Learning About Cancer

While often thought of as a single disease, cancer is actually a group of more than 100 different diseases characterized by the uncontrolled, abnormal growth of cells that can potentially spread throughout the body. Cancer begins when normal cells in the body live beyond their normal life cycle, enabling them to continue to divide and reproduce uncontrollably, invading and destroying healthy tissue.

Most types of cancer cells form a lump or mass called a tumor, although some cancers, such as blood cancers, do not form tumors. Not all tumors are cancerous (malignant). A tumor that is not malignant is called benign. Benign tumors can grow locally, but don't spread and are usually not life-threatening.

Cells from malignant tumors can also break away and travel to other parts of the body where they can continue to grow, a process called metastasis. When cancer metastasizes it is named for the part of the body where it originated. For example, if breast cancer spreads to the lungs, it is called breast cancer, not lung cancer.

Cancer is the second leading cause of death in the United States after heart disease. In 2004, approximately 1,368,030 new cancer cases are expected to be diagnosed and about 563,700 cancer deaths are expected to occur in the United States. Today, there are more than 9 million cancer survivors in the United States.

The most common types of cancer are breast, prostate, lung, and colorectal cancer. Lung cancer remains the leading cause of cancer death and will account for more than 173,000 deaths this year. Tobacco is the cause of the majority of these deaths.

The risk of developing cancer can be greatly reduced by avoiding activities that contribute to its development. Abstaining from smoking, heavy drinking, and excessive sun exposure can significantly reduce the risk of developing cancer. Good nutrition and regular exercise may lower the risk, as well.

In some instances, the risk of developing cancer may be reduced through chemoprevention, the use of certain synthetic drugs or natural agents to reduce the risk of developing cancer. Several chemoprevention agents are being researched.

The causes of the majority of cancers can be divided into external and internal factors. The causes of many cancers, however, are not yet known.

External factors include cigarette smoking and heavy alcohol use, as well as environmental causes such as exposure to radiation and chemicals. Some cancers are caused by viral infections. The best known example is the human papillomavirus (HPV), which causes cervical cancer.

While all people are at risk of developing cancer, some people are at higher risk than others. The most significant risk factor is age. People over the age of 55 account for 80% of all new cancer cases. In the United States, men have a one in two risk of either developing or dying from cancer over the course of their lifetime, while the risk for women is one in three.

Certain lifestyle choices, including smoking, heavy alcohol consumption, and prolonged sun exposure, can also increase risk for lung cancer, liver cancer, skin cancers, and head and neck cancers.

Approximately 5% to 10% of cancers are hereditary, so if a person has close relatives with cancer, he or she may be at higher risk. For example, a woman whose mother or sister or had breast cancer is twice as likely to develop the disease. See Genetic Testing for more information.

Cancer treatment is much more likely to be successful when it is diagnosed early, before it has spread. One of the most important ways to detect cancer early is to get regular screenings by a trained physician. Currently, doctors can screen for cancers of the breast, colon, rectum, cervix, prostate, testicles, mouth and mouth area, and skin.

Screening tests for other cancers, such as ovarian cancer, are currently being evaluated. Self-examination is also recommended for breast, skin, and testicular cancer, among others. People at high risk for developing cancer should talk with their doctor about starting a regular screening.

While there has been controversy in the past over the value of mammography for breast cancer screening, most experts agree that mammography can significantly lower the risk of death from breast cancer, and that cancers caught early can be treated less aggressively. For example, women may need to undergo a local excision (lumpectomy) and radiation therapy if cancer is caught early, instead of a mastectomy and general chemotherapy if the breast cancer is caught in its later stages, after it has spread.

There are many different types of tests to diagnose cancer and to find out how far the disease has spread. Some tests can also determine what treatments will work best for a particular patient. The types of tests that a doctor selects depends on the type of cancer that is suspected.

Biopsy: A test where abnormal tissue is removed and examined under a microscope to determine if cancer cells are present.

Bone marrow testing: An examination of a bone marrow sample under a microscope to test for certain bone marrow or blood cancers.

Fecal occult blood test: An examination of stool for traces of blood not visible to the naked eye to test for colon cancer.

Magnetic resonance imaging (MRI): A test that produces internal pictures of the body using radiofrequency waves, electromagnetic fields, and a computer.

Positron emission tomography (PET scans): A test that uses a low-dose radioactive sugar to determine the metabolic activity in cells.

Computerized tomography (CT or CAT scans): A test that uses an x-ray to develop a three-dimensional picture of a part of the body.

Staging is the process of describing the local extent of the disease or the spread of cancer from the original site, and is essential in determining the choice of therapy and assessing prognosis. A cancer's stage is based on information about the tumor's size and location in the body, and whether or not it has spread to other areas of the body.

