Glioblastoma multiforme with epithelial differentiation: a potential diagnostic pitfall in cerebrospinal fluid cytology.

Cerebrospinal fluid (CSF) cytology provides valuable diagnostic and prognostic information for diseases of the central nervous system (CNS) and remains the gold standard for the detection of neoplastic meningitis. Metastatic involvement of the CSF by non-CNS neoplasms far surpasses that of primary brain tumors, although conventional glioblastoma multiforme (GBM) can occasionally be identified in the CSF. GBM with epithelial differentiation is an uncommon variant that may contain features such as adenoid structures, signet ring cells, or squamous metaplasia. Herein, we present a case of GBM with epithelial differentiation to highlight a potential diagnostic pitfall in CSF cytology. A 55-year-old man presented with neurological symptoms and a 6.4 cm left temporal lobe cystic mass. Primary resection revealed GBM with focal epithelial differentiation confirmed by cytokeratin, epithelial membrane antigen, and glial fibrillary acidic protein immunohistochemical studies. Four months following primary resection, the patient developed severe headache for which a lumbar puncture with CSF cytologic evaluation was performed. The cytospin preparation showed numerous malignant epithelioid cells with high nuclear-cytoplasmic ratio and prominent cytoplasmic vacuoles resembling metastatic carcinoma. However, the lesional cells were cytomorphologically identical to the epithelial component present in the patient's recently diagnosed GBM. This case illustrates the potential for GBM with epithelial differentiation to closely mimic metastatic carcinoma from a non-CNS site in CSF cytology, which expands the differential diagnosis and emphasizes the necessity of clinical correlation.