Cancer Essentials
Learn all about the latest in breast cancer research.
What is a cancer?
The term "cancer" describes diseases of uncontrolled tissue growth. The probabilities differ, but cancer can arise in any organ, in almost any tissue, and at any age. Although more than one hundred forms of cancer exist and each has unique characteristics, all cancers arise from defects in the same basic mechanism of cell division.
Cancer occurs when a cell no longer responds to the signals that normally prevent it from dividing uncontrollably. As a result of unregulated growth, the offspring of the diseased cell may form a tumor that interferes with the functioning of neighboring structures. The offspring may also invade or destroy neighboring tissues or spread to other regions of the body and form tumors there. In addition, the offspring of the diseased cell may impair the normal functioning of the blood. Combinations of these effects are also possible.
Except for lung cancer, which is caused almost exclusively by smoking, there is no cancer epidemic. When lung cancer is excluded, the death rates from cancer have declined since 1950. Death from lung cancer is decreasing for men and increasing for women, although the increase has begun to slow.
How do cancerous tumors differ from benign tumors?
A tumor is called malignant if it invades neighboring tissue or spreads (metastasizes) to other parts of the body. Generally, a tumor is considered benign if it stays in one place where it may be removed surgically. However, if such a tumor occurs in a confined space such as inside the skull, its effects can be far from benign.
Why do cancers form?
Cancer is generally attributed to two kinds of influences, those that start cancers by damaging certain genes, and those that promote the growth of cancer cells. Here are some of the factors that are responsible for starting cancers, and some factors that have been accused unjustly.
Aging
Could a normal, healthy person with intact genes who is lucky enough to be able to avoid exposure to all external carcinogens still get cancer? Yes, if the person lives long enough. Rough estimates suggest that one-quarter of all cancers are caused by carcinogens that are produced inside the body in the normal course of living. It is estimated that the genetic code in a normal human cell sustains about 10,000 damaging oxidative hits a day. Most of this damage will be repaired quickly. However, because repair is not perfect, if the amount of unrepaired damage becomes sufficient, enough may accumulate to eventually cause cancer in one of the cells.
Smoking and diet
Smoking and diet are thought to be the major culprits in deaths from cancer in industrialized countries. Carcinogens in tobacco smoke are held responsible for a combined 25% of cancer deaths in the United States. Not eating enough fruits and vegetables may account for another 25–30%. High consumption of saturated fats and red meat are associated with some cancers. Overeating and insufficient exercise are likely contributors to cancer deaths, amounting to about 3 percent of deaths from cancer. Medium to heavy consumption of alcohol may contribute as much as another 3 percent to total cancer deaths.
Chronic infections
Roughly 5 percent of cancer deaths in the United States and other industrialized countries may be attributable to infections. The most frequent cancer-causing microbes are human papilloma viruses, which cause cancer of the uterine cervix, and the hepatitis B virus, which can cause liver cancer.
Medical treatments
Chemotherapy and radiation used in cancer treatment may themselves be carcinogenic. Immune suppressant drugs and hormone treatments may cause cancers. Some researchers estimate that medical treatments contribute to around 1 percent of cancer deaths but emphasize that the usefulness of these treatments outweighs the risks.
Radiation
Skin cancer from exposure to the sun is by far the most frequent cause of death from radiation-induced cancer. For the general population, exposure to radon is insignificant, and the contribution from other sources of radiation has often been exaggerated. High-frequency electromagnetic radiation in the form of X rays or gamma rays from certain radioactive processes can pack enough energy to produce harmful free radicals and damage the genetic code. Energetic electrons and alpha particles that are emitted in radioactive decay also have the same potential for harm. For most people, most exposure to ionizing radiation comes from the background radiation to which we are all exposed.
Radio- and low-frequency electromagnetic radiation
Low-frequency electromagnetic radiation (for example, from power lines) and high-frequency radiation (for example, radio waves) are another matter. Neither has been found to cause cancer. It is extremely unlikely that they would, because the energies from exposure to this type of radiation are thousands to millions of times smaller than would be needed to damage the genetic code or produce harmful free radicals. There are numerous other agents that can cause cancer under certain circumstances and, usually, only in isolated instances. Examples are occupational exposure to certain chemicals such as benzene, formaldehyde, and some pesticides. The impact of these exposures on the general population is dwarfed by the risk posed by carcinogens in the diet and in tobacco smoke, and by insufficient consumption of fruits and vegetables.
