The Great Pretender in the Sella: A Rare Doughnut-Shaped Partially Thrombosed Internal Carotid Artery Aneurysm Mimicking Pituitary Macroadenoma

Sellar and suprasellar lesions are usually attributed to pituitary adenomas, craniopharyngiomas, and other nonvascular pathologies. Rarely, however, a partially thrombosed intracranial aneurysm can present as a tumor-like mass and create a dangerous diagnostic trap. We report a 53-year-old woman who presented with multiple episodes of vomiting for three days and intermittent headache. She had no history of loss of consciousness, seizure-like activity, or trauma, and her examination was neurologically intact, including normal visual acuity. Magnetic resonance imaging performed at another hospital showed an enlarged sella containing a 15.8 × 17 × 16 mm sellar-suprasellar lesion with a peripheral low-signal rim, central isointensity, marked T2-weighted hypointensity, and rim enhancement, leading to an initial impression of pituitary macroadenoma or craniopharyngioma. Further vascular imaging demonstrated a medially directed aneurysmal outpouching from the supraclinoid segment of the right internal carotid artery with central thrombus, producing the classic doughnut-shaped appearance. The patient underwent successful endovascular flow-diverter placement through a right femoral approach with excellent wall apposition and marked contrast stasis within the aneurysm sac. This case emphasizes that atypical sellar lesions, especially those showing T2-weighted hypointensity, hyperdensity on computed tomography, and a peripheral enhancement pattern, require vascular evaluation before any surgical plan is made. Early recognition prevents catastrophic hemorrhage and allows minimally invasive endovascular treatment with excellent anatomical results. The present case is clinically instructive because the lesion was small enough to resemble a routine sellar mass, yet dangerous enough to alter management completely once its vascular origin was recognized.