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A brain tumor begins when normal cells in the brain change and grow
uncontrollably, forming a mass. A tumor can be benign (noncancerous) or
malignant (cancerous). In general, primary CNS tumors do not spread
outside of the CNS. Malignant brain tumors are further classified using a
grade: low, intermediate, or high. More information can be found in Staging.
This section describes primary brain tumors, which are tumors that
begin in the brain. Secondary brain tumors (also called brain
metastases) are much more common than primary tumors. A secondary brain
tumor is a cancerous tumor that started in another part of the body
(such as the breast, lung, or colon) and then spread to the brain. Learn
more about cancer that started elsewhere in the body and spread to the
brain by reading about that specific type of cancer.
Anatomy of the brain
The brain is made up of four main parts: the cerebrum, the cerebellum, the brain stem, and the meninges.
The cerebrum. This is the largest part of the brain.
It contains two cerebral hemispheres and is divided into four lobes
where specific functions occur:
- The frontal lobe controls reasoning, emotions, problem-solving, expressive speech, and movement
- The parietal lobe controls the sensations of touch, such as pressure, pain, and temperature, and parts of speech, visual-spatial orientation, and calculation
- The temporal lobe controls memory, the special senses such as hearing, and the ability to understand spoken or written words
- The occipital lobe controls vision
The cerebellum. The cerebellum is located at the back part of the brain below the cerebrum. It is responsible for coordination and balance.
The brain stem. This is the portion of the brain
that connects to the spinal cord, controls involuntary functions
essential for life, such as the beating of the heart and breathing. In
addition, messages for all the functions controlled by the cerebrum and
cerebellum travel through the brain stem to the connections in the body.
The meninges. These are the membranes that surround
and protect the brain and spinal cord. There are three meningeal layers,
called the dura mater, arachnoid, and pia mater. The cerebrospinal
fluid (CSF) is made near the center of the brain, in the lateral
ventricles, and circulates around the brain and spinal cord between the
arachnoid and pia layers.
View illustrations of the anatomy of the brain.
Types of brain tumors
There are more than 100 types of primary brain tumors, and about 5%
of all brain tumors cannot be assigned an exact type. For a complete
list of the types of brain tumors and how often they are diagnosed,
please refer to the Central Brain Tumor Registry of the United States.
This section covers brain tumors diagnosed in adults. (Learn about brain
tumors in children.) For practical purposes, this section's coverage is
divided into gliomas and non-glioma types of tumors in adults:
Gliomas
As a group, a glioma is one of the most common types of brain tumor. A
glioma is a tumor that grows from a glial cell, which is a supportive
cell in the brain. There are two main types of supportive cells:
astrocytes and oligodendrocytes. Most gliomas are called either
astrocytoma or oligodendroglioma, or a mix of both. A glioma is given a
grade (a measure of how much the tumor appears like normal brain tissue)
from I to IV (one to four) based how likely they are to grow quickly. A
grade I glioma is often considered a benign tumor, while grades II
through IV are tumors with an increasing likelihood of growing and
spreading quickly and are therefore considered possibly cancerous.
Types of gliomas include:
Astrocytoma. Astrocytoma is the most common type of
glioma and begins in cells called astrocytes in the cerebrum or
cerebellum. There are four grades of astrocytoma.
- Grade I or pilocytic astrocytoma is a slow-growing tumor that is most often benign and rarely spreads into nearby tissue. It accounts for about 2% of all brain tumors.
- Grade II or low-grade diffuse astrocytoma is a slow-growing tumor that can often spread into nearby tissue and can become a higher grade. It accounts for about 3% of all brain tumors.
- Grade III or anaplastic astrocytoma is a cancerous tumor that can quickly grow and spread to nearby tissues. It accounts for about 2% of all brain tumors.
- Grade IV or glioblastoma multiforme is a very aggressive form of astrocytoma that accounts for about 16% of all brain tumors.
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