Unusual localization of pilocytic astrocytoma at the foramen of Monro mimicking a colloid cyst: a case report and literature review

Pilocytic astrocytoma is a circumscribed central nervous system (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, non-enhancing mass at the foramen of Monro 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 lesions at the foramen of Monro should preserve a route for tissue confirmation to secure the correct diagnosis and guide management.