What Is Cancer?
What Is Cancer?
Differences between Cancer Cells and Normal Cells
Cancer cells differ from normal cells in many ways that allow them to grow out of control and become invasive. One important difference is that cancer cells are less specialized than normal cells. That is, whereas normal cells mature into very distinct cell types with specific functions, cancer cells do not. This is one reason that, unlike normal cells, cancer cells continue to divide without stopping.In addition, cancer cells are able to ignore signals that normally tell cells to stop dividing or that begin a process known as programmed cell death, or apoptosis, which the body uses to get rid of unneeded cells.
Cancer cells may be able to influence the normal cells, molecules, and blood vessels that surround and feed a tumor—an area known as the microenvironment. For instance, cancer cells can induce nearby normal cells to form blood vessels that supply tumors with oxygen and nutrients, which they need to grow. These blood vessels also remove waste products from tumors.
Cancer cells are also often able to evade the immune system, a network of organs, tissues, and specialized cells that protects the body from infections and other conditions. Although the immune system normally removes damaged or abnormal cells from the body, some cancer cells are able to “hide” from the immune system.
Tumors can also use the immune system to stay alive and grow. For example, with the help of certain immune system cells that normally prevent a runaway immune response, cancer cells can actually keep the immune system from killing cancer cells.
How Cancer Arises
Cancer is a genetic disease—that is, it is caused by changes to genes that control the way our cells function, especially how they grow and divide.Genetic changes that cause cancer can be inherited from our parents. They can also arise during a person’s lifetime as a result of errors that occur as cells divide or because of damage to DNA caused by certain environmental exposures. Cancer-causing environmental exposures include substances, such as the chemicals in tobacco smoke, and radiation, such as ultraviolet rays from the sun. (Our Cancer Causes and Risk Factors page has more information.)
Each person’s cancer has a unique combination of genetic changes. As the cancer continues to grow, additional changes will occur. Even within the same tumor, different cells may have different genetic changes.
In general, cancer cells have more genetic changes, such as mutations in DNA, than normal cells. Some of these changes may have nothing to do with the cancer; they may be the result of the cancer, rather than its cause.
"Drivers" of Cancer
The genetic changes that contribute to cancer tend to affect three main types of genes—proto-oncogenes, tumor suppressor genes, and DNA repair genes. These changes are sometimes called “drivers” of cancer.Proto-oncogenes are involved in normal cell growth and division. However, when these genes are altered in certain ways or are more active than normal, they may become cancer-causing genes (or oncogenes), allowing cells to grow and survive when they should not.
Tumor suppressor genes are also involved in controlling cell growth and division. Cells with certain alterations in tumor suppressor genes may divide in an uncontrolled manner.
DNA repair genes are involved in fixing damaged DNA. Cells with mutations in these genes tend to develop additional mutations in other genes. Together, these mutations may cause the cells to become cancerous.
As scientists have learned more about the molecular changes that lead to cancer, they have found that certain mutations commonly occur in many types of cancer. Because of this, cancers are sometimes characterized by the types of genetic alterations that are believed to be driving them, not just by where they develop in the body and how the cancer cells look under the microscope.
When Cancer Spreads
In metastasis, cancer cells break away from where they first
formed (primary cancer), travel through the blood or lymph system, and
form new tumors (metastatic tumors) in other parts of the body. The
metastatic tumor is the same type of cancer as the primary tumor.
Metastatic cancer has the same name and the same type of cancer cells as the original, or primary, cancer. For example, breast cancer that spreads to and forms a metastatic tumor in the lung is metastatic breast cancer, not lung cancer.
Under a microscope, metastatic cancer cells generally look the same as cells of the original cancer. Moreover, metastatic cancer cells and cells of the original cancer usually have some molecular features in common, such as the presence of specific chromosome changes.
Treatment may help prolong the lives of some people with metastatic cancer. In general, though, the primary goal of treatments for metastatic cancer is to control the growth of the cancer or to relieve symptoms caused by it. Metastatic tumors can cause severe damage to how the body functions, and most people who die of cancer die of metastatic disease.
Tissue Changes that Are Not Cancer
Not every change in the body’s tissues is cancer. Some tissue changes may develop into cancer if they are not treated, however. Here are some examples of tissue changes that are not cancer but, in some cases, are monitored:Hyperplasia occurs when cells within a tissue divide faster than normal and extra cells build up, or proliferate. However, the cells and the way the tissue is organized look normal under a microscope. Hyperplasia can be caused by several factors or conditions, including chronic irritation.
Dysplasia is a more serious condition than hyperplasia. In dysplasia, there is also a buildup of extra cells. But the cells look abnormal and there are changes in how the tissue is organized. In general, the more abnormal the cells and tissue look, the greater the chance that cancer will form.
Some types of dysplasia may need to be monitored or treated. An example of dysplasia is an abnormal mole (called a dysplastic nevus) that forms on the skin. A dysplastic nevus can turn into melanoma, although most do not.
An even more serious condition is carcinoma in situ. Although it is sometimes called cancer, carcinoma in situ is not cancer because the abnormal cells do not spread beyond the original tissue. That is, they do not invade nearby tissue the way that cancer cells do. But, because some carcinomas in situ may become cancer, they are usually treated.
Normal cells may become cancer cells. Before cancer cells form in
tissues of the body, the cells go through abnormal changes called
hyperplasia and dysplasia. In hyperplasia, there is an increase in the
number of cells in an organ or tissue that appear normal under a
microscope. In dysplasia, the cells look abnormal under a microscope but
are not cancer. Hyperplasia and dysplasia may or may not become cancer.
