A Pilocytic Astrocytoma Mimicking a Clinoidal Meningioma

Pilocytic astrocytomas and meningiomas are benign, primary brain tumors that may involve the optic tract. Classically, the presence of a dural “tail” sign may differentiate a meningioma from other intracranial lesions. In this report, we describe a mass with the typical appearance of a clinoidal meningioma on magnetic resonance imaging (MRI) but postoperatively diagnosed as a pilocytic astrocytoma. This case illustrates the rare occurrence of a pilocytic astrocytoma mimicking a meningioma on MRI. 1. Background Pilocytic astrocytomas are benign, World Health Organization (WHO) grade I gliomas that most frequently arise in adolescents and young adults and usually involve the cerebellum, hypothalamus, and optic pathways. Radiographically, they are relatively well demarcated and strongly enhance on MRI as they are well vascularized and often exhibit glomeruloid vascular proliferation. Although rare, pilocytic astrocytomas can originate from the optic pathway, anterior or posterior to the optic chiasm. These lesions have a strong association with neurofibromatosis type 1 (NF1) and are estimated to occur in 15% of all patients with this inherited syndrome [1]. In contrast, meningiomas are benign growths of arachnoid cap cells, usually found at sites of dural reflections within the skull, and classically exhibit a homogenously enhancing thickening of the dural margin with peripheral tapering on MRI, known as the “tail” sign [2]. Because of these characteristic features, meningiomas are usually readily identified based on clinical and radiographic findings alone, often precluding a need for tissue biopsy prior to treatment. In this case report, we describe a young patient whose clinical and radiographic presentation strongly supported a diagnosis of a meningioma but upon neurosurgical intervention was found to have a pilocytic astrocytoma. We describe the clinical course of our patient and highlight the unusual features that were subsequently found. 2. Case Report An 18-year-old female, 36 weeks into pregnancy, presented to ophthalmology with poor right-sided vision evolving over few years. Her exam was notable for right optic nerve pallor and atrophy. A brain MRI without contrast due to her pregnancy showed a right supraclinoid mass measuring 2.5 × 2.2 × 2.5？cm, for which she was referred for neurosurgical evaluation. No other neurological deficits were present on examination. Considering the stability of the visual deficit and the late gestational age, a contrast study was postponed until after her delivery. The subsequent MRI confirmed an extra-axial mass in