Next-generation sequencing clarified why first-line treatment with osimertinib was ineffective in an autopsied case of EGFR-mutated lung squamous cell carcinoma.

Epidermal growth factor receptor (EGFR)-mutated squamous cell carcinoma (SCC) is less common than adenocarcinoma. The third-generation EGFR-tyrosine kinase inhibitor, osimertinib, is effective in EGFR-mutated lung adenocarcinoma, but its efficacy in EGFR-mutated lung SCC is unclear. The patient was an 83-year-old male. He was diagnosed with SCC of the lung, and molecular analysis revealed that the tumor was positive for EGFR exon19 deletion. He was treated with osimertinib 80 mg/day. No adverse events were observed, but after 18 days of therapy, he complained of dyspnea, and a computed tomography scan showed enlarged lung cancer. The case was categorized as a progressive disease. The patient died 3 weeks later. The autopsy findings confirmed the diagnosis of lung SCC, with morphology and immunohistochemical staining identical to the tumor obtained by bronchoscopy. Next-generation sequencing showed the presence of TP53 R158L, CDK6, and KRAS amplifications. The current case report shows that next-generation sequencing can explain why osimertinib is ineffective in EGFR-mutated SCC.