab, and a partial response was maintained for more than 20 months. To predict EGFR‐TKI efficacy, confirmation of gene mutations and VAF using next‐generation sequencing is helpful.