What is the likely diagnosis?
The features are those of a giant, mostly thrombosed, aneurysm most likely from the ACA / Acom complex.
If the patient was a child, what differential should be entertained?
Craniopharyngioma. They can have variable density and in 90% of cases (adamantinomatous type) calcification is present.
Non contrast axial images through the brain demonstrate a large peripherally calcified mass in the suprasellar region. It is hyperdense.