Meningiomas

OVERVIEW

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).

Risk Factors

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: