Brain Tumors

Tumor Types

Primary brain tumors can form in any of the cells, membranes, nerves or glands in or around the brain. The type of brain tumor is determined by the location of the tumor, the type of tissue involved, whether it is benign (noncancerous) or malignant (cancerous) and whether it is fast-growing (aggressive) or slow-growing.

There are more than 120 types of brain and central nervous system (CNS) tumors. Some of the more common types include gliomas, meningiomas, schwannomas, pituitary adenomas, pineal gland tumors and primary germ cell tumors of the brain. The following includes information about each of these types, where they are located and some of their notable characteristics.

Gliomas

Glioma is a general term for a group of tumors that begin in glial cells. In the CNS, glial cells include astrocytes, ependymal cells, microglia and oligodendrocytes. Peripheral nervous system glial cells include Schwann cells and satellite cells. Glial cells have four main functions:

  • Surround neurons and hold them in place
  • Supply nutrients and oxygen to neurons
  • Insulate one neuron from another
  • Destroy pathogens (agents that cause infection or disease) and remove dead neurons

Up to 30 percent of all brain tumors are gliomas, and approximately 75 percent of gliomas are malignant (cancerous).

Astrocytomas

Astrocytomas begin in the glial cells called astrocytes, which are star-shaped cells that make up the supportive tissue of the brain. Approximately 20 percent of all brain tumors are astrocytomas, and this type comprises nearly 75 percent of all gliomas.

Astrocytomas can develop in various parts of the brain and CNS, including the cerebellum (back part of the brain), the cerebrum (large front portion of the brain), and central areas of the brain, brain stem and spinal cord.

Astrocytomas may be difficult to remove surgically because they spread widely throughout the brain and blend with normal brain tissue. In some cases, they spread along the cerebrospinal fluid (CSF) pathways, but they rarely spread outside of the brain or spinal cord.

Astrocytomas are graded on a scale from I to IV based on how abnormal the cells look under a microscope. They range from low-grade astrocytomas to high-grade astrocytomas. Low-grade astrocytomas are typically contained in one location and grow slowly. High-grade astrocytomas grow rapidly. Astrocytomas are typically high grade in adults.

Following are the most common types of astrocytomas in order of low grade to high grade.

Pilocytic astrocytomas are typically classified as Grade I astrocytomas. Also called juvenile pilocytic astrocytomas because they tend to occur in children, they are considered the most benign (noncancerous) of all of the astrocytomas. They typically stay in the location where they begin and do not spread. They generally form sacs of fluid (cysts), or may be enclosed within a cyst. Although they are usually slow-growing, these tumors can become very large.

Low-grade astrocytomas are also known as diffuse astrocytoma or astrocytoma Grade II. Types of low-grade astrocytomas include fibrillary, gemistocytic and protoplasmic astrocytoma. With Grade II, more cells look abnormal when examined with the use of a microscope than with Grade I. The tumor may be in the form of a cyst and usually does not spread to nearby tissues, but it is more likely to invade surrounding tissue than a Grade I tumor. They tend to grow at a relatively slow pace.

Anaplastic astrocytomas are considered Grade III tumors and are fairly rare. These tumors are malignant and often require more aggressive treatment than pilocytic astrocytomas. Anaplastic astrocytomas may develop in any area of the CNS. They are more likely to begin in the cerebrum, including any of the lobes (frontal, temporal, parietal or occipital). Other common sites for anaplastic astrocytomas include the part of the brain that contains the thalamus and hypothalamus, the lower area of the brain near the back of the neck that controls movement and balance (cerebellum), and the spinal cord. These tumors tend to have tentacle-like projections that grow into surrounding tissue, making them difficult to completely remove during surgery. This tumor is most often found in adults but can also affect children.

Glioblastomas (GBM) are considered high-grade tumors (Grade IV). They may also be known as astrocytoma Grade IV or glioblastoma multiforme. There are two types of glioblastoma — primary (or de novo) and secondary. Primary tumors are very aggressive and are the most common form of astrocytoma Grade IV. The secondary tumors begin as lower-grade tumors and evolve into Grade IV tumors. This tumor represents about 15 percent of all primary brain tumors.

