RationaleLeptomeningeal metastasis (LM) is an important cause of mortality in patients with non-small cell lung cancer (NSCLC). As the symptoms of LM and its early clinical manifestations are nonspecific, early diagnosis of LM is difficult. However, there are few treatment options for LM, which leads to a poor prognosis; thus, increased clinical attention is necessary. The effects of most systemic chemotherapies on metastatic brain tumors (brain metastases and LMs) are limited as they cannot pass the blood–brain barrier; therefore, whole-brain radiation therapy is a therapeutic option. Osimertinib is a potent and irreversible third-generation oral epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI). It binds to EGFR with high affinity when the EGFR T790M mutation is present together with sensitizing mutations. The clinical efficacy of osimertinib in NSCLC patients carrying the T790M mutation has been demonstrated in clinical trial NCT02468661. Intrathecal injection of chemotherapeutic drugs can be directed to a specific lesion. Temozolomide is one such drug, and its effect has been confirmed.Patient and interventionsWe treated a 38-year-old patient with NSCLC who carried the EGFR L858R mutation. We administered a combination of oral osimertinib and oral temozolomide plus an intrathecal injection of cytarabine and whole-brain radiation therapy for symptomatic multiple brain metastases.OutcomesThe patient showed a marked response to this combination therapy. To date (after ~18 months), no recurrence or new lesions have been observed and he is asymptomatic. His disease-free survival surpasses that achieved with any monotherapy for LM.LessonsThis is the first report to demonstrate the response to combination therapy in an NSCLC patient with LM. These findings indicate the potential utility of chemotherapy combined with radiotherapy combined with targeted therapy combined with local treatment, as each treatment acts via a different mechanism, enhancing their therapeutic effects.