Cancer is an abnormal growth of cells. Cancer cells rapidly reproduce despite restriction of space, nutrients shared by other cells, or signals sent from the body to stop reproduction. Cancer cells are often shaped differently from healthy cells, do not function properly, and can spread to many areas of the body. Tumors, abnormal growth of tissue, are clusters of cells that are capable of growing and dividing uncontrollably; their growth is not regulated.
Oncology is the branch of medicine concerned with the diagnosis and treatment of cancer.
Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors tend to grow slowly and do not spread. Malignant tumors can grow rapidly, invade and destroy nearby normal tissues, and spread throughout the body.
Cancer is malignant because it can be "locally invasive" and "metastatic":
- locally invasive - the tumor can invade the tissues surrounding it by sending out "fingers" of cancerous cells into the normal tissue.
- metastatic - the tumor can send cells into other tissues in the body, which may be distant from the original tumor.
The original tumor is called the "primary tumor." Its cells, which travel through the body, can begin the formation of new tumors in other organs. These new tumors are referred to as "secondary tumors." The cancerous cells travel through the blood (circulatory system) or lymphatic system to form secondary tumors. The lymphatic system is a series of small vessels that collect waste from cells, carrying it into larger vessels, and finally into lymph nodes. Lymph fluid eventually drains into the bloodstream.
Cancer is named after the part of the body where it originated. When cancer spreads, it keeps this same name. For example, if kidney cancer spreads to the lungs, it is still kidney cancer, not lung cancer. (The cancer in the lung would be an example of a secondary tumor.) Staging is the process of determining whether cancer has spread and, if so, how far. There is more than one system used for staging cancer, and the definition of each stage will depend on the type of cancer.
Cancer is not just one disease but rather a group of diseases, all of which cause cells in the body to change and grow out of control. Cancers are classified either according to the kind of fluid or tissue from which they originate, or according to the location in the body where they first developed. In addition, some cancers are of mixed types. The following five broad categories indicate the tissue and blood classifications of cancer:
A carcinoma is a cancer found in body tissue known as epithelial tissue that covers or lines surfaces of organs, glands, or body structures. For example, a cancer of the lining of the stomach is called a carcinoma. Many carcinomas affect organs or glands that are involved with secretion, such as breasts that produce milk. Carcinomas account for 80 percent to 90 percent of all cancer cases.
A sarcoma is a malignant tumor growing from connective tissues, such as cartilage, fat, muscle, tendons, and bones. The most common sarcoma, a tumor on the bone, usually occurs in young adults. Examples of sarcoma include osteosarcoma (bone) and chondrosarcoma (cartilage).
Lymphoma refers to a cancer that originates in the nodes or glands of the lymphatic system, whose job it is to produce white blood cells and clean body fluids. Some lymphomas start in lymph tissue in organs such as the brain or stomach. Lymphomas are classified into two categories: Hodgkin’s lymphoma and non-Hodgkin’s lymphoma.
Leukemia, also known as blood cancer, is a cancer of the bone marrow that keeps the marrow from producing normal red and white blood cells and platelets. White blood cells are needed to resist infection. Red blood cells are needed to prevent anemia. Platelets keep the body from easily bruising and bleeding. Examples of leukemia include acute myelogenous leukemia, chronic myelogenous leukemia, acute lymphocytic leukemia, and chronic lymphocytic leukemia. The terms myelogenous and lymphocytic indicate the type of cells that are involved.
Myeloma grows in the plasma cells of bone marrow. In some cases, the myeloma cells collect in one bone and form a single tumor, called a plasmacytoma. However, in other cases, the myeloma cells collect in many bones, forming many bone tumors. This is called multiple myeloma.
There is no one single cause for cancer. Scientists believe that it is the interaction of many factors together that produces cancer. The factors involved may be genetic, environmental, or lifestyle characteristics of the individual.
