ABSTRACT. A 5-year-old female cross-breed dog was presented with a 1-month history of progressive changes in the posture of the head and in the gait. At neurological examination the dog showed a central vestibular syndrome lateralized to the left. MRI showed a space occupying lesion within the fourth ventricle, characterized by iso-to hypointensity in T1 and hyperintensity in T2 with a heterogeneous contrast uptake. Histologically, a neoplasia composed of meningothelial cells forming compact whorls with slight atypia, and stellate cells delimitating microcysts containing eosinophilic fluid was observed. Neoplastic cells were positive for vimentin and negative for GFAP and FVIII. A diagnosis of intraventricular microcystic meningioma was achieved. Intraventricular meningiomas in dogs are rarely encountered and reports of meningiomas within the fourth ventricle have not yet been described. Although choroid plexus tumor is the most frequent neoplasia localized in the fourth ventricle, intraventricular meningioma should be included in the differential diagnoses.KEY WORDS: canine, central nervous system tumors, intraventricular meningioma, microcystic meningioma.J. Vet. Med. Sci. 73(3): 367-370, 2011 Meningiomas are the most common canine intracranial tumors and differently from human neuro-oncology are frequently classified as atypical meningioma (grade II) with a poorer therapeutic response [14]. Meningiomas involve most commonly the anterior half of the dorsal surface of the brain near the falx [11]. Other common locations are tentorium cerebelli, ventral or lateral aspect of the brainstem, mainly at hypothalamus and optic chiasm level, and other external surfaces of the brain with a typical extra-axial growth pattern [9,11]. Although the intraventricular location involving the tela choroidea of the third ventricle represents a common site for meningiomas in cats [15], it is rare in dogs and the presence of meningiomas in the fourth ventricle has not yet been described in the literature. A secondary involvement of the fourth ventricle by a meningioma consequent to local extension has been described in a dog [6]. In human beings, intraventricular meningiomas are rare and meningiomas of the fourth ventricle are exceptional [4].Histologically, meningiomas in animals are classified as meningothelial, fibroblastic, transitional, psammomatous, angiomatous, papillary, granular cell, myxoid and anaplastic [5]. Rare reports of cystic and microcystic meningiomas are also described [1,8,12,14].To our knowledge, no cases of microcystic meningiomas involving the fourth ventricle in dogs have been described so far. This report describes the clinical, neuroradiological and neuropathological findings of a dog with a primary meningioma of the fourth ventricle with histological features suggestive of microcystic subtype.A 5-year-old female cross-breed dog was presented with a 1-month history of progressive changes in the head posture, frequent fallings, circling to the left side and excessive anxiety. Neurological evaluation rev...