Leptomeningeal carcinomatosis in non-small-cell lung cancer: initial response to erlotinib followed by relapse despite continuing radiological resolution of disease

A 60-year-old male was diagnosed with T3, N3, M1b epidermal growth factor receptor (EGFR) mutant lung adenocarcinoma. Five months later he developed significant headaches, weakness and numbness of the left leg, and unsteadiness of gait. Magnetic resonance imaging (MRI) brain demonstrated subtle gyral enhancement indicative of early leptomeningeal infiltration. He was commenced on second-line erlotinib which improved his lower limb symptoms. Three months later he developed increased urinary frequency and redeveloped leg symptoms. MRI brain showed improvement in the gyral enhancement. Four weeks later, the patient developed new onset confusion and decrease in mobility. Examination of the cerebrospinal fluid (CSF) demonstrated leptomeningeal carcinomatosis. This case demonstrates radiological and clinical response of leptomeningeal disease to erlotinib in EGFR mutant lung cancer with subsequent clinical relapse despite continued radiological resolution of leptomeningeal disease. This suggests that CSF examination should be considered when monitoring leptomeningeal disease response following treatment as the disease can be undetectable on repeat radiological imaging.