As mentioned, some cancers, particularly in adults, have been associated with certain risk factors. A risk factor is anything that may increase a person's chance of developing a disease. A risk factor does not necessarily cause the disease, but it may make the body less resistant to it. Persons who have an increased risk of developing cancer can help to protect themselves by scheduling regular screenings and check-ups with their physician and avoiding certain risk factors. Cancer treatment has been proven to be more effective when the cancer is detected early. The following risk factors and mechanisms have been proposed as contributing to the development of cancer:
- lifestyle factors
Lifestyle factors such as smoking, high-fat diet, and exposure to ultraviolet light (UV radiation from the sun) may be risk factors for some adult cancers. Most children with cancer, however, are too young to have been exposed to these lifestyle factors for any extended time.
- genetic factors
Family history, inheritance, and genetics may play an important role in some adult and childhood cancers. It is possible for cancer of varying forms to be present more than once in a family. Some gene alterations are inherited. However, this does not necessary mean that the person will develop cancer. It indicates that the chance of developing cancer increases. It is unknown in these circumstances if the disease is caused by a genetic mutation, other factors, or simply coincidence.
- virus exposure
Exposures to certain viruses, such as the human papillomavirus (HPV) and human immunodeficiency virus (HIV; the virus that causes acquired immune deficiency, or AIDS), have been linked to an increased risk of developing certain types of cancers. Possibly, the virus alters a cell in some way. That cell then reproduces an altered cell and, eventually, these alterations become a cancer cell that reproduces more cancer cells. Cancer is not contagious and a person cannot contract cancer from another person who has the disease.
- environmental exposures
Environmental exposures have been linked to some cancers. For example, people who have certain jobs (such as painters, farmers, construction workers, and those in the chemical industry) seem to have an increased risk for some cancers, likely due to exposure to certain chemicals. Other exposures may occur in the home or elsewhere, such as radon (a radioactive gas) in some homes.
The discovery of certain types of genes that contribute to cancer has been an extremely important development for cancer research. Virtually all cancers are observed to have some type of genetic alteration. A small percentage (5 percent to 10 percent) of these alterations are inherited, while the rest are sporadic, which means they occur by chance or occur from environmental exposures (usually over many years). There are three main types of genes that can affect cell growth, and are altered (mutated) in certain types of cancers, including the following:
These genes regulate the normal growth of cells, causing them to grow. Scientists commonly describe oncogenes as similar to a cancer "switch" that most people have in their bodies. What "flips the switch" to make these oncogenes suddenly allow abnormal cancer cells to begin to grow is unknown.
- tumor suppressor genes
These genes are able to recognize abnormal growth and reproduction of damaged cells, or cancer cells, and can interrupt their reproduction until the defect is corrected. If the tumor suppressor genes are mutated, however, and they do not function properly, tumor growth may occur.
- mismatch-repair genes
These genes help recognize errors when DNA is copied to make a new cell. If the DNA does not "match" perfectly, these genes repair the mismatch and correct the error. If these genes are not working properly, however, errors in DNA can be transmitted to new cells, causing them to be damaged.
Usually the number of cells in any of our body tissues is tightly controlled so that new cells are made for normal growth and development, as well as to replace dying cells. Ultimately, cancer is a loss of this balance due to genetic alterations that "tip the balance" in favor of excessive cell growth.
Diagnosis, treatment, and prognosis for childhood cancers are different than for adult cancers. The main differences are the survival rate and the cause of the cancer. The survival rate for childhood cancer is about 80 percent, while in adult cancers the survival rate is 66 percent. This difference is thought to be because childhood cancer is more responsive to therapy, and a child can tolerate more aggressive therapy.
Childhood cancers often occur or begin in the stem cells, which are simple cells capable of producing other types of specialized cells that the body needs. A sporadic (occurring by chance) cell change or mutation is usually what causes childhood cancer. In adults, the type of cell that becomes cancerous is usually an "epithelial" cell, which is one of the cells that line the body cavity, including the surfaces of organs, glands, or body structures, and cover the body surface. Cancer in adults usually occurs from environmental exposures to these cells over time. Adult cancers are sometimes referred to as "acquired" for this reason.