The meningioma is a tumor that arises from the lining cells that are outside of the brain. The meningioma can be benign (most often) or malignant. The meningioma most often has a "pushing" border (does not send tentacles into normal brain). Both the symptoms and the appropriate treatments are highly dependent upon the location from which the meningioma arises.
Origin of Meningiomas
Meningiomas arise from the dura mater (fibrous covering of the brain) or the "leptomeninges" (arachnoid and pia mater).
The dura is composed of two layers.
an outer periosteal (next to the skull) layer and
an inner layer of dural border cells.
The arachoid is also composed of two layers.
The outer, or arachoid barrier layer is composed of the arachoind cap cells.
The inner layer is a loose network of fibroblasts lined by meningothelial cells.
Tumors arising from these layers span the full range of "mesenchymal" neoplasms. The meninigioma is the most commmon mesenchymal tumor to arise from these layers.
Meningiomas account for 13 to 17 per cent of intracranial tumors in the United States. They are probably more common than realized and comprise 33 per cent of asymptomatic brain tumors in a series of 300 autopsies. The relatively benign nature is suggested by the fact that as few as 7 per cent of patients dying of brain tumors have meningiomas.
Meningiomas occur most commonly in the fourth through the sixth decades of life with a peak incidence at 45 years. About 65 per cent occur in females, in whom the average age of occurrence may be less than that for the males (42 versus 52 years).
For patients over 65 years of age, the female predominance is lost and the relative incidence of meningioma to other tumors is higher and is 35 per cent of the brain tumors. About 1.5 per cent of all meningiomas occur in childhood and adolescence.
For malignant meningiomas the female-to-male ratio is approximately 1:1. Malignant meningiomas may occur in younger patients.
In adults the clinical presentation of meningioma depends primarily on the location of the tumor and its rate of growth. More slowly growing tumors become larger before producing symptms. Slowly growing tumors can present with the signs of "increased intracranial pressure" (headache, decreased vision).
Seizures occur in about 50 per cent of patients with meningioma. Focal seizures (arm or leg versus the whole body) are most common with tumors that are close to the "motor" region of the brain (posterior frontal region).
Vascular events are rare. The most common is dural venous sinus occlusion, which is often asymptomatic. Clotting or thrombosis of the large vein at the top of the brain (superior sagittal sinus) can result in headache and increased intracranial pressure. In about 5 per cent of meningiomas some bleeding (hemorrhage) can be shown by MRI. Arterial occlusion (stroke) is rare.
Parasagittal meningiomas arise from arachnoid villi fo the superior sagittal sinus and are usually visible on the surface of the brain. They grow laterally and downward, displacing but not invading the brain. Forty to fifty per cent invade the superior sagittal sinus, about 50 per cent have secondary attachment to the falx and 25 per cent are bilateral. Falcine meningiomas arise from the falx or inferior sagittal sinus and rarely involve the superior sagittal sinus. The falcine meningiomas are less common than the parasagittal meningiomas. The falcine meningiomas can grow through the falx to become bilateral.
Headache may be present for years, followed by slowly progressive personality changes, dementia, increasing apathy, flattening of the affect, ataxia and tremor. Urinary incontinence can develop. Seizures are seen in 25 to 50 per cent of patients.
The most common locations for convexity menningiomas are the parasagittal cortex (near the midline superiorly), the region under the coronal suture, and the pterion (anterior to the motor region).
The clinical presentation depends on the location of the tumor. Harvey Cushing (famous neurosurgeon) delineated nine different tumor sites that are recognizable by the symptoms. Pre-coronal (frontal) meningiomas can present with alterations in personality. Coronal meningiomas (near midline) can present with increased intracranial pressure and weakness of the arm or face.
The sphenoidal meningiomas can be clinoidal, alar and pterional (inner, middle and outer sphenoid). Meningiomas of the inner sphenoid are of two types.
The first grows from the anterior clinoid and medial sphenoid. There can be a history of progressive loss of vision over several years and optic atrophy on examination. The visual deficit involves unilateral loss of acuity due to optic nerve involvement. The visual field can show a field cut. There may be unilateral eye pain as well. Increased intracranial pressure may result in the Foster-Kennedy syndrome (optic atrophy and scotoma in the ipsilateral eye with pailledema in the other eye). In addition, palsies of cranial nerves II, IV, V and VI may occur.
