Skull base, tumour of

Primary skull base tumours are mainly chordomas, chondromas (see chondroma intracranial) and sarcomas.

Intracranial tumours involving the skull base are mainly meningiomas, pituitary adenomas, neurinomas (VI:1), epidermoid tumour, dermoid tumour and glomus tumours.

Tumours invading the skull base from the extracranial compartment include nasopharygeal carcinoma, juvenile nasopharyngeal angiofibroma and lymphoma.

Topographically, the skull base can be divided into three regions: anterior, including the floor of the anterior cranial fossa or the orbital roofs and the cribriform region; central, including the clivus, sella, planum sphenoidale, cavernous sinus and, more laterally, the greater wing of the sphenoid; and the posterolateral region comprising the temporal bone.

Tumours involving the anterior skull base arise from the meninges, the olfactory nerves and mucosa, the upper nasal cavity and paranasal sinuses, the orbit, or the bone itself. Thus, the tumours that occur may be meningiomas, olfactory neuroblastomas (esthesioneuroblastomas) and various sinus, orbit and bone tumours. Primary tumours of the nasal cavity include squamous cell carcinoma, adenocarcinoma, and a variety of rare lesions such as melanoma and plasmacytoma. Rhabdomyosarcoma, usually seen in children, can extend through the skull base, usually from the orbit.

Most malignancies invading the central skull base from below are nasopharyngeal carcinomas, though lymphoma and a variety of rare tumours must also be considered.

The cavernous sinus and Meckel's cave region are the sites of tumours arising from the cranial nerves that traverse this area. Meckel's cave contains the trigeminal ganglion and its divisions that communicate via numerous foramina and fissures with the regions immediately beneath the skull base. These extracranial regions include the pterygopalatine fossa and infratemporal fossa (specifically the masticator space). Thus, the cavernous sinus and Meckel's cave become the target site for tumours spreading along the branches of the trigeminal nerve.

The tumour most commonly implicated in perineural extension is adenoid cystic carcinoma.

MR is the modality of choice to show the detailed topography of the complex spread of skull base tumours. The signs of transforaminal spread include obliteration of the fat just outside the neural foramen and enlargement of the actual bony canal. Occasionally, slight enhancement of the tumour can be seen within the canal itself.

MR clearly shows the bulk of the primary intra- or extracranial tumour as well as its relationship with the skull base and surrounding structures, including the carotid siphon.

Angiography may be necessary, particularly when surgery or endovascular occlusion is envisaged. Encasement and narrowing of the carotid siphon as well as hypertrophy of meningeal branches are well demonstrated.

CT, however, is better at demonstrating the existence and extent of any bony involvement.

GS