GBMs usually contain a mix of cell types along with cystic mineral, calcium deposits, blood vessels or a mixed grade of cells. They increase in frequency with age and affect more men than women. GBMs have finger-like tentacles and are difficult to completely remove, particularly when they are near parts of the brain that control functions such as language and coordination (see figure below).

Although GBMs can be found anywhere in the brain or spinal cord, they are generally found in the cerebral hemispheres of the brain.

GBMs are usually highly malignant (cancerous) because the cells reproduce quickly and migrate into the brain substance, and they are supported by a large network of blood vessels. These cells are able to easily invade and live within normal brain tissue. However, they rarely spread elsewhere in the body.

Oligodendrogliomas

Oligodendrogliomas are tumors that develop from cells called oligodendrocytes. These cells are responsible for making myelin. Myelin surrounds the nerves and is rich in protein and fatty substances called lipids. Oligodendrogliomas are subclassified as either oligodendrogliomas, which is considered low grade (Grade II), or anaplastic oligodendroglioma, which is more aggressive (Grade III).

Approximately 2 percent of primary brain tumors are oligodendrogliomas, and they represent about 10 to 15 percent of the gliomas. Overall, this tumor is more likely to occur in adults 35 to 45 years old and is found more often in men than women. Only about 6 percent of these tumors are found in infants and children. Because of their generally slow growth, oligodendrogliomas often are present for years before they are diagnosed.

This type of tumor can be found anywhere within the cerebral hemisphere of the brain. The frontal and temporal lobes are the most common locations. They typically appear as soft, grayish-pink tumors and often contain mineral deposits (calcifications), areas of hemorrhage (bleeding), and/or cysts. The cells may appear to have “short arms” or a fried-egg shape. Occasionally, oligodendrogliomas are mixed with other cell types.

Ependymomas

Ependymomas develop from ependymal cells, which line the ventricles of the brain and the middle of the spinal cord. The ventricles are the passageways where cerebrospinal fluid is made and stored. These tumors are divided into four major types that range from low grade (less aggressive) to high grade (aggressive).

  • Subependymomas (Grade I): This slow-growing tumor usually develops near a ventricle.
  • Myxopapillary ependymomas (Grade I): This slow-growing tumor tends to develop in the lower part of the spinal cord.
  • Ependymomas (Grade II): Ependymomas usually are located along, within or next to the ventricular system. This is the most common ependymal tumor and can be further classified as cellular, papillary, clear cell and tancytic.
  • Anaplastic ependymomas (Grade III): This fast-growing tumor is most commonly found in the brain in adults. It is rarely found in the spinal cord.

These tumors account for about 2 to 3 percent of primary brain tumors and are relatively rare in adults.

Because ependymomas form in the cells that line the ventricles, they most often spread along the cerebrospinal fluid pathways. Ependymomas rarely spread outside the brain or spinal cord. These tumors typically are soft and grayish or red. Ependymomas may prevent cerebrospinal fluid from leaving the ventricles, which causes the ventricles to enlarge.

Surgery often is successful in completely removing an ependymoma because it rarely grows into brain tissue. However, since they can spread along the cerebrospinal fluid pathways, they may be difficult to treat if they have spread.

Meningiomas

Meningiomas begin in the meninges instead of brain tissue. Meninges consist of three layers of tissue that cover the outer part of the brain and spinal cord. These tumors account for about 30 to 35 percent of primary brain and spinal cord tumors. Since the meninges surround the brain, these tumors are usually found in the layers on the top of the brain and the outer curve (see Figure below). However, these tumors may form at the base of the skull as well. Meningiomas are the most common primary brain tumor in adults.

Meningiomas tend to occur more often in women, and sometimes they occur in families, especially in families who have neurofibromatosis, a syndrome in which many benign tumors of nerve tissue develop. These tumors are more likely to be found in adults older than 60, and the incidence appears to increase with age. Meningiomas rarely develop in children.

