Meningiomas are the most common benign intracranial tumor. They originate from arachnoid cap cells, which are cells within the thin, spider web-like membrane that covers the brain and spinal cord. The arachnoid is one of three protective layers, collectively known as the meninges, which surround the brain and the spinal cord. The other two layers of the meninges are the dura mater and pia mater. Although the majority of meningiomas are benign, these tumors can grow slowly until they are very large, if left undiscovered, and, in some locations, can be severely disabling and life-threatening. Other forms of meningioma may be more aggressive. Most patients develop a single meningioma; however, some patients may develop several tumors growing simultaneously in other locations of the brain or spinal cord.
Some meningiomas are found along the dural lining in the venous sinuses of the brain and skull base – locations where arachnoid cap cells are most abundant. The following subtypes are based on the location of the tumor.
- Cavernous Sinus Meningioma: Occurs near the area that drains deoxygenated blood to the heart from the brain.
- Cerebellopontine Angle Meningioma: Located near the margin of the cerebellum; acoustic neuromas (vestibular schwannoma) are also frequently found in this area.
- Cerebral Convexity Meningioma: Located on the upper surface of the cerebral convexity.
- Foramen Magnum Meningioma: Located near the opening at the base of the skull through which the lower portion of the brainstem passes.
- Intraorbital Meningioma: Located in or around the eye sockets.
- Intraventricular Meningioma: Located in the fluid chambers that produce and carry cerebrospinal fluid throughout the brain.
- Olfactory Groove Meningioma: Located along the nerves connecting the nose to the brain.
- Parasagittal/Falx Meningioma: Located adjacent to the dural fold that separates the two brain hemispheres.
- Petrous Ridge Meningioma: Portion of the temporal bone (which supports the temple) that contain sections of the organs that facilitate hearing.
- Posterior Fossa Meningioma: Occurs near the back of the brain.
- Sphenoid Meningioma: Located near the sphenoid bone behind the eyes.
- Spinal Meningioma: Located in the spine, in some cases against the spinal cord.
- Suprasellar Meningioma: Located near the area of the skull where the pituitary gland is found.
- Tentorium Meningioma: Located near where the brain connects to the brainstem, an area known as the tentorium cerebelli.
Types and Classification
The World Health Organization (WHO) classification of brain tumors is the most widely utilized tool in grading tumor types. The WHO classification scheme recognizes 15 variations of meningiomas according to their cell type as seen under a microscope. These variations are called meningioma subtypes – the technical term for these cell variations is histological subtypes. These histological subtypes are organized into three grades that generally reflects the rate of growth and likelihood of recurrence based on cytological features.
World Health Organization (WHO) Meningioma Classifications
Atypical meningiomas (WHO grade II, which account for 18% of meningioma cases) exhibit increased tissue and cell abnormalities. These tumors grow at a faster rate than benign meningiomas and are often characterized by brain invasion. Atypical meningiomas have a higher likelihood of recurrence than benign meningiomas (WHO grade I).
Malignant meningiomas (WHO grade III) show increased cellular abnormalities and grow at a faster rate than benign and atypical meningiomas. Malignant meningiomas are the most likely to invade the brain and recur more frequently than the other two subtypes.
Prevalence and Incidence
According to the Central Brain Tumor Registry of the United States Statistical Report, of tumors diagnosed in the U.S. in 2012-2016, meningiomas were the most frequently reported overall histology (37.6%) of all primary central nervous system tumors with 33,560 cases projected in 2019. The average annual age-adjusted incidence rate was also highest for meningiomas (8.6 per 100,000 people) of all primary brain and spinal cord tumors. Additionally, these incidence rates for meningioma were observed to increase with age, with a median age at diagnosis of 66 years. The majority of meningiomas with tissue confirmation are non-malignant, with 1.7% confirmed to be malignant (WHO grade III).
The risk of meningioma increases with age with a dramatic increase after 65 years. Children aged 0-14 are at the lowest risk. African Americans have been observed to have higher rates of meningioma than other ethnic groups in the U.S.
