What Is Cancer?

A Collection of Related Diseases

Cancer is the name given to a collection of related diseases. In all types of cancer, some of the body’s cells begin to divide without stopping and spread into surrounding tissues.

Cancer can start almost anywhere in the human body, which is made up of trillions of cells. Normally, human cells grow and divide to form new cells as the body needs them. When cells grow old or become damaged, they die, and new cells take their place.

When cancer develops, however, this orderly process breaks down. As cells become more and more abnormal, old or damaged cells survive when they should die, and new cells form when they are not needed. These extra cells can divide without stopping and may form growths called tumors.

Many cancers form solid tumors, which are masses of tissue. Cancers of the blood, such as leukemias, generally do not form solid tumors.

Cancerous tumors are malignant, which means they can spread into, or invade, nearby tissues. In addition, as these tumors grow, some cancer cells can break off and travel to distant places in the body through the blood or the lymph system and form new tumors far from the original tumor.

Unlike malignant tumors, benign tumors do not spread into, or invade, nearby tissues. Benign tumors can sometimes be quite large, however. When removed, they usually don’t grow back, whereas malignant tumors sometimes do. Unlike most benign tumors elsewhere in the body, benign brain tumors can be life threatening.

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 Prevention section 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

A cancer that has spread from the place where it first started to another place in the body is called metastatic cancer. The process by which cancer cells spread to other parts of the body is called metastasis.

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.

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.

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

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).

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.

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.

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.

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).

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.

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.

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.

There are more than 200 known cancers, some of these are listed below:

