Osimertinib Administration as the Primary Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitor Therapy for Brain Metastasis of De Novo T790M-positive Lung Cancer

A 69-year-old woman underwent left upper lobectomy for left upper lobe lung adenocarcinoma. She later perceived a left visual field defect, and a brain metastasis was detected on head magnetic resonance imaging (MRI). Epidermal growth factor receptor (EGFR) testing identified two separate EGFR mutations: an L858R mutation in exon 21 and a de novo T790M mutation in exon 20. Treatment with osimertinib was started. After one month, head MRI showed that the brain metastasis had shrunk, and the visual field defect had also improved. In this case, first-line osimertinib was effective for treating brain metastasis of de novo T790M-positive lung cancer.