The majority of meningiomas are benign, but some may be classified as malignant. Although rare, malignant meningiomas may be highly aggressive. Even if the meningioma is benign, it can cause problems if it grows into nearby areas of the brain.

These tumors typically grow slowly and inward, putting pressure on the brain or spinal cord, which can interfere with normal brain functioning. However, they also can grow outward toward the skull and cause it to thicken. Some contain sacs of fluid (cysts), mineral deposits (calcifications), or tightly packed bunches of blood vessels.

It is estimated that between 40 and 80 percent of meningiomas contain an abnormal chromosome 22 that typically suppresses tumor growth. The exact cause of this abnormality is unknown. Research has found that meningiomas typically have extra copies of the platelet-derived growth factor (PDGFR) and epidermal growth factor receptor (EGFR) genes, which may cause these tumors to grow. This may be the subject of further research in clinical trials.

Pituitary Adenomas

The pituitary is a small gland located inside the brain. It sits above the nasal passages, which are above the fleshy back part of the roof of the mouth (known as the soft palate). Optic nerves that send visual information from the eyes to the brain are near the pituitary gland and may be affected by a tumor in this region.

Tumors that start in the pituitary gland can be called pituitary adenomas or pituitary carcinomas. The majority of pituitary adenomas develop in the front two-thirds of the pituitary gland. The tumors may be classified as “secreting” and “non-secreting” because the pituitary gland connects the brain with the endocrine system, which is responsible for directing the body to make hormones. A “secreting tumor” produces larger than normal amounts of hormones. Most pituitary tumors are the “non-secreting tumor” type, and don’t produce any hormones. Pituitary tumors are further classified by the type(s) of hormone they produce.

Almost all pituitary tumors are benign glandular tumors called pituitary adenomas, and they grow slowly. Pituitary carcinoma is the rare malignant form of pituitary adenoma. Pituitary adenomas are considered benign because they do not spread to other parts of the body. However, they still can cause problems if they grow large enough to press on nearby structures or if they make too much of any kind of hormone (see Figure below). These tumors represent about 10 percent of primary brain tumors. Pituitary tumors can occur at any age (including in children), but they are most often found in older adults.

Sometimes pituitary adenomas grow into the walls of the sella turcica and surrounding blood vessels, nerves and meninges (coverings of the brain). They don’t typically grow very large, but they can affect a person’s health because there is very little room for tumors to grow in this part of the skull. Therefore, if the tumor becomes larger than about 1 cm (about half an inch) across, it may grow upward, where it can compress and damage nearby parts of the brain and the nerves that arise from it.

Pituitary adenomas are divided into two categories based on size.

  • Microadenomas are tumors that are less than 1 cm in diameter. These tumors typically do not damage the rest of the pituitary gland or nearby tissues. However, if they produce too much of certain hormones, symptoms can occur.
  • Macroadenomas are tumors 1 cm or more in diameter. Macroadenomas can cause symptoms if they make too much of a certain hormone or by expanding to the extent they press on normal parts of the pituitary gland or on nearby nerves, such as the optic nerves.

Schwannomas

Schwannomas form in the Schwann cells that primarily cover and protect cranial nerves but can include other nerves as well. When these tumors develop on the nerve that controls hearing and balance, they are also known as vestibular schwannomas or acoustic neuromas. Although they can form on any cranial nerve, they typically develop on the eighth cranial nerve, which is also known as the acoustic or vestibulocochlear nerve. This nerve is near the cerebellum. In addition to cranial nerves, schwannomas also can begin on spinal nerves where they extend from the spinal cord. They account for about 8 percent of all primary brain tumors. Middle-aged adults are more likely to have schwannomas than children are, and the tumors are twice as likely to occur in women as they are in men.

Schwannomas are almost always benign and usually very slow growing. The vast majority of schwannomas occur spontaneously and as a single tumor. In these cases, people typically do not have affected family members. Most schwannomas are not inherited

Multiple schwannomas may develop in approximately 5 to 10 percent of people. In these cases, the person may have an inherited condition that can be passed from parent to child. For example, neurofibromatosis type 2 and schwannomatosis are two conditions that can cause multiple schwannomas.

