Unusual Localization of Pilocytic Astrocytoma at the Foramen of Monro Mimicking a Colloid Cyst: A Case Report and Literature Review

Arturs Balodis^1,2,3*Sigita Zalite⁴Inese Briede⁵Jurijs Nazarovs⁵Liga Jaunozolina⁶Evija Bergfelde⁷Tõnu Rätsep⁸Kaspars Auslands^7,9

• ¹Institute of Diagnostic Radiology, Pauls Stradins Clinical university hospital, Riga, Latvia
• ²Department of Radiology, Riga Stradins University, Riga, Latvia
• ³Faculty of Medicine and Life Sciences, University of Latvia, Riga, Latvia
• ⁴Faculty of Medicine, Riga Stradins University, Riga, Latvia
• ⁵Department of Pathology, Riga Stradins University, Riga, Latvia
• ⁶Center of Radiology, Riga East University hospital, Riga, Latvia
• ⁷Clinic of Neurosurgery, Riga East University hospital, Riga, Latvia
• ⁸Tartu University Hospital, Tartu, Estonia
• ⁹Department of Neurology and Neurosurgery, Riga Stradins University, Riga, Latvia

Pilocytic astrocytoma is a circumscribed CNS WHO grade 1 glioma that typically arises in the cerebellum or optic–hypothalamic pathways; intraventricular occurrence at or near the foramen of Monro is exceptional and can mimic a colloid cyst. Although tissue diagnosis is ultimately required, certain imaging features should keep neoplasm in the differential when a "colloid cyst" is suspected. We report a 22-year-old man with a well-circumscribed, nonenhancing foramen-of-Monro mass initially favored radiologically as a colloid cyst; radiology–pathology correlation after microsurgical resection established pilocytic astrocytoma with characteristic histology and a supportive immunophenotype. This case emphasizes that location and CT density are not pathognomonic and that surgical planning for equivocal foramen-of-Monro lesions should preserve a route for tissue confirmation to secure the correct diagnosis and guide management.