Exposure to ionizing radiation, especially high doses, has been associated with a higher incidence of intracranial tumors, particularly meningiomas. There is also evidence indicating a connection between meningiomas and low doses of radiation. The most well-known case involves children in Israel who were given radiation for scale ringworm between 1948 and 1960. Within the U.S., dental X-rays are the most common form of exposure to ionizing radiation. A number of studies have linked the number of full mouth dental radiographs to increased risk of meningioma.
The genetic disorder Neurofibromatosis type 2 (NF2) is believed to put people at a higher risk of developing meningioma. Patients with NF2 also may be more likely to develop malignant or multiple meningiomas.
Per the Brain Science Foundation, a number of studies have suggested a correlation between meningiomas and hormones, such as the following:
- Increased occurrence of meningioma in post-pubertal women compared with men.
- A higher female to male incidence ratio during reproductive years that disappears with increasing age.
- The detection of estrogen, progesterone and androgen receptors in a significant number of meningiomas.
- A link between breast cancer and meningioma.
- A connection between meningioma growth, menstrual cycles and pregnancy.
Researchers are beginning to explore the possible connection between meningioma risk and the use of oral contraceptives and hormone-replacement therapy procedures.
Furthermore, an association between obesity and meningioma incidence in several large studies indicates a possible underlying relationship.
Because meningiomas commonly are slow-growing tumors, they often do not cause noticeable symptoms until they are quite large. Some meningiomas may remain asymptomatic for a patient’s lifetime or be detected unexpectedly when a patient has a brain scan for unrelated symptoms. Presenting signs and symptoms depend on the size and location of the tumor. Symptoms of meningiomas may include:
- Change in personality or behavior
- Progressive focal neurologic deficit
- Hearing loss or ringing in the ears
- Muscle weakness
- Nausea or vomiting
- Visual disorders
Symptoms can be related more specifically to the location of the meningioma. Examples include:
- Falx and Parasagittal: Impaired levels of brain functioning, such as in reasoning and memory. If located in the middle section, it would likely cause leg weakness/numbness or seizures.
- Convexity: May cause seizures, headaches and neurological deficits.
- Sphenoid: Vision problems, loss of sensation in the face or facial numbness and seizures.
- Olfactory Groove: Loss of smell due to compression of the nerves that run between the brain and the nose. If the tumor grows large enough, vision problems may occur due to compression of the optic nerve.
- Suprasellar: Vision problems due to compression of the optic nerves/chiasm.
- Posterior Fossa: Facial symptoms or loss of hearing due to compression of cranial nerves, unsteady gait and problems with coordination.
- Intraventricular: May block the flow of cerebrospinal fluid, resulting in obstructive hydrocephalus, potentially leading to headaches, lightheadedness and changes in mental function.
- Intraorbital: Buildup of pressure in the eyes, leading to a bulging appearance and potential loss of vision.
- Spinal: Back pain or pain in the limbs caused by compression of the nerves that run into the spinal cord.
It can be difficult to diagnose meningiomas for several reasons. Because the majority of meningiomas are slow-growing tumors and primarily affect adults, symptoms may be so subtle that the patient and/or doctor may attribute them to the normal signs of aging. Adding to the confusion is that some of the symptoms associated with meningiomas can also be due to other medical conditions. Misdiagnosis is not uncommon and, in fact, may take several years to diagnosis correctly.
When a patient presents slowly increasing signs of mental dysfunction, new seizures or persistent headaches or if there is evidence of pressure inside the skull (e.g. vomiting, swelling of the optic nerve head in the back of the eye), the first step should be a thorough neurological evaluation, followed by radiological studies, if needed.
Sophisticated imaging techniques can help diagnose meningiomas. Diagnostic tools include computed tomography (CT or CAT scan) and magnetic resonance imaging (MRI). Intraoperative MRI is also used during surgery to guide tissue biopsies and tumor removal. Magnetic resonance spectroscopy (MRS) may be used to examine the tumor’s chemical profile and determine the nature of the lesions seen on the MRI. PET-MRI of the brain and body may also be useful in the diagnosis and exclusion of a brain metastases mimicking a menigiomas explains Dr Prem Pillay a Singapore Neurosurgeon who is an expert on meningiomas.