A second type grows "en plaque" as a sheet of tumor and may invade the cavernous sinus, causing ocular venous congestion. Patients can present with slow ipsilateral loss of vision and cranial nerve palsies.
Sphenoid meningiomas can grow on each side of the sphenoid ridge, compressing both the frontal and the temporal lobes as the tumor opens the Sylvian fissure. These tumors are frequently large at the time of diagnosis.
Olfactory groove meningiomas arise over the "lamina cribrosa" of the ethmoid bone, in front of the pituitary gland and anterior or between the optic canals.
The clinical presentation is subtle mental deterioration, often with euphoria and diminished concentration, and occasionally urinary incontinence. Unilateral or bilateral inability to smell (anosmia) is frequent.
These meningiomas arise from the meninges of the anterior clinoid or tuberculum sellae about 1 centimeter posterior to the origin of the olfactory meningiomas. They can lift or push back the optic nerve and the chiasm. Consequently they can be diagnosed earlier as they result in unilateral loss of vision. The most frequent deficit is asymmetrical bitemporal visual field defect. The presence of optic atrophy and the absence of papilledema, anosmia and mental disturbance distinguish this tumor from the olfactory meningioma.
Cerebellopontine angle meningiomas contstitute 30 to 50 per cent of all posterior fossa meningiomas and 3 to 12 per cent of all cerebellopontine angle tumors. They can arise anywhere on the posterior surface of the petrous bone, but the majority arise anterior to the internal auditory meatus (containing the auditory and vestibular nerves).
The symptoms can include hearing loss, vertigo, tinnitus, facial numbness and pain, and headache. The neurologic findings on examination (signs) are decreased hearing, nystagmus, ataxia, facial numbness (hypesthesia) and weakness.
Tentorial meningiomas are 20 to 30 per cent of all posterior fossa meningiomas. They can extend both above and below the tentorium, causing symptoms of cerebellar and brain stem compression as well as symptoms of occipital and temporal lobe compression such as seizure, visual hallucinations or homonymous visual field defects.
Meningiomas of the clivus account for 3 to 10 per cent of the posterior fossa meningiomas and arise from the arachnoid in the region of the junction between the sphenoid and occipital bones of the skull. The symptoms progress very slowly without a characteristic syndrome with an average duration of almost 3 years before diagnosis. At diagnosis the most common symptoms are headache, gait disturbance, hearing loss, vertigo, visual disturbance and dysphagia.
The most common signs on examination are pailledema, facial sensory deficit, ataxia, decreased hearing, unilateral weakness, faical weakness or difficulty swallowing.
Patients with cavernous sinus meningiomas complain of retro-ocular pain for several months to years prior to diagnosis. They may develop protrusion of the eye (exophthalmos) and decreased vision. There is impairment of cranial nerve VI (abducens; provides lateral eye movement) followed by the other cranial nerves that reside in the cavernous sinus (trigeminal (V), trochlear (IV), oculomotor (III)).
How does the patient receive fractionated stereotactic radiosurgery (FSR)?
The patient has the non-invasive creation of the plastic mask for localization and the non-invasive scan. The treatment planning is then performed on the computer. The patient then
returns to the treatment area, is positioned on the treatment table and
receives the FSR. In our institution, almost all meningioma treatments are
fractionated. Fractionation results in safer treatment when compared to single "shot" modalities. Thus, the frame is attached to a plastic mask that is created at the time of the "simulation" and that precisely contours the facial skeletal features. This allows precise "repeat fixation"
of the patient for multiple, outpatient treatments that result in no scars or placement of surgical pins in the scalp and skull as for single dose (gamma knife) modalities. The patient feels nothing as the beam treats the meningioma. Usually there are none of the side effects that can be associated with radiotherapy, such as nausea, red skin or hair loss. Most patients carry on
their normal daily activities before and after the daily treatment.