Credit: Terese Winslow
Types of Cancer
There are more than 100 types of cancer. Types of cancer are usually named for the organs or tissues where the cancers form. For example, lung cancer starts in cells of the lung, and brain cancer starts in cells of the brain. Cancers also may be described by the type of cell that formed them, such as an epithelial cell or a squamous cell.You can search NCI’s website for information on specific types of cancer based on the cancer’s location in the body or by using our A to Z List of Cancers. We also have collections of information on childhood cancers and cancers in adolescents and young adults.
Here are some categories of cancers that begin in specific types of cells:
Carcinoma
Carcinomas are the most common type of
cancer. They are formed by epithelial cells, which are the cells that
cover the inside and outside surfaces of the body. There are many types
of epithelial cells, which often have a column-like shape when viewed
under a microscope.
Carcinomas that begin in different epithelial cell types have specific names:
Adenocarcinoma is a cancer that forms in
epithelial cells that produce fluids or mucus. Tissues with this type
of epithelial cell are sometimes called glandular tissues. Most cancers
of the breast, colon, and prostate are adenocarcinomas.
Basal cell carcinoma is a cancer that
begins in the lower or basal (base) layer of the epidermis, which is a
person’s outer layer of skin.
Squamous cell carcinoma is a cancer that
forms in squamous cells, which are epithelial cells that lie just
beneath the outer surface of the skin. Squamous cells also line many
other organs, including the stomach, intestines, lungs, bladder, and
kidneys. Squamous cells look flat, like fish scales, when viewed under a
microscope. Squamous cell carcinomas are sometimes called epidermoid
carcinomas.
Transitional cell carcinoma is a cancer
that forms in a type of epithelial tissue called transitional
epithelium, or urothelium. This tissue, which is made up of many layers
of epithelial cells that can get bigger and smaller, is found in the
linings of the bladder, ureters, and part of the kidneys (renal pelvis),
and a few other organs. Some cancers of the bladder, ureters, and
kidneys are transitional cell carcinomas.
Sarcoma
Soft tissue sarcoma forms in soft tissues of the body, including
muscle, tendons, fat, blood vessels, lymph vessels, nerves, and tissue
around joints.
Sarcomas are cancers that form in bone and soft tissues, including muscle, fat, blood vessels, lymph vessels, and fibrous tissue (such as tendons and ligaments).
Osteosarcoma is the most common cancer of bone. The most common types of soft tissue sarcoma are leiomyosarcoma, Kaposi sarcoma, malignant fibrous histiocytoma, liposarcoma, and dermatofibrosarcoma protuberans.
Leukemia
Cancers that begin in the blood-forming
tissue of the bone marrow are called leukemias. These cancers do not
form solid tumors. Instead, large numbers of abnormal white blood cells
(leukemia cells and leukemic blast cells) build up in the blood and bone
marrow, crowding out normal blood cells. The low level of normal blood
cells can make it harder for the body to get oxygen to its tissues,
control bleeding, or fight infections.
There are four common types of leukemia,
which are grouped based on how quickly the disease gets worse (acute or
chronic) and on the type of blood cell the cancer starts in
(lymphoblastic or myeloid).
Our page on leukemia has more information.
Lymphoma
Lymphoma is cancer that begins in
lymphocytes (T cells or B cells). These are disease-fighting white blood
cells that are part of the immune system. In lymphoma, abnormal
lymphocytes build up in lymph nodes and lymph vessels, as well as in
other organs of the body.
There are two main types of lymphoma:
Hodgkin lymphoma – People with this
disease have abnormal lymphocytes that are called Reed-Sternberg cells.
These cells usually form from B cells.
Non-Hodgkin lymphoma – This is a large
group of cancers that start in lymphocytes. The cancers can grow quickly
or slowly and can form from B cells or T cells.
Our page on lymphoma has more information.
Multiple Myeloma
Multiple myeloma is cancer that begins in plasma cells,
another type of immune cell. The abnormal plasma cells, called myeloma
cells, build up in the bone marrow and form tumors in bones all through
the body. Multiple myeloma is also called plasma cell myeloma and Kahler
disease.
Our page on multiple myeloma and other plasma cell neoplasms has more information.
Melanoma
Melanoma is cancer that begins in cells
that become melanocytes, which are specialized cells that make melanin
(the pigment that gives skin its color). Most melanomas form on the
skin, but melanomas can also form in other pigmented tissues, such as
the eye.
Our pages on skin cancer and intraocular melanoma have more information.
Brain and Spinal Cord Tumors
There are different types of brain and
spinal cord tumors. These tumors are named based on the type of cell in
which they formed and where the tumor first formed in the central
nervous system. For example, an astrocytic tumor begins in star-shaped brain cells called astrocytes, which help keep nerve cells healthy. Brain tumors can be benign (not cancer) or malignant (cancer).
Our page on brain and spinal cord tumors in adults has more information, as does our overview of brain and spinal cord tumors in children.
Other Types of Tumors
Germ Cell Tumors
Germ cell tumors are a type of tumor
that begins in the cells that give rise to sperm or eggs. These tumors
can occur almost anywhere in the body and can be either benign or
malignant.
Our page of cancers by body location/system includes a list of germ cell tumors with links to more information.
Neuroendocrine Tumors
Neuroendocrine tumors form from cells
that release hormones into the blood in response to a signal from the
nervous system. These tumors, which may make higher-than-normal amounts
of hormones, can cause many different symptoms. Neuroendocrine tumors
may be benign or malignant.
Our definition of neuroendocrine tumors has more information.
Carcinoid Tumors
Carcinoid tumors are a type of
neuroendocrine tumor. They are slow-growing tumors that are usually
found in the gastrointestinal system (most often in the rectum and small
intestine). Carcinoid tumors may spread to the liver or other sites in
the body, and they may secrete substances such as serotonin or
prostaglandins, causing carcinoid syndrome.
Our page on gastrointestinal carcinoid tumors has more information.
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