How do cancers form?
The development of a cancer progresses through a series of stages during which a cell accumulates changes in the genetic code of the molecule that are called mutations. Development may stop at any of these stages. Progression from one stage to the next is triggered by additional damage to a cell. The following are the stages of development of a cancer.
Hyperplasia
Malignant growth of a solid tumor is usually set in motion when a previously normal cell begins to lose its ability to respond to restraining chemical signals from other cells. The cell is then more likely to start to divide when it would otherwise rest. At this stage, the cell and its offspring still look and behave normally, except that they divide at a somewhat faster-than-normal rate. It may take years for a visible result to occur, namely a mass of cells where normally there would be fewer cells. This abnormal increase in the number of normal cells is called hyperplasia.
Dysplasia
While hyperplasia develops, one cell among millions may suffer additional damage that further impairs its ability to respond to restraining signals. In addition to accelerated growth, the offspring of this cell now look and behave abnormally. This abnormal increase in the number of abnormal cells is called dysplasia.
Cancer in situ
On rare occasion, a dysplastic cell mutates further and becomes more abnormal in appearance and growth. As long as the mass of cells it produces does not spread into neighboring tissue, it is called an in situ (in place) cancer. Again, growth may stop at this stage.
Invasive cancer
Additional mutations may develop in one or more cells at the in situ stage and enable the resultant cells to break into neighboring tissue or to shed cells into the bloodstream or lymph vessels. These disseminated cells are likely to produce new tumors (metastases) elsewhere in the body. At this stage, the cancer may begin to interfere with the normal functions of the body and is called malignant.
Can cancerous tumors just go away?
There are case reports of cancers going away by themselves. Such events, called spontaneous remissions, are very rare, hard to prove, and difficult to study. One team of researchers compiled reports on a total of 755 spontaneous remissions of malignant tumors. A review of the international literature on spontaneous remission of breast cancer found only 32 cases, of which only six were sufficiently documented to be considered dependable.
Although an understanding of these events could be valuable, there is no centralized system or medical journal devoted to the systematic recording and investigation of spontaneous remissions.
Is cancer preventable?
Many of the known causes of cancer can be avoided by not smoking, by eating a diet rich in fruits and vegetables, by avoiding overeating, and by exercising. These are the most powerful general preventive measures known to date.
Smoking
About one-quarter of all cancers in the United States and about 90% of lung cancers can be attributed to smoking.
Diet
For most cancers, the 25% of Americans who eat the fewest fruits and vegetables have twice the risk as the 25% who eat the most fruits and vegetables.
With increased understanding of the development of cancer at the molecular level, many possible ways of preventing cancer at various stages have been identified. Several clinical trials are under way to look at the possible preventive effects of nutrients and other substances.
Is cancer genetic?
Some cancers run in families. This does not make cancer a genetic disease. Some familial clustering of cancers results from shared habits or a shared environment. In other families, cancers may occur more often than expected because of shared genes. Carrying such genes does not mean that a family member will get cancer. It merely means that the risk of cancer is higher than it would be without the gene. Sometimes this increase in risk can be avoided—for example, when a person who inherits fair skin, which increases the risk of skin cancer, avoids excessive exposure to the sun.
In other instances, people may inherit a mutated gene that has lost its ability to regulate cell growth and division. This event will more directly increase the risk of getting cancer, because it will take fewer additional mutations to transform a normal cell into a cancerous cell. Even so, if one of these additional mutations fails to occur, no cancer will develop.
Is cancer always deadly?
About 40% of Americans can expect to be diagnosed as having cancer at least once during his or her lifetime, but fewer than 25% of Americans will die of cancer. Since 1950, death rates have decreased for stomach, cervical, endometrial, and colorectal cancers, and for a number of less frequently occurring cancers.
Improvements in treatment have led to great and steady reductions in deaths from some childhood cancers, certain cancers of bone and muscle, testicular cancer, and some blood cell cancers.
Most skin cancers are treated successfully because they are usually found early or because they grow slowly. Especially in later years, some cancers may go undetected and grow so slowly that the person dies of another cause. For example, autopsy studies suggest that most men die with prostate cancer but not from prostate cancer.
