Brain Tumors

Types of Brain Tumors

The brain is made up of many different types of cells, and primary tumors may originate in any of them. These cells may be in the brain tissue or in any of the membranes, nerves or glands in and around the brain. As a result, many different types of primary central nervous system (CNS) tumors can occur in any of these areas. Some estimates say there are more than 120 types.

The tumor types are named according to the tissue or cell type involved, the location of the tumor, whether it is benign (noncancerous) or malignant (cancerous), and whether it is fast-growing (aggressive) or slow-growing. Although brain tumors may contain one type of cell, they may also be a mixture of cell types.

Some of the most common types include gliomas, meningiomas, schwannomas, pituitary tumors, 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

Gliomas, which develop from glial cells, are the most common type of brain tumors. Any cancer that develops from the glial cells is referred to as a glioma. Glial cells are the most common and abundant cell type within the CNS, and they have many functions, including surrounding, protecting and supporting neuron cells with nutrients and oxygen. There are multiple types of glial cells: astrocytes, oligodendrocytes and ependymal cells. Tumors of these glial cells are known as astrocytomas, oligodendrogliomas and ependymomas.

Gliomas are graded based on how aggressive the tumor is. They can range from Grade I (noncancerous, slow-growing) to Grade IV (cancerous, fast-growing) (see Pathology & Grading). More information about the three types of gliomas is listed below.

Astrocytomas

Astrocytomas begin in astrocytes, which are star-shaped cells that comprise the supportive tissue of the brain. Astrocytomas make up the majority 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 and spread into surrounding tissues. Astrocytomas are typically high grade in adults.

Following are the most common types of astrocytomas, from low grade to high grade.

Pilocytic astrocytomas are typically classified as Grade I astrocytomas. 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. 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. With Grade II, more cells look abnormal when examined under a microscope than with Grade I. The tumors usually do not spread to nearby tissues but are more likely to invade surrounding tissue than a Grade I tumor. They tend to grow at a relatively slow pace. They also typically contain the IDH+ and 1p19q- molecular markers.

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.

Glioblastomas (GBMs) are considered high-grade tumors (Grade IV). They may also be known as astrocytoma Grade IV or glioblastoma multiforme because the cells may take a variety of appearances under the microscope. 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. Secondary tumors begin as lower-grade tumors and evolve into Grade IV 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 fried egg-shaped cells called oligodendrocytes, which are a type of glial cell. Oligodendrogliomas, which contain the IDH+ and 1p19q+ molecular markers, are classified as oligodendrogliomas and anaplastic oligodendrogliomas.

Oligodendrogliomas are considered low grade (Grade II), and anaplastic oligodendrogliomas are malignant and more aggressive (Grade III). 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. Tumors typically appear soft and grayish-pink, and they often contain mineral deposits (calcifications), areas of hemorrhage (bleeding), and/or cysts.

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. Ependymomas may prevent cerebrospinal fluid from leaving the ventricles, which causes the ventricles to enlarge. These tumors range from low grade, which are less aggressive, to high grade, which are more aggressive.

  • Ependymomas can be either Grade I or II, are less aggressive and typically grow slowly with mostly normal cells when viewed under a microscope. Ependymomas usually are located along, within or next to the ventricular system. Grade I ependymomas can be either myxopapillary ependymomas or subependymomas. Grade II ependymomas grow into the ventricles.
  • Anaplastic ependymomas grow quickly and may spread into surrounding tissues. When viewed under a microscope, the cells look different than normal cells. This type is rarely found in the spinal cord.

Because ependymomas form in the cells that line the ventricles, they most often spread along the cerebrospinal fluid pathways but rarely spread outside the brain or spinal cord. They are typically soft and grayish or red.

Meningiomas

Unlike gliomas that form in the cells that make up the brain’s tissues, meningiomas develop in the meninges. The meninges consist of three layers of tissue that cover the outer part of the brain inside the skull and spinal cord. Because 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 sometimes occur in families who have neurofibromatosis, a genetic 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.

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 function. 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.

These tumors are graded from Grade I to Grade III. Grade I meningiomas are benign and look mostly normal under a microscope. Grade II meningiomas may be considered atypical or invasive, and the cells look more abnormal than Grade I tumors. These can grow beyond the meninges and into the brain’s tissues. Grade III meningiomas may be considered anaplastic or malignant and look mostly abnormal under a microscope. These grow quickly, can grow into other parts of the brain and bone, and can return after treatment.

It is estimated that a majority 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 (PDGF) and its receptor (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.

Pineal Gland Tumors

The pineal gland 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. Although the pineal gland is located inside the brain, it is not considered part of the brain. But a tumor that develops in this area often impacts other areas of the brain that surround this gland. Tumors in the pineal region are rare.

Although these tumors typically remain confined to the pineal gland and nearby tissues, a small percentage of the tumors, particularly pineoblastomas, can potentially spread through the cerebrospinal fluid. When they spread to the cerebrospinal fluid, it is typically late in the disease. The tumors rarely spread to other regions of the body.

There are several common types.