Acute Lymphoblastic Leukemia (ALL)
Acute Myeloid Leukemia (AML)
Adolescents, Cancer in
Adrenocortical Carcinoma
Childhood Adrenocortical Carcinoma - Unusual Cancers of Childhood
AIDS-Related Cancers
Kaposi Sarcoma (Soft Tissue Sarcoma)
AIDS-Related Lymphoma (Lymphoma)
Primary CNS Lymphoma (Lymphoma)
Anal Cancer
Appendix Cancer - Gastrointestinal Carcinoid Tumors
Astrocytomas, Childhood (Brain Cancer)
Atypical Teratoid/Rhabdoid Tumor, Childhood, Central Nervous System (Brain Cancer)
Basal Cell Carcinoma of the Skin - Skin Cancer
Bile Duct Cancer
Bladder Cancer
Childhood Bladder Cancer - Unusual Cancers of Childhood
Bone Cancer (includes Ewing Sarcoma and Osteosarcoma and Malignant Fibrous Histiocytoma)
Brain Tumors
Breast Cancer
Childhood Breast Cancer - Unusual Cancers of Childhood
Bronchial Tumors, Childhood - Unusual Cancers of Childhood
Burkitt Lymphoma - Non-Hodgkin Lymphoma
Carcinoid Tumor (Gastrointestinal)
Childhood Carcinoid Tumors - Unusual Cancers of Childhood
Carcinoma of Unknown Primary
Childhood Carcinoma of Unknown Primary - Unusual Cancers of Childhood
Cardiac (Heart) Tumors, Childhood - Unusual Cancers of Childhood
Central Nervous System
Atypical Teratoid/Rhabdoid Tumor, Childhood (Brain Cancer)
Embryonal Tumors, Childhood (Brain Cancer)
Germ Cell Tumor, Childhood (Brain Cancer)
Primary CNS Lymphoma
Cervical Cancer
Childhood Cervical Cancer - Unusual Cancers of Childhood
Childhood Cancers
Cancers of Childhood, Unusual
Cholangiocarcinoma - Bile Duct Cancer
Chordoma, Childhood - Unusual Cancers of Childhood
Chronic Lymphocytic Leukemia (CLL)
Chronic Myelogenous Leukemia (CML)
Chronic Myeloproliferative Neoplasms
Colorectal Cancer
Childhood Colorectal Cancer - Unusual Cancers of Childhood
Craniopharyngioma, Childhood (Brain Cancer)
Cutaneous T-Cell Lymphoma - Lymphoma (Mycosis Fungoides and Sézary Syndrome)
Ductal Carcinoma In Situ (DCIS) - Breast Cancer
Embryonal Tumors, Central Nervous System, Childhood (Brain Cancer)
Endometrial Cancer (Uterine Cancer)
Ependymoma, Childhood (Brain Cancer)
Esophageal Cancer
Childhood Esophageal Cancer - Unusual Cancers of Childhood
Esthesioneuroblastoma (Head and Neck Cancer)
Ewing Sarcoma (Bone Cancer)
Extracranial Germ Cell Tumor, Childhood
Extragonadal Germ Cell Tumor
Eye Cancer
Childhood Intraocular Melanoma - Unusual Cancers of Childhood
Intraocular Melanoma
Retinoblastoma
Fallopian Tube Cancer
Fibrous Histiocytoma of Bone, Malignant, and Osteosarcoma
Gallbladder Cancer
Gastric (Stomach) Cancer
Childhood Gastric (Stomach) Cancer - Unusual Cancers of Childhood
Gastrointestinal Carcinoid Tumor
Gastrointestinal Stromal Tumors (GIST) (Soft Tissue Sarcoma)
Childhood Gastrointestinal Stromal Tumors - Unusual Cancers of Childhood
Germ Cell Tumors
Childhood Central Nervous System Germ Cell Tumors (Brain Cancer)
Childhood Extracranial Germ Cell Tumors
Extragonadal Germ Cell Tumors
Ovarian Germ Cell Tumors
Testicular Cancer
Gestational Trophoblastic Disease
Hairy Cell Leukemia
Head and Neck Cancer
Heart Tumors, Childhood - Unusual Cancers of Childhood
Hepatocellular (Liver) Cancer
Histiocytosis, Langerhans Cell
Hodgkin Lymphoma
Hypopharyngeal Cancer (Head and Neck Cancer)
Intraocular Melanoma
Childhood Intraocular Melanoma - Unusual Cancers of Childhood
Islet Cell Tumors, Pancreatic Neuroendocrine Tumors
Kaposi Sarcoma (Soft Tissue Sarcoma)
Kidney (Renal Cell) Cancer
Langerhans Cell Histiocytosis
Laryngeal Cancer (Head and Neck Cancer)
Leukemia
Lip and Oral Cavity Cancer (Head and Neck Cancer)
Liver Cancer
Lung Cancer (Non-Small Cell and Small Cell)
Childhood Lung Cancer - Unusual Cancers of Childhood
Lymphoma
Male Breast Cancer
Malignant Fibrous Histiocytoma of Bone and Osteosarcoma
Melanoma
Childhood Melanoma - Unusual Cancers of Childhood
Melanoma, Intraocular (Eye)
Childhood Intraocular Melanoma - Unusual Cancers of Childhood
Merkel Cell Carcinoma (Skin Cancer)
Mesothelioma, Malignant
Childhood Mesothelioma - Unusual Cancers of Childhood
Metastatic Cancer
Metastatic Squamous Neck Cancer with Occult Primary (Head and Neck Cancer)
Midline Tract Carcinoma With NUT Gene Changes
Mouth Cancer (Head and Neck Cancer)
Multiple Endocrine Neoplasia Syndromes - Unusual Cancers of Childhood
Multiple Myeloma/Plasma Cell Neoplasms
Mycosis Fungoides (Lymphoma)
Myelodysplastic Syndromes, Myelodysplastic/Myeloproliferative Neoplasms
Myelogenous Leukemia, Chronic (CML)
Myeloid Leukemia, Acute (AML)
Myeloproliferative Neoplasms, Chronic
Nasal Cavity and Paranasal Sinus Cancer (Head and Neck Cancer)
Nasopharyngeal Cancer (Head and Neck Cancer)
Neuroblastoma
Non-Hodgkin Lymphoma
Non-Small Cell Lung Cancer
Oral Cancer, Lip and Oral Cavity Cancer and Oropharyngeal Cancer (Head and Neck Cancer)
Osteosarcoma and Malignant Fibrous Histiocytoma of Bone
Ovarian Cancer
Childhood Ovarian Cancer - Unusual Cancers of Childhood
Pancreatic Cancer
Childhood Pancreatic Cancer - Unusual Cancers of Childhood
Pancreatic Neuroendocrine Tumors (Islet Cell Tumors)
Papillomatosis (Childhood Laryngeal)
Paraganglioma
Childhood Paraganglioma - Unusual Cancers of Childhood
Paranasal Sinus and Nasal Cavity Cancer (Head and Neck Cancer)
Parathyroid Cancer
Penile Cancer
Pharyngeal Cancer (Head and Neck Cancer)
Pheochromocytoma
Childhood Pheochromocytoma - Unusual Cancers of Childhood
Pituitary Tumor
Plasma Cell Neoplasm/Multiple Myeloma
Pleuropulmonary Blastoma - Unusual Cancers of Childhood
Pregnancy and Breast Cancer
Primary Central Nervous System (CNS) Lymphoma
Primary Peritoneal Cancer
Prostate Cancer
Rectal Cancer
Recurrent Cancer
Renal Cell (Kidney) Cancer
Retinoblastoma
Rhabdomyosarcoma, Childhood (Soft Tissue Sarcoma)
Salivary Gland Cancer (Head and Neck Cancer)
Sarcoma
Childhood Rhabdomyosarcoma (Soft Tissue Sarcoma)
Childhood Vascular Tumors (Soft Tissue Sarcoma)
Ewing Sarcoma (Bone Cancer)
Kaposi Sarcoma (Soft Tissue Sarcoma)
Osteosarcoma (Bone Cancer)
Soft Tissue Sarcoma
Uterine Sarcoma
Sézary Syndrome (Lymphoma)
Skin Cancer
Childhood Skin Cancer - Unusual Cancers of Childhood
Small Cell Lung Cancer
Small Intestine Cancer
Soft Tissue Sarcoma
Squamous Cell Carcinoma of the Skin - Skin Cancer
Squamous Neck Cancer with Occult Primary, Metastatic (Head and Neck Cancer)
Stomach (Gastric) Cancer
Childhood Stomach (Gastric) Cancer - Unusual Cancers of Childhood
T-Cell Lymphoma, Cutaneous - Lymphoma (Mycosis Fungoides and Sèzary Syndrome)
Testicular Cancer
Childhood Testicular Cancer - Unusual Cancers of Childhood
Throat Cancer (Head and Neck Cancer)
Nasopharyngeal Cancer
Oropharyngeal Cancer
Hypopharyngeal Cancer
Thymoma and Thymic Carcinoma
Thyroid Cancer
Transitional Cell Cancer of the Renal Pelvis and Ureter (Kidney (Renal Cell) Cancer)
Unknown Primary, Carcinoma of
Childhood Cancer of Unknown Primary - Unusual Cancers of Childhood
Unusual Cancers of Childhood
Ureter and Renal Pelvis, Transitional Cell Cancer (Kidney (Renal Cell) Cancer
Urethral Cancer
Uterine Cancer, Endometrial
Uterine Sarcoma
Vaginal Cancer
Childhood Vaginal Cancer - Unusual Cancers of Childhood
Vascular Tumors (Soft Tissue Sarcoma)
Vulvar Cancer
Wilms Tumor and Other Childhood Kidney Tumors
Young Adults, Cancer in