Sometimes, the only way to make a definitive diagnosis of the meningioma is through a biopsy. The neurosurgeon performs the biopsy to obtain tissue for examination by the neuropathologist to establish the diagnosis, determine whether the tumor is benign or malignant (and establish a tumor grade) so doctors can recommend an appropriate clinical management plan.
Meningiomas are primarily benign tumors with defined borders that enables complete surgical removal, which offers the best chance for a cure. The neurosurgeon opens the skull through a craniotomy to enable full access to the meningioma. The goal of surgery is to remove the meningioma completely, including the fibers that attach it to the coverings of the brain and bone. However, complete removal can carry potential risks that may be significant, especially when the tumor has invaded brain tissue or surrounding veins.
Although the goal of surgery is to remove the tumor, the first priority is to preserve or improve the patient’s neurological functions. With patients for whom total removal of the tumor carries significant risk of morbidity (any side effect that can cause decreased quality of life), it may be better to leave some of the tumor in place and observe future growth with regular imaging studies. In such cases, the patient will be observed over a period of time with regular examinations and MRIs, while for other patients, radiation therapy may be deemed the best approach. It is common for patients to undergo preoperative embolization of the tumor to ensure safety during the surgical procedure. The embolization procedure is similar to a cerebral angiogram except that the surgeon fills the blood vessels in the tumor with a compound to stop blood supply to the tumor.
Observation over a period of time may be the appropriate course of action in patients who meet the following criteria:
- Patients with few symptoms and little or no swelling in the adjacent brain areas
- Patients with mild or minimal symptoms who have a long history of tumors without much negative effect on their quality of life
- Older patients with very slow-progressing symptoms
- Patients for whom treatment carries a significant risk
- Patients who choose not to have surgery after being offered alternate treatment options
Radiation therapy uses high-energy X-rays to kill cancer cells and abnormal brain cells, and to shrink tumors. Radiation therapy may be an option if the tumor cannot be treated effectively through surgery.
Standard external beam radiotherapy uses a variety of radiation beams to create a conformal coverage of the tumor, while limiting the dose to surrounding normal structures. The risk of long-term radiation injury with modern delivery methods is very low. Newer techniques of delivery aside from 3-dimensional conformal radiotherapy (3DCRT) include intensity-modulated radiotherapy (IMRT).
employs a specific type of radiation in which protons, a form of radioactivity, are directed specifically to the tumor. The advantage is that less tissue surrounding the tumor incurs damage.
Stereotactic radiosurgery (such as Gamma Knife, Micro-Radiosurgery, Novalis ) is a technique that focuses the radiation with many different beams on the target tissue. This treatment tends to incur less damage to tissues adjacent to the tumor. Proton Therapy is also another emerging technology to treat meningiomas especially recurrent, aggressive and malignant meningiomas states Dr Prem Pillay a Singapore Neurosurgeon who was previously at the Cleveland Clinic and MD Anderson Cancer Center.
Chemotherapy is rarely used to treat meningioma, except in atypical or malignant subtypes that cannot be adequately treated with surgery and/or radiation therapy. Hormonal therapy may be used for selected meningiomas states Dr Prem Pillay.
In adults, the patient’s age at the time of diagnosis is one of the most powerful predictors of outcome. In general, the younger the adult, the better his or her prognosis tends to be. There generally is a better outcome if the entire tumor is surgically removed; however, this is not always possible due to the location of the tumor.
Data from the Central Brain Tumor Registry of the United States Statistical Report indicates an overall ten-year survival rate for non-malignant meningioma of 84%. Individuals with malignant meningiomas have an overall ten-year survival rate of 62%. Non-malignant meningiomas of the spine conferred a better ten-year survival (96%) than non-malignant meningiomas of the cerebral meninges (83%). Furthermore, malignant spinal meningiomas had higher ten-year survival rates (73%) than malignant brain meningiomas (55.7%).
References and Acknowledgements
American Association of Neurological Surgeons Patient information
Singapore Brain Spine Nerves Center Treatment Protocols and information
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