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.
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.
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.
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.
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 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 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).
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 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 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.
Causes and Prevention:
Risk Factors for Cancer
It is usually not possible to know exactly why one person develops cancer and another doesn’t. But research has shown that certain risk factors may increase a person’s chances of developing cancer. (There are also factors that are linked to a lower risk of cancer. These are sometimes called protective risk factors, or just protective factors.)
Cancer risk factors include exposure to chemicals or other substances, as well as certain behaviors. They also include things people cannot control, like age and family history. A family history of certain cancers can be a sign of a possible inherited cancer syndrome. The list below includes the most-studied known or suspected risk factors for cancer. Although some of these risk factors can be avoided, others—such as growing older—cannot. Limiting your exposure to avoidable risk factors may lower your risk of developing certain cancers.
- Chronic Inflammation
- Infectious Agents
Genes control how your cells work by making proteins that have specific functions and act as couriers for the cell. Therefore, each gene must have the correct instructions or “code” for making its protein. This is so the protein can perform the correct function for the cell. All cancers begin when one or more genes in a cell are mutated, or changed. This creates an abnormal protein or no protein at all. An abnormal protein provides different information than a normal protein, which can cause cells to multiply uncontrollably and become cancerous.
There are two basic types of genetic mutations:
- Sematicmutations are the most common cause of cancer. These occur from damage to genes during a person’s life. They are not passed from parent to child. Factors such as tobacco, ultraviolet (UV) radiation, viruses, and age cause these mutations.
- Germ line mutations are less common and are passed directly from a parent to a child. In these situations, the mutation can be found in every cell of a person’s body, including the reproductive sperm cells in a boy’s body and egg cells in a girl’s body. Because the mutation affects reproductive cells, it passes from generation to generation. Cancer caused by germ line mutations is called inherited cancer, and it makes up about 5% to 10% of all cancers.
Mutations and cancer
Mutations happen often, and the human body is normally able to correct most of them. Depending on where in the gene the change occurs, a mutation may be beneficial, harmful, or make no difference at all. So, one mutation alone is unlikely to lead to cancer. Usually, it takes multiple mutations over a lifetime to cause cancer. This is why cancer occurs more often in older people who have had more opportunities for mutations to build up.
Types of genes linked to cancer
Many of the genes that contribute to the development of cancer fall into broad categories:
- Tumor suppressor genes are protective genes. Normally, they limit cell growth by monitoring how quickly cells divide into new cells, repairing mismatched DNA, and controlling when a cell dies. When a tumor suppressor gene is mutated, cells grow uncontrollably and may eventually form a mass called a tumor. BRCA1, BRCA2, and p53 are examples of tumor suppressor genes. Germline mutations in BRCA1 or BRCA2 genes increase a woman’s risk of developing hereditary breast or ovarian cancers. The most commonly mutated gene in people who have cancer is p53. In fact, more than 50% of all cancers involve a missing or damaged p53 Most p53 gene mutations are acquired mutations. Germline p53mutations are rare.
Oncogenes turn a healthy cell into a cancerous cell. Mutations in these genes are not inherited. Two common oncogenes are:
- HER2, which is a specialized protein that controls cancer growth and spread, and it is found on some cancer cells, such as breast and ovarian cancer cells
- The ras family of genes, which make proteins involved in cell communication pathways, cell growth, and cell death.
- DNA repair genes fix mistakes made when DNA is copied. But if a person has an error in a DNA repair gene, these mistakes are not corrected. And then they become mutations, which may eventually lead to cancer. This is especially true if the mutation occurs in a tumor suppressor gene or oncogene. Mutations in DNA repair genes can be inherited or acquired.
Despite all that is known about the different ways cancer genes work, many cancers cannot be linked to a specific gene. Cancer likely involves multiple gene mutations. Some evidence also suggests that genes interact with their environment, further complicating genes’ role in cancer.
Genetic Test Results
Genetic tests are usually requested by a person’s doctor. Genetic counseling can help people consider the risks, benefits, and limitations of genetic testing in their particular situations.
The results of genetic tests can be positive, negative, or uncertain. A genetic counselor, doctor, or other health care professional trained in genetics can help an individual or family understand their test results. These professionals can also help explain the incidental findings that a test may yield, such as a genetic risk factor for a disease that is unrelated to the reason for administering the test. And they can clarify the implications of test results for other family members.
Clinical DNA Sequencing
Until recently, most genetic testing for cancer focused on testing for individual inherited mutations. But, as more efficient and cheaper DNA sequencing technologies have become available, sequencing of an individual’s entire genome or the DNA of an individual’s tumor is becoming more common.
Clinical DNA sequencing can be useful in detecting many genetic mutations at one time. Targeted multiple-gene panels test for many inherited mutations or somatic mutations at the same time. These panels can include different genes and be tailored to individual tumor types. Targeted gene panels limit the data to be analyzed and include only known genes, which makes the analysis more straightforward than in broader approaches that assess the whole genome (or tumor genome) or significant parts of it. Multiple-gene panel tests are becoming increasingly common in genetic testing for hereditary cancer syndromes.
Tumor sequencing can identify somatic mutations that may be driving the growth of particular cancers. It can also help doctors sort out which therapies may work best against a particular tumor. For instance, patients whose lung tumors harbor certain mutations may benefit from drugs that target these particular changes.
Testing tumor DNA may reveal a mutation that has not previously been found in that tumor type. But if that mutation occurs in another tumor type and a targeted therapy has been developed for the alteration, the treatment may be effective in the “new” tumor type as well.
Tumor sequencing can also identify germline mutations. Indeed, in some cases, the genetic testing of tumors has shown that a patient’s cancer could be associated with a hereditary cancer syndrome that the family was not aware of.
As with testing for specific mutations in hereditary cancer syndromes, clinical DNA sequencing has implications that patients need to consider. For example, they may learn incidentally about the presence of germline mutations that may cause other diseases, in them or in their family members.