Periosteal osteoblastoma of the calvaria mimicking a meningioma

While osteoblastoma of the cranial vault is rare, the periosteal form of the tumor is highly unusual, with only one case reported in the English literature. We report on a 24-year-old woman presenting with headache and tinnitus. Magnetic resonance imaging of the brain showed an extra-axial temporal mass with findings that were suggestive of a meningioma. The mass was excised completely, and histological examination revealed a periosteal osteoblastoma arising from the inner surface of the temporal bone and adhering to the dura. The tumor was strongly positive for epithelial membrane antigen, a feature not previously described in osteoblastoma, and one that could lead to a mistaken diagnosis of metaplastic meningioma in a limited sample. A detailed literature review of 40 other reported cases of calvarial osteoblastoma is presented. Apart from being slightly more common in females, calvarial osteoblastoma is similar in all other respects to that arising at conventional skeletal sites. Accurate histological diagnosis of a calvarial osteoblastoma requires adequate sampling of the tumor, including its interface with adjacent structures. Correlation with the radiological findings is crucial for the diagnosis in most cases; however, it is not helpful in differentiating the rare intracranial periosteal variant from a meningioma.