Pineal Gland Tumors

A pineal gland tumor is classified as a brain tumor even though the pineal gland is not considered part of the brain. It is a small endocrine gland that sits between the cerebral hemispheres. It makes melatonin, a hormone that regulates sleep, in response to changes in light. Tumors in the pineal region are rare and represent less than 1 percent of all primary brain tumors; but they represent 3 to 8 percent of childhood brain tumors. Pineal gland tumors are more likely to develop in young adults between 20 and 40 years old. Although these tumors typically remain confined to the pineal gland and nearby tissues, about 10 to 20 percent of the tumors, particularly pineoblastoma, can potentially spread through the cerebrospinal fluid. When it spreads to the cerebrospinal fluid, it is typically late in the disease. The tumors rarely spread to other regions of the body.

The most common tumors of the pineal gland are called pineoblastomas, pineocytomas and mixed pineal tumors. Other tumors that may occur in this region include germinomas, non-germinomas (e.g., teratoma, endodermal sinus tumor, embryonal cell tumor, choriocarcinoma and mixed tumors), meningioma, astrocytoma, ganglioglioma and dermoid cysts.

Pineoblastomas are more aggressive, Grade IV malignant tumors, but a Grade III intermediate form also has been described. Pineocytomas are slow growing, Grade II tumors. Mixed pineal tumors contain a combination of cell types.

 

Additional Types of Brain Tumors
Astrocyctic tumors Gliomas Other neuroepithelial tumors
  Cerebellar astrocytoma   Brain stem glioma   Angiocentric glioma
  Desmoplastic infantile
  astrocytoma
  Diffuse intrinsic pontine glioma
  (DIPGs)
  Chordoid glioma of the third
  ventricle
  Giant cell glioblastoma   Optic nerve glioma Pineal tumors
  Gliosarcoma Meningeal tumors   Papillary tumor of the pineal
  region
  Pilomyxoid astrocytoma   Anaplastic hemangiopericytoma   Pineal parenchymal tumor of
  intermediate differentiation
  Pineal astrocytic tumor   Anaplastic/malignant meningioma   Pineoblastoma
  Pleomorphic xanthoastrocytoma   Atypical meningioma Schwannomas (neurilemmomas)
  Subependymal giant cell
  astrocytoma
  Hemangioblastoma   Acoustic neuroma
Chordomas   Hemangiopericytoma   Vestibular schwannoma
  Chondroidchordoma Neuronal and mixed neuronal-glial tumors Tumors of the cerebellum
  Conventional (sometimes
  called classic) chordoma
  Anaplastic ganglioglioma   Hemangioblastoma
  Dedifferentiated chordoma   Central neurocytoma Tumors of the cranial and paraspinal nerves
Choroid plexus tumors   Cerebellar liponeurocytoma   Malignant peripheral nerve sheath
  tumor (MPNST)
  Atypical choroid plexus
  papilloma
  Desmoplastic infantile astrocytoma
  and ganglioglioma
  Neurofibroma
  Choroid plexus carcinoma   Dysembryoplastic-neuroepithelial
  tumor
  Perineurioma
  Choroid plexus papilloma   Extraventricular neurocytoma Tumors of the sellar region
Embryonal tumors   Gangliocytoma   Adamantinomatous
  craniopharyngioma
  Atypical teratoid/rhabdoid tumor   Ganglioglioma   Craniopharyngioma
  Primitive neuroectodermal
  tumor (PNET)
  Papillary glioneuronal tumor   Granular cell tumor of the
  neurohypophysis
Ependymal tumors   Paraganglioma of the spinal cord   Papillary craniopharyngioma
  Cellular ependymoma   Rosette-forming glioneuronal
  tumor of the fourth ventricle
  Pituicytoma
  Clear cell ependymoma   Oligoastrocytic tumors   Spindle cell oncocytoma of the
  adenohyphophysis
  Papillary ependymoma   Anaplastic oligoastrocytoma Tumors of the skull
  Tancytic ependymoma   Oligoastrocytoma   Glomus jugulare
  Oligodendroglioma

Additional Resources

 

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