Background: A huge spherical intracranial mass can sometimes be misdiagnosed, due to the lack of typical radiographic features. Thrombosed giant intracranial aneurysms (GIAs) are an uncommon but still a possible differential diagnosis that must be kept in mind to guarantee the best surgical approach and resection of the lesion. We describe an extremely rare case of a huge bifrontal mass mimicking a cystic echinococcosis, in which the surgery unveiled a completely thrombosed GIA of the left anterior cerebral artery (ACA).

Case Presentation: A 61-year-old patient complained about intermittent weakness of the right leg, mild holocephalic headache, beginning cognitive deficits and lethargy. Magnetic resonance imaging (MRI) showed a huge partially calcified and bilobed frontal mass with peripheral edema. Based on a time-resolved angiography with interleaved Stochastic trajectories MRI (TWIST-MRI), a vascular origin of the lesion was considered unlikely. Therefore, the surgery was performed under the suspicion of a cystic echinococcosis but revealed a bilobed GIA of the left ACA with a parent vessel thrombosis. Although only a limited left frontal craniotomy was performed, a proximal control of the parent vessel could be ensured, and the aneurysm was successfully clipped. The patient showed postoperatively no new neurological deficits.

Conclusions: Completely thrombosed GIAs with parent vessel thrombosis are rare lesions that might be misdiagnosed if typical radiographic features are missing. Thus, in case of an intracranial spherical mass with signs of intralesional hemorrhage and mural calcifications, presence of a completely thrombosed GIA should be considered as a possible differential diagnosis.