Skull Base Tumors

Fig. 15.1. Coronal MRI of patient with extensive fibrous dysplasia involving the anterior cranial fossa.

The common denominator is the replacement of normal bone architecture by tissue composed of collagen, fibroblasts and varying amounts of osteoid or bone. The two most common conditions falling within this group are ossifying fibroma and fibrous dysplasia.

• Ossifying fibroma is an encapsulated benign neoplasm consisting of fibrous tissue and containing varied amounts of metaplastic bone and mineralized masses. It mainly affects bones which are ossified in membrane, usually presenting as a painless mass. Radiological changes are of a discrete mass with a distinct boundary and thinning of the cortical bone, resulting in an eggshell appearance. In most instances, the mass is surrounded by smooth, well defined cortical bone, which differentiates it from fibrous dysplasia where "blending" into the surrounding bone is universal. Its behaviour is that of a benign neoplasm and hence it continues to grow after skeletal maturity is reached. It normally, therefore, warrants surgical treatment at some point.

• Fibrous dysplasia is a benign, localized bone disorder of unknown aetiology that results in extreme thickening of bone owing to the presence of intraosseous proliferating connective tissue. It may be monostotic (affecting one bone) or polyostotic (affecting several bones). It is a self-limiting process that starts in childhood but, due to a very slow growth rate, may not cause symptoms until adulthood. Growth can be expected to slow or stop after puberty in monostotic disease. The most characteristic radio-logic feature is a diffusely blending margin in marked contrast to the sharply demarcated ossifying fibroma (although the features are non-specific, they depend upon the age of the lesion and degree of metaplastic bone formation).

Differential diagnosis of these conditions relies on the correlation of histopathologic and radiologic findings. Both are usually required to provide a definitive diagnosis.

The most common presentation would be with a slow-growing, asymmetrical, painless swelling (see Fig. 15.1). It manifests craniofacial involvement in about 10% of cases and gives

Fig. 15.1. Coronal MRI of patient with extensive fibrous dysplasia involving the anterior cranial fossa.

rise to cosmetic deformity, proptosis and, more seriously, compression of the optic nerve in the optic canal. Cosmetic concerns of the patient may give rise to pressure for consideration of surgical treatment but, on the whole, the management would be conservative in view of the slow natural history. However, signs of optic nerve compression are an absolute indication for surgery, as is the development of other intracranial complications, e.g. cerebrospinal fluid rhinorrhea.

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