Largest documented cerebral aneurysm mimicking a giant brain tumor and treated by highly selective occlusion of inflow: illustrative case.

BACKGROUND: Giant intracranial aneurysms (GIAs) are rare vascular lesions associated with significant morbidity and mortality. Their management is particularly challenging due to the absence of universal treatment guidelines, necessitating individualized strategies tailored to each case's unique morphology and hemodynamics. OBSERVATIONS: The authors report the case of a 66-year-old woman who presented with progressive aphasia, gait disturbance, and hemiparesis over several months. Initial CT imaging revealed a large, left-sided, partially calcified mass measuring 11 cm in diameter with significant perilesional edema, initially suspected to be a brain tumor. Further evaluation with angiography confirmed a giant, partially thrombosed aneurysm, the largest cerebral aneurysm documented to date, to the authors' knowledge. Hemodynamic analysis demonstrated a delayed intra-aneurysmal "washout" phenomenon and an anteroposterior pressure gradient, with adequate collateralization. A targeted approach using highly selective anterograde and retrograde coil embolization only of the aneurysm's inflow and outflow points achieved complete parent artery occlusion. This resulted in full aneurysm thrombosis, resolution of perilesional edema, significant reduction in mass effect, and complete neurological recovery. LESSONS: Despite their size, GIAs may still allow for a straightforward, safe, and effective treatment strategy. This case underscores the critical importance of thoroughly assessing the morphology and hemodynamics of GIAs to guide treatment decisions. https://thejns.org/doi/10.3171/CASE2539.