Progressive leptomeningeal metastases (LM) are associated with intractable neurological symptoms and a poor prognosis, and effective treatment options are limited. Intrathecal (IT) pemetrexed has been shown to confer clinical benefit in lung adenocarcinoma, yet our understanding of the efficacy and safety of the treatment is limited. We report a patient with a long-standing history of leptomeningeal disease due to ALK-positive adenocarcinoma of the lung, previously controlled by increased …