That's a sequestration dermoid, congenital in origin with intracranial extension along the line of metopic suture.
I would like to keep osteoid osteoma as a strong possibility....since the 3d reconstruction shows the outer covering to be sclerotic and having the same density as the rest of the skull. Seperation from the skin as seen on sagittlal ct, will be seen in both osteoid osteoma and dermoid and cannot be used to differentiate the two. A sclerotic rim goes in favor of - osteoid osteoma and a bony indentation goes in favor of a dermoid which is trying to plunge in. Even when there is intracranial extension the indentation and bone defect will clinge the diagnosis in favor of dermoid. On the other hand a 'sclerotic rim' with density similar to bone will go in favor of osteoma as opposed to a dermoid whose lining is invariably squamous similar to the overlying skin / ectoderm from which it is derived. i am posting an animation to further clarify this... Thanks Hesham, Nice one (Y)
