P11.25.B CARCINOMATOUS ENCEPHALITIS (CE) AS FIRST MANIFESTATION OF OCCULT EGFR-MUTATED LUNG ADENOCARCINOMA MIMICKING LEUKOENCEPHALOPATHY ON MRI.

Abstract BACKGROUND Leptomeningeal carcinomatosis is an uncommon complication of solid tumors, occurring in a minority of patients with advanced malignancies. However, carcinomatous encephalitis—defined as perivascular (Virchow-Robin space) infiltration of malignant cells without mass formation—is even more rare. The simultaneous presence of both leptomeningeal and perivascular parenchymal involvement is exceptional and described only in isolated case reports. These manifestations usually occur in patients with known systemic cancer, particularly lung adenocarcinoma. Here, we present a unique case of dual central nervous system (CNS) involvement as the initial and only manifestation of an occult EGFR-mutated lung carcinoma, with an atypical leukoencephalopathy-like magnetic resonance imaging (MRI) pattern. MATERIAL AND METHODS A 60-year-old female, light smoker with no other significant medical history presented with a 3-month course of headache, disorientation and weight loss. Neurological examination was unremarkable. RESULTS Brain MRI demonstrated symmetrical hyperintensities in the periventricular white matter and brainstem on FLAIR sequences, resembling a leukoencephalopathy pattern. Diffusion-weighted imaging (DWI) revealed multiple millimetric foci of restricted diffusion with a peripheral distribution, while no abnormal enhancement was observed on post-gadolinium T1-weighted images. Thoracic-abdominal-pelvic computerized tomography (CT) showed a 12 millimeters subsolid lesion in the right upper lobe without pathological enhancement on positron emission tomography (PET). CSF analysis demonstrated mononuclear-predominant pleocytosis and undetectable glucose. Cytology detected malignant cells consistent with lung adenocarcinoma, and molecular testing confirmed an epidermal growth factor receptor (EGFR) exon 19 deletion. Brain biopsy confirmed malignant infiltration of subarachnoid space and Virchow-Robin space consistent with simultaneous leptomeningeal carcinomatosis and carcinomatous encephalitis. A diagnosis of CE was established and target therapy was initiated with a daily dose of osimertinib but it was unsuccessful, as she developed seizures and rapidly progressive altered mental state. The clinical evolution was not favorable and unfortunately, she died two months after admission. CONCLUSION This case illustrates the diagnostic challenges of early CNS-only metastatic dissemination. The absence of mass lesions, lack of contrast enhancement, and atypical imaging can mimic non-oncological conditions. Carcinomatous encephalitis, though rare, should be considered in the differential diagnosis of rapidly progressive neurological syndromes—even in patients without a known malignancy.