<h4>Background</h4>Leptomeningeal metastasis (LM) after the development of third-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) resistance is indicative of a poor prognosis in EGFR-mutant non-small cell lung cancer (NSCLC), and no standardised treatments are currently available. The gene discussed is EGFR; the disease is lymphangioma.