Osteoma of the inner table of the skull

Clinical History

An 82-year-old lady presented to the emergency department with features suggestive of small bowel obstruction. In lieu of altered sensorium a noncontrast CT head was performed to rule out any intracranial pathology.

Imaging Findings

Noncontrast CT brain failed to reveal any significant cerebral parenchymal lesion. Incidental note was made of a pedunculated mushroom-like osseous lesion arising from the inner table of skull. It demonstrated a trabeculated appearance on the bone window settings. Adjacent frontal cerebral parenchyma displayed normal attenuation and grey-whitte matter differentiation. Remaining skull vault appeared normal.

Discussion

Osteoma is a benign neoplasm of membranous bones mostly involving the skull and facial bones. It represents the most common primary benign neoplasm of the calvarium, and the nose and paranasal sinuses. Amongst the sinuses the frontal sinus is the most frequently affected. Osteomas are classified into three types: (i) compact/dense/ ivory; (ii) trabeculated or spongy; and (iii) mixed. Compact osteomas have Haversian system, are very slow growing and usually demonstrate a wide base. In contrast spongy osteomas have trabecular bone and are more likely to be pedunculated and relatively faster growing. Calvarial osteomas are usually of trabecular variety [2, 3]. Calvarial osteomas predominantly arise from the external table (exostotic). The inner table is rarely involved (enostotic variety), probably, due to some protection endowed by the dura mater [2]. The exostotic variety usually manifests with cosmetic deformity while the enostoctic variety tends to present with features of raised intracranial pressure. Rarely, osteomas may occur purely within the diploic space or at times may involve both the cortices [2]. Enostotic osteoma manifest as a mushroom-like mass with well-defined borders attached via a bony stalk or neck to the inner table of the skull. Two important differential diagnoses of enostotic osteoma include ossified meningioma and metaplastic dural ossification. Metaplastic dural ossifications tend to be multicentric, generally located on both sides of the dural falx, displaying a plaque-like or nodular morphology [4]. Avrahami E. et al studied 38 patients with proven enostotic osteoma (either by operation or by post-mortem examination) and 100 patients with proven meningioma. They concluded that CT examination is usually sufficient for the diagnosis of enostotic osteoma evading the need of unnecessary surgical intervention [1]. These appear as bone density, mushroom-like mass with well-defined borders attached to the inner table of the skull by a bony stalk or neck. In contrast, meningiomas show contrast enhancement and do not present with a bony stalk or neck. They may display cystic changes and/or concomitant osseous changes [1].

The image morphology in the present case corresponds to the classical findings of inner skull table osteoma as described by Avrahami E. et al. As this was simply an incidental finding on neuroimaging, no surgical intervention was planned for this patient.

Differential Diagnosis List

Final Diagnosis

Osteoma of the inner table of the skull

URL:	https://www.eurorad.org/case/9497
DOI:	10.1594/EURORAD/CASE.9497
ISSN:	1563-4086