The rationale for
fractionation of radiosurgery is the same as that for conventional radiation:
It results in the highest "therapeutic ratio" (highest killing of tumor cells
with the lowest effect on normal brain). We know that conventional radiation
could never be done in a single fraction, and we have therefore taken advantage
of the benefit of fractionation for the radiosurgical cases. For most
Meningiomas, 5 to 10 consecutive daily treatments (one per day, M-F, no
weekends) are given. For larger Meningiomas, 10 or even 20 fractions are given,
sometimes in conjunction with the conventional external beam radiotherapy (see
above). Each treatment lasts 15-20 minutes. The patient is awake and
comfortable during the treatments.
The issues for
both radiosurgery and the fractionated stereotactic radiotherapy (FSR) of
meningiomas are the total dose, number of fractions (individual treatments),
the volume of the meningioma, and the proximity of the meningioma to
radiosensitive, critical structures such as the optic chiasm. The nomenclature
has become confusing for the patients. "Radiosurgery" has come to mean single
large treatments. FSR means multiple smaller treatments, stereotactically
applied via the relocatable head frame, whose total dose exceeds that possible
with single dose techniques. FSR exploits the benefits of fractionation in
order to both spare the critical neighboring structures and to increase the
total dose (rad) to the meningioma, beyond that which single shot techniques
demonstrate the rationale for the differing schedules of fractionation: the
cavernous sinus / petroclival meningioma vs. the lateral shenoid wing
(convexity) meningioma. For the cavernous sinus meningioma, the cranial nerves
including the optic nerves and optic chiasm are dose limiting (prevent giving a
higher dose). For radiosurgery (single shot) the tolerance for the optic
structures is approximately 800 rad (reference: Clin Neurosurg 1995;42:99-118).
The rates of cranial nerve deficit following gamma knife was 8% in one series
(reference Neurosurgery 1998 Mar;42(3):437-43). In another gamma knife series
the rates of trigeminal neuropathy (facial numbness) was 10.5% (reference: J
Neurosurg 1999 Jan;90(1):42-9). Still, the rates of control of the treated
meningiomas are very high in these series. The tolerance for the optic
structures following conventional radiotherapy is approximately 5400 rad when
given in 30 treatments. Further, we know that the safety and durability of
control of meningiomas has been high for conventional external beam
radiotherapy (also appx. 5400 rad in 30 fractions) (reference:Int J Radiat
Oncol Biol Phys 1999 Apr 1;44(1):75-9).
FSR utilizes the
relocatable head frames to provide both the stereotactic technique and the
radiobiologically advantageous fractionation. This means that the critical
structures are spared by virtue of the different response to small vs. large
doses (this relationship is very steep and the tolerance of normal tissues
increases exponentially with reduction of fractional dose. Conversely,
tolerance of normal tissues decreases exponentially with fraction size). Our
publication regarding the optic nerve sheath meningioma shows the potential
advantage of fractionated stereotactic radiotherapy in this regard (Reference:
J Neuroophthalmol 1998 Jun;18(2):117-20). Our publication regarding atypical
and malignant meningiomas that received FSR in this location may also be of
interest (reference: J. Radiosurgery 1:4 251-256 (1998).
For the lateral
sphenoid wing meningioma or convexity meningioma, the treatment volume is
distant from more radiosensitive structures such as the optic nerve and chiasm.
The "fractional dose dependence" of normal cortical brain is less severe when
compared to optic nerve and trigeminal nerve. For the same risk, a smaller
number of larger fractions to normal brain (not optic chiasm) is very similar
to the effect of 5400 rad given in 30 fractions. The selection of total dose
and fractional dose is further determined by the "differential dose-volume
histogram (DVH)" that we calculate during our treatment planning. This display
shows dose for each volume element of normal brain vs. meningioma. By
selectively and iteratively improving the distribution of dose, the dose to
normal brain is minimized, again allowing reduction of the number of fractions
and increasing the dose for each fraction. The total dose for the FSR of
convexity meningiomas is less and the fractional doses are higher when compared
to 5400 rad in 30 fractions. Because the fractional doses can be higher for
convexity meningiomas, the intensity is the same or even higher when compared
to the 5400 rad in 30 fractions. The relationship between fractional dose and
intensity of the effect is not linear: In fact it is exponential. A lesser
total dose in much fewer fractions can be as powerful, or more powerful than
the 5400 rad in 30 fractions. We use the "integrated BED (biological equivalent
dose) formalism" to compare intensities of the fractionation schedules for the
small volume of normal brain vs. the tumor effect.