Patients undergo cerebrospinal fluid (CSF) and blood sample collection at baseline (at least 4 days after starting erlotinib hydrochloride and prior to radiosurgery) for pharmacokinetic and biomarker correlative studies. Samples are analyzed for concentrations of erlotinib hydrochloride by 2-dimensional-liquid chromatography/mass spectrometry and antithrombin by enzyme-linked immunosorbent assay.