Fourth ventricular pilocytic astrocytoma mimicking an arachnoid cyst: A case report and review of literature.

Background: Arachnoid cysts and pilocytic astrocytomas (PAs) are pathologically distinct lesions. Arachnoid cysts are essentially cerebrospinal fluid (CSF)-filled collections lined by the arachnoid membrane, mostly congenital but sometimes developing later in life. In contrast, PAs are low-grade, solid-cum-cystic brain tumors arising from astrocytes that mainly affect children and young adults. Fourth ventricular PAs are exceptionally rare, with approximately 56 cases reported in the literature since the early 1990s. Case Description: We report a case of a 20-year-old female who presented with progressive headache, nausea, and gait imbalance. Magnetic resonance imaging (MRI) revealed a nonenhancing cystic lesion in the fourth ventricle, initially suspected to be an arachnoid cyst due to the absence of mural nodules or enhancing solid components. Given her symptomatic presentation, she underwent a midline suboccipital craniotomy with cyst fenestration. Intraoperatively, the appearance of the cyst did not resemble an arachnoid cyst; hence, the cyst wall was sent for histopathological evaluation, which confirmed the diagnosis of PA. Postoperatively, the patient experienced significant improvement in her symptoms, and her neurological examination normalized. Follow-up MRI at 1 year demonstrated no evidence of progression. Conclusion: This case emphasizes the diagnostic difficulty of distinguishing cystic, nonenhancing PAs from arachnoid cysts on imaging alone, particularly in the fourth ventricle of adults. Our findings highlight the critical role of surgical exploration both for symptom relief and for obtaining tissue to establish a definitive diagnosis, as well as the importance of long-term radiological surveillance due to the limited resection achievable in this anatomically constrained region.