Skull Base Tumors
What are Skull Base Tumors?
The term “skull base tumors” refers to a group of tumors that have a tendency to grow along different regions of the bottom part of the skull, mostly on the inside but occasionally also on the outside of the skull. Several tumors that occur throughout the skull base include meningiomas, schwannomas, chordomas, glomus tumors, chondrosarcomas, and metastatic tumors, including malignant head and neck tumors. Skull base tumors are relatively rare. The reported frequency of the more commonly encountered tumors are as follows: fewer than 10% of all primary intracranial tumors are schwannomas, approximately 0.1% of all intracranial tumors are chondrosarcomas and approximately 0.1% of all intracranial tumors are chordomas. Although these are more common types of skull base tumors, different tumors can exist and each surgical approach and procedure is different.
Meningiomas can arise in the sphenoid bone and optic sheath. In the middle cranial base, meningiomas occur in the olfactory groove and planum tuberculum. Posterior fossa meningiomas include tumors of the petrous bone. Meningiomas can be divided into en plaque and en masse tumors. En plaque tumors are flatter and grow along the dura. En masse tumors bulge into the intracranial compartment and often have a dural tail on contrast-enhanced images from radiologic studies.
Schwannomas are also referred to as neuromas, neurinomas, and neurilemomas. These tumors almost always develop from sensory nerves. Because the olfactory and optic nerves do not have a Schwann cell layer, they do not develop these tumors. The most common intracranial schwannoma develops from the vestibular nerve and occupies the posterior cranial fossa. It is referred to as a “vestibular schwannoma” or “acoustic neuroma.” The second most common intracranial schwannomas develop from the trigeminal nerve and account for fewer than 8% of intracranial schwannomas. Trigeminal schwannomas usually arise from the root or ganglion and occupy the middle fossa and, sometimes, the posterior fossa. These schwannomas may occupy both the middle and posterior fossa with a dumbbell shape. Schwannomas of the other cranial nerves are rare.
The clivus is the second most common location, after the sacrococcygeal region, for chordomas. They may develop in persons of any age, but they manifest most commonly in persons aged 20-40 years. A slight male predominance exists. Chordomas develop from remnants of the embryonic notochord and are usually extradural in origin. Dural invasion is possible, but usually occurs late in the course with aggressive or recurrent tumors if a dural defect was left after the initial operation. Chordomas rarely metastasize, but they often invade local structures; therefore, patients may die from the regional spread of the tumor. Approximately 10% of chordomas show malignant histologic features, which may be related to previous irradiation.
Another benign tumor found in the posterior skull base is the “glomus tumor” or paraganglioma. The paraganglia are small aggregates of cells derived from embryonic neuroepithelium that is distributed throughout the body. These tumors are closely associated with the sympathetic (fight or flight nervous system).
Most of the paragangliomas of the skull base are found along the course of the tympanic branch of cranial nerve IX, in the adventitia of the jugular bulb, in the tympanic canaliculus, or along the lesser petrosal nerve. These tumors are slow-growing hypervascular tumors that usually occur in the temporal bone. Patients usually present with gradual hearing loss, unilateral pulsatile tinnitus, and lower cranial nerve deficits. Approximately 1-3% of paragangliomas produce catecholamines, and they may be locally invasive but rarely metastasize.
Chondrosarcomas can occur anywhere in the skeletal system, including intracranially at the petrosphenoclival junction. Most commonly, patients are diagnosed with chondrosarcomas during the third or fourth decade of life. Males are affected more often than females. Chondrosarcomas can be divided into classic, mesenchymal, and dedifferentiated tumors.
Classic chondrosarcomas are composed of large cells with single or multiple nuclei. Microscopically, on low power, abundant cartilaginous matrix is observed. These tumors can be subdivided into 3 grades (I-III). The lower-grade tumors are less aggressive, have minimal malignant potential, and act clinically similar to chordomas. These tumors do not stain for epithelial markers or oncofetal antigens, they occur most commonly as a subtype of chondrosarcomas, and they usually carry a good prognosis.
The dedifferentiated and mesenchymal subtypes occur less frequently and are both more aggressive tumors. The dedifferentiated variety has features in common with an anaplastic sarcoma. The mesenchymal subtype has islands of undifferentiated mesenchymal cells and islands of cartilage.
The symptoms are completely dependent on the size and location of the tumor. They range greatly from small periodic changes; such as headaches, dizziness or hearing changes, to much larger symptoms; such as a complete loss of vision and/or hearing, facial paralysis, or diminished cognitive function. Please consult our physicians if your present symptoms are staying the same or worsening, as nearly any symptom can be a result of a tumor. Only through a thorough evaluation by the physician and appropriate diagnostic tests can this be effectively evaluated.
As always, the first steps in diagnosis is a careful history and physical examination. A CT and/or MRI scan may be performed with and without intravenous contrast enhancement to define these tumors. Additional testing may include audiometric testing (different types of hearing tests), balance testing, and vision testing.
Our physicians use minimally invasive approaches for certain skull base tumors when indicated. Other types of tumors or locations may involve a more invasive surgery. Frequently, our neurotologists work with a team of physicians including neurosurgeons, ophthalmologists, internists and others. Further treatment can include radiation therapy and or stereotactic radiosurgery.