4) Tumor Types

Tumor Types Intro

When most normal cells grow old or get damaged, they die, and new cells take their place. Sometimes, this process goes wrong. New cells form when the body doesn't need them, and old or damaged cells don't die as they should. The buildup of extra cells often forms a mass of tissue is called a growth or tumor.

Today there are more than 120 different types of brain tumors that have been identified. While there are some prognosis information for some types of tumors, each person’s tumor is different. Emerging information about biological differences in tumors may explain why outcomes for some people are different than others. [1]

All brain tumors can be categorized as either a benign or malignant/cancerous tumor. Cancerous tumors can then be divided into primary tumors that start within the brain, and secondary tumors that have spread from somewhere else, known as brain metastasis tumors.

Benign Brain Tumors

Benign brain tumors are usually defined as a group of similar cells that do not follow normal cell division and growth patterns and develop into a mass of cells that microscopically do not have the characteristic appearance of a cancer. Most benign brain tumors are found by CT or MRI brain scans. These tumors usually grow slowly, do not invade surrounding tissues or spread to other organs, and often have a border or edge that can be seen on CT scans. These tumors rarely develop into metastatic (cancerous or spreading) tumors. Most benign brain tumors can be removed; the benign tumors usually do not reoccur after removal. The exact causes of benign brain tumors are not known, but investigators have suggested that family history, radiation exposure, or exposure to chemicals (for example, vinyl chloride, formaldehyde) may be risk factors. Benign brain tumors, however, can be life-threatening because they can compress brain tissue and other structures inside the skull, so the term "benign" can be misleading. Terminology is further complicated by some investigators who classify low-grade cancerous tumors as either "benign" or "relatively benign." [4]

Benign Tumor [9]

Cancerous Tumors

Primary Tumors

Primary brain tumours start in the brain. Tumours can start in any part of the brain or related structures.

The most common types of brain tumours in adults start in the main part of the brain called the cerebrum. About a quarter of tumours (24%) start in the meninges. These are the membranes that surround and protect the brain and spinal cord. Around 1 in 10 (10%) tumours start in the glands of the brain such as the pituitary gland or pineal gland.

In children the picture is slightly different. 6 out of 10 (60%) childhood tumours start in part of the brain called the cerebellum or the brain stem. Only 4 out of 10 (40%) start in the cerebrum. [3]

Secondary Tumors

Cancers that have spread to the brain from somewhere else in the body are called secondary brain tumours or brain metastases.

Cancers of the lung, breast, kidney, stomach, bowel (colon), and melanoma skin cancer can all spread to the brain. This happens because cancer cells break away from the primary cancer and travel through the bloodstream to lodge in the brain. There they can begin to grow into new tumours. [3]

Most Common Tumors

The top 4 tumor types listed below account for 94.2% of all brain tumors.

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. A glial cell is a supportive cell in the brain. The main types of supportive cells in the brain include astrocytes, oligodendrocytes, and ependymal cells. Gliomas may be considered astrocytomas, oligodendrogliomas, or ependymomas. Gliomas are assigned a grade, which is an indication of how aggressive a tumor is likely to be. A higher grade is usually more aggressive and more likely to grow quickly. However, doctors are moving toward using tumor genetics to better classify gliomas. [6]

Meningiomas

A meningioma is a tumor that forms on membranes that cover the brain and spinal cord just inside the skull. Specifically, the tumor forms on the three layers of membranes that are called meninges. These tumors are often slow-growing. As many as 90% are benign (not cancerous). Most meningiomas occur in the brain. But they can also grow on parts of the spinal cord. Often, meningiomas cause no symptoms and require no immediate treatment. But the growth of benign meningiomas can cause serious problems. In some cases, such growth can be fatal. [7]

Type Of Brain Tumors [9].

Pituitary Adenomas

Pituitary adenomas are benign tumors of the pituitary gland - a small structure located behind the eyes, beneath the brain. Most pituitary tumors are benign, meaning that they do not spread to distant locations in the body. Cancerous pituitary tumors are extremely rare. The pituitary gland is a principal part of the endocrine system. The pituitary is called the “master endocrine gland,” because it produces specific hormones that control the activities of other endocrine glands and organs in the body. Pituitary adenomas usually develop in older adults, but they may occur in children. These tumors are very common, occurring in up to 20% of adults. Although most pituitary adenomas are harmless, some cause serious problems.

Nerve Sheath Tumors

A nerve sheath tumor is a type of tumor arising from the nervous system (nervous system neoplasm) which is primarily made up of the myelin (it forms a layer around the axon of a neuron) surrounding nerves. On the other hand, peripheral neural sheath tumors are those originating from the peripheral nervous system (it extends outside the central nervous system consisting of the brain and spinal cord). Peripheral neural sheath lesions can be divided into both benign and malignant. The malignant peripheral nerve sheath tumors (MPNSTs) are cancerous in nature. The lesions may appear as white, firm nodules. They tend to be locally aggressive. Although rare, these tumors can cause potential damage. Early detection is very important for better diagnosis. [8]

World Health Organization (WHO) Classification System

Today, most medical institutions use the World Health Organization classification system to identify brain tumors. The WHO classifies brain tumors by cell origin and how the cells behave, from the least aggressive (benign) to the most aggressive (malignant).

WHO Tumor Classification [2].

Some tumor types are assigned a grade (discussed in next section), ranging from Grade I (least malignant) to Grade IV (most malignant), which signifies the rate of growth. There are variations in grading systems, depending on the tumor type. The classification and grade of an individual tumor help predict its likely behavior. [2]