Metastatic breast cancer in primary lung cancer with compound <i>EGFR</i> mutations: A case report

The epidermal growth factor receptor (<i>EGFR</i>) is the most common driver gene in the development and progression of non-small cell lung cancer (NSCLC). Mutations in <i>EGFR</i> exons 18-21 are frequently observed, particularly exon 19 deletions and the exon 21 L858R point mutations. The T790M mutation in <i>EGFR</i> exon 20 was the first resistance mechanism to tyrosine kinase inhibitors (TKIs) identified in <i>EGFR</i>-mutant NSCLC. The coexistence of an exon 19 deletion and the exon 20 T790M missense mutation in <i>EGFR</i> is relatively rare, with a low incidence. The incidence of breast metastasis from primary lung cancer ranges from 0.5 to 6.0%, making it an infrequent occurrence. The present study reported a patient with primary NSCLC harboring both an <i>EGFR</i> exon 19 deletion and the exon 20 T790M missense mutations, who developed metastatic breast cancer after being progression-free for 1 month. Following the Chinese Society of Clinical Oncology and National Comprehensive Cancer Network guidelines for lung cancer, the patient was treated with furmonertinib, a third-generation targeted antitumor therapy. The present case provides notable insights for the diagnosis and treatment of NSCLC with coexisting <i>EGFR</i> exon 19 deletion and exon 20 T790M missense mutation, and a rare clinical example of breast metastasis from lung cancer.