An Acquired Epidermal Growth Factor Receptor T790M Mutation after the Addition of Bevacizumab to Preceding Erlotinib Monotherapy in a Lung Cancer Patient with Leptomeningeal Metastases

A 53-year-old man with advanced lung adenocarcinoma harboring epidermal growth factor receptor (EGFR) exon 19 deletion received erlotinib. After 12 months of disease control with erlotinib monotherapy, leptomeningeal metastases (LM) occurred. A cerebrospinal fluid examination demonstrated a pre-existing EGFR exon 19 deletion. Bevacizumab was combined with erlotinib, and the LM improved. After six months of combination therapy, however, the LM was exacerbated. A re-examination of the cerebrospinal fluid revealed a T790M mutation and exon 19 deletion. Osimertinib was administered, and the LM improved. The combination of bevacizumab and erlotinib was effective for erlotinib-resistant LM and resulted in the expression of a newly acquired T790M mutation, which enabled successful treatment with osimertinib.