The epidermal growth factor receptor (EGFR) is the most common driver gene in the development and progression of non‑small cell lung cancer (NSCLC). Mutations in EGFR exons 18‑21 are frequently observed, particularly exon 19 deletions and the exon 21 L858R point mutations. The T790M mutation in EGFR exon 20 was the first resistance mechanism to tyrosine kinase inhibitors (TKIs) identified in EGFR‑mutant NSCLC. The coexistence of an exon 19 deletion and the exon 20 T790M missense mutation in EGFR 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 EGFR 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 EGFR exon 19 deletion and exon 20 T790M missense mutation, and a rare clinical example of breast metastasis from lung cancer.