A number of different staging systems are currently used to classify tumors. For example, the "TNM" staging system measures tumors in three ways: size of the primary tumor (T), absence or presence of cancer in the lymph nodes (N), and absence or presence of metastasis (M), or cancer in other areas of the body. Once the T, N, and M are determined, a "stage" of I, II, III, or IV is assigned, with stage I being early stage and IV being advanced stage.

The precursor state of cancer represents its earliest stage that is confined to the place or site where it started and is called in situ. This means that it has not invaded any surrounding structures. These cancers are highly curable, and are given a stage of "0" (zero). Cancer that has invaded any surrounding structures is considered invasive (Stages I-IV).

There are many approaches to treating cancer, and each cancer is treated differently. The most common types include surgery, radiation, and chemotherapy, used either alone or in combination. More recent treatment options such as targeted molecular therapies and biological treatments are also being used to treat specific types of cancer.

Chemotherapy. Chemotherapy is the use of drugs to destroy cancer cells. These drugs fight cancer by interfering with the metabolism or growth process of cancer cells, eventually causing the cells to die. Damage can also occur in some healthy cells, which can cause side effects. In many cases, combinations of two or more drugs are used together to achieve the best possible effect.

Radiation. Radiation therapy uses beams of high-energy waves or streams of particles including x-rays, gamma rays, and electrons to destroy cancer cells. Radiation can either be directed at a tumor from outside the body (external-beam radiation therapy) or supplied by radioactive materials implanted next to cancer cells inside the body (internal radiation therapy or brachytherapy). Like chemotherapy, radiation therapy halts the growth and division of cancer cells and causes them to die, so radiation therapy can also damage normal cells and can cause side effects.

Surgery. Surgery involves the removal of tissue from the body, and is an important part of treatment and detection for many types of cancer. Most curable cancers are cured in surgery. Surgery can also confirm a diagnosis (biopsy), determine how far a person's cancer has advanced (staging), prevent symptoms, or help make a patient more comfortable (palliative surgery).

Hormone therapy. Several types of cancer, including some breast and prostate cancers, can only grow and spread in the presence of natural chemicals in the body called hormones. Other types of cancer are slowed by specific hormones. Hormone therapy fights cancer by altering the amounts of hormones in the body. Hormone therapy is especially useful for treating many cancers of the breast and reproductive system. For example, tamoxifen, an anti-estrogen drug, is used extensively in the treatment of some hormone-responsive breast cancers.

Targeted treatments. Every cell of the body has thousands of genes that provide instructions that enable all the body's cells to function. Mutations, or defects, in a cell's genes can prevent cells from behaving normally, allowing them to grow and divide uncontrollably, causing cancer. Many new cancer treatments halt the growth of cancer by preventing mutated genes from functioning, or by starving a tumor's blood supply. These include:

Anti-angiogenesis agents: The formation of new blood vessels that feed tumors is known as angiogenesis. By cutting off a tumor's blood supply, it may be possible to starve the tumor of the oxygen, energy, and blood vessels it needs to grow and spread.

Antisense therapy: This therapy utilizes small, chemically-modified strands of DNA that block gene expression by binding to messenger RNA before it can produce a protein. Essentially, the technology acts to selectively "knock out" the production of a single protein.

Signal transduction inhibitors: These drugs hinder the activation of specific enzymes called tyrosine kinases, which can send faulty signals that trigger tumor cell growth. Signal transduction inhibitors, which act like "circuit breakers," block these enzymes, thereby preventing the signal from being activated. For example, imatinib mesylate (Gleevec) selectively blocks the abnormal tyrosine kinase found in chronic myelogenous leukemia (CML), as well as the altered KIT protein that causes gastrointestinal stromal tumors (GIST).

Biological treatments. Biological treatments stimulate the disease-fighting ability of the body's immune system. These therapies are often used to help restore the functioning of the immune system. Immunotherapy is a type of biological treatment.

Cancer vaccines: Unlike vaccines that can prevent infectious diseases, cancer vaccines attempt to treat the cancer by training the immune system to recognize cancer cells and attack them. Some cancer vaccines alter and reintroduce the patient's own cancer cells to stimulate an immune response.

Monoclonal antibodies: These laboratory-produced substances can locate and bind to specific receptors or characteristics on the surface of cancer cells, blocking their growth. Monoclonal antibodies can be used alone, in combination with other therapies, or to deliver drugs, toxins, or radioactive material. For example, Herceptin is a monoclonal antibody therapy for breast cancers that "overexpress" the protein HER2/neu. The antibody binds with HER2/neu on the outer surface of tumor cells, preventing the growth and division of cancer cells.