Multivariable analysis of 84 patients, excluding a patient with unknown brain metastasis, revealed that EGFR exon 21 L858R mutation, malignant pleural effusion or pleural metastasis, liver metastasis, and dose reduction within 6 months were independent factors affecting PFS. Early dose reduction of osimertinib is an effective therapeutic strategy for prolonging PFS in patients with EGFR-mutated NSCLC. The gene discussed is EGFR; the disease is non-small cell lung carcinoma.