  • Pineocytomas are considered Grade I or II, are less aggressive and typically grow slowly with mostly normal cells when viewed under a microscope. They can be relatively benign.
  • Pineal parenchymal tumors of intermediate differentiation are considered Grade II or III and fall between pineocytomas and pineoblastomas, which are Grade IV. Parenchymal cells (pineocytes) are the cells that make up most of the pineal gland. The tumor cells may spread into nearby tissue or the cerebral spinal fluid. These cells look very different under a microscope.
  • Papillary tumors of the pineal region are relatively uncommon tumors and have only recently been recognized by the World Health Organization. These do not develop within the pineal gland but, instead, may grow from regions or structures around the pineal gland. Grades have yet to be determined for this group, but it is believed they are close to Grade II or III.
  • Pineoblastomas are Grade IV, malignant (cancerous) and the most aggressive. They may resemble medulloblastomas and retinoblastomas. Their cells look very different under a microscope, and their cells may even contain dead regions.

Pituitary Tumors

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). The pituitary gland is responsible for releasing multiple types of hormones that affect many bodily functions. Sometimes referred to as the “master endocrine gland,” it produces hormones for the thyroid, adrenal gland, testicles, ovaries and breasts, as well as melanin, oxytocin and growth hormones.

Tumors that start in the pituitary gland are categorized into two groups.

  • Pituitary adenomas are divided into benign pituitary adenomas and invasive pituitary adenomas. Benign pituitary adenomas are not considered cancerous, grow very slowly and do not spread outside of the pituitary gland. However, invasive pituitary adenomas may spread to the skull or the sinus cavity below the pituitary gland.
  • Pituitary carcinomas, also known as pituitary cancer, are malignant (cancerous) and are very rare. They can grow into other areas of the brain and spinal cord or outside of the CNS.

Almost all pituitary tumors are pituitary adenomas, which are benign. 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). For example, 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.

Pituitary tumors can occur at any age, but they are most often found in older adults. The majority of pituitary adenomas develop in the front two-thirds of the pituitary gland.

In addition to dividing these tumors by whether they are benign or malignant, the tumors may be classified as “functioning” or “non-functioning” because the pituitary gland connects the brain with the endocrine system, which is responsible for directing the body to make hormones. Most pituitary tumors are the “functioning” type, producing larger than normal amounts of one or more hormones.

When pituitary tumors grow outside of the gland, there is very little room 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.

Schwannomas

Schwannomas are tumors that 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 (known as the acoustic or vestibulocochlear nerve), they are also known as vestibular schwannomas or acoustic neuromas. Although they can form on any cranial nerve, they typically develop on the acoustic or vestibulocochlear nerve, which is near the cerebellum. In addition to cranial nerves, schwannomas can begin on spinal nerves where they extend from the spinal cord. When schwannomas press on the spinal cord, they can cause weakness, sensory loss, and bowel and bladder problems.

Schwannomas are almost always benign and usually very slow growing. They are considered Grade I tumors. The vast majority of schwannomas occur spontaneously and as a single tumor. Multiple schwannomas may develop in a small percentage of people. In these cases, the person may have an inherited condition that can be passed from parent to child.

 

Other Types of Brain Tumors*
Diffuse astrocytic and oligodendroglial tumors Diffuse astrocytoma
Gemistocytic astrocytoma
Giant cell glioblastoma
Gliosarcoma
Epithelioid glioblastoma
Diffuse midline glioma
Other astrocytic tumors Pilomyxoid astrocytoma
Pleomorphic xanthoastrocytoma
Anaplastic pleomorphic xanthoastrocytoma
Ependymal tumors Papillary ependymoma
Clear cell ependymoma
Tanycytic ependymoma
Other gliomas Astroblastoma
Choroid plexus tumors Choroid plexus carcinoma
Neuronal and mixed neuronal-glial tumors Anaplastic ganglioglioma
Embryonal tumors Embryonal tumor with multilayered rosettes
CNS embryonal tumor
Atypical teratoid/rhabdoid tumor
CNS embryonal tumor with rhabdoid features
Tumors of the cranial and paraspinal nerves Malignant peripheral nerve sheath tumor (MPNST)
Epithelioid MPNST
MPNST with perineural differentiation
Meningiomas Papillary meningioma
Rhabdoid meningioma
Anaplastic (malignant) meningioma
Mesenchymal, nonmeningothelial tumors Solitary fibrous tumor/hemangiopericytoma
Epithelioid hemangioendothelioma
Angiosarcoma
Kaposi sarcoma
Ewing sarcoma/peripheral primitive neuroectodermal tumor
Liposarcoma
Fibrosarcoma
Undifferentiated pleomorphic sarcoma/malignant fibrous histiocytoma
Leiomyosarcoma
Rhabdomyosarcoma
Chondrosarcoma
Osteosarcoma
Melanocytic lesions Meningeal melanoma
Meningeal melanomatosis
Lymphomas Diffuse large B-cell lymphoma of the CNS
Anaplastic large cell lymphoma
Intravascular large B-cell lymphoma
MALT lymphoma of the dura
Histiocytic tumors Langerhans cell histiocytosis
Erdheim-Chester disease
Histiocytic sarcoma
Germ cell tumors Embryonal carcinoma
Yolk sac tumor
Choriocarcinoma
Immature teratoma
Teratoma with malignant transformation
Mixed germ cell tumor
*Source: American Joint Committee on Cancer (AJCC), Eighth Edition (2017)

Additional Resources

 

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