What Is Cancer?
What Is Cancer?Normal Cells and Cancer Cells
DNA: The Cell's Blueprint
Oncogenes and Suppressor Genes
How Tumor Cells Trick the Body
Kinds of Cancers
Cancer Causes: Heredity or Environment?
Carcinogens as a Cause of Cancer
Dealing with Cancer
Cancer is not a single disease; it is a name given to at least two hundred different diseases. Cancer occurs when normal cells in the body behave in abnormal ways. The human body consists of trillions of cells that are so tiny they can only be seen under a microscope. The organs and tissues of the body are made up of groups of these cells. All cells reproduce by dividing—a process called mitosis. Usually, when cells are no longer needed by the body for growth or repair of tissues, they die. Some cells, however, continue to divide and refuse to die when they are no longer needed.
Each cancer starts with one cell that reproduces uncontrollably. This behavior is called mutation. As the out-of-control mutant cells pile up, they form masses called tumors. When these tumors become aggressive and invasive, they are said to be malignant, or cancerous. Cancer can develop in almost any organ or tissue of the body. From its original site, a cancer may spread, or metastasize, to other parts of the body. When this happens, the cancer is more difficult to treat and may become life threatening.
Normal cells and cancer cells are different in several ways. Normal cells reproduce themselves exactly and stop reproducing when they are supposed to. They self-destruct when they have completed their job or if they become damaged. Cancer cells keep on reproducing and do not obey signals to stop. They have been called immortal because they refuse to die on their own. Normal cells stick together. Cancer cells do not, which allows them to become detached from their neighboring cells. Finally, normal cells mature and become specialized in doing their job. Cancer cells remain immature and even become more immature or primitive over time; they do not perform any helpful function in the body.
Each cell in the human body contains long coils of deoxyribo-nucleic acid (DNA). Strands of DNA look like twisted strings of beads. Similar to the way letters are arranged in a sentence to provide information, the way the beads in the DNA are arranged provides information to the cells. Sections of DNA called genes are blueprints or designs for making new cells. Genes contain instructions that are passed down from generation to generation. The information in genes controls how a person will look: eye and hair color, height, and many other physical traits as well as some behavioral traits.
Genes in the DNA strand also contain complicated instructions that control the growth, division, and repair of cells. When cells divide, each new cell gets a complete copy of all the DNA instructions from the original cell. If the original cell's genes are damaged, the damage is passed on to the new cell. The new cell can also acquire more damage, which it passes along to its offspring. When the damaged cells multiply wildly, cancer may result. Some substances that can damage cells' DNA are cigarette smoke, chemicals, radiation, and excessive sunlight. Substances that cause damage to DNA genes in such a way that cancer may occur are called carcinogens.
Some genes encourage cells to multiply. This is important in young people, who are still growing, but in adults it normally does not happen very often. Exceptions to this are after a wound or operation, when cells multiply to repair the damage. When genes become overactive and tell cells to keep on multiplying when there is no reason for it, scientists call them oncogenes, or cancer genes. These are the villains that direct cells to multiply excessively, ignoring normal stop signals. Oncogenes have been found to contribute to cancers in many places in the body, including the breasts, lungs, liver, bladder, and colon. Along with the term oncogene comes the name oncologist for a doctor who specializes in treating cancer.
Genetic Medicine: Blessing or Curse?
In “Genetic Medicine: Powerful Opportunities for Good and Greed,” Michael Dalzell says that while genetic treatments could spawn incredible improvements in health care, they also raise complex questions. He explains:
The theory of genetic diagnosis and treatment turns the practice of Western medicine inside out—literally. Instead of starting with disease and searching for its origin, genom-ics begins with a genetic variation and relies on treatments that manipulate it, often before the gene can express itself in terms of illness. It bespeaks a change in the nature of health care from treating symptoms to predicting health status and taking steps accordingly.
Dalzell points out that while this concept fits the disease prevention goals of managed health care, it requires expensive, highly individualized treatment. Furthermore, it can be shocking to patients to know what their future is likely to hold. He asks, “At what point do you say to a 13-year-old girl: ‘We've done genetic testing on you, and somewhere around age 60, you're going to develop Alzheimer's. You'll have osteoporosis by around 40. And, oh, by the way, you have an 80 percent chance of getting breast cancer'?”
Michael Dalzell, “Genetic Medicine: Powerful Opportunities for Good and Greed,” Managed Care, May 2001, p. 4.
Fortunately, the body normally has a way to combat these unruly oncogenes. Suppressor genes come to the rescue by limiting cell growth or division. If oncogenes are like accelerators in cars, then suppressor genes are like brakes. They stop cells with damaged genes from reproducing and encourage them to self-destruct. However, in cancerous cells, the suppressor gene may be damaged, making it unable to stop cell multiplication. One well-known suppressor gene is called p53, and in most human cancers this suppressor is damaged or missing. When genes transform into oncogenes and suppressor genes like p53 become inactive through damage, tumors may form.
A tumor is a large group of cells that have clumped together to form a mass of tissue. Some tumors are benign. A benign tumor usually grows slowly. When it stops growing, it does not spread beyond the place where it started. Benign tumors are usually contained inside a wall or barrier called a basement membrane, which is made up of proteins (molecules of amino acids). Once a benign tumor is removed, it is not likely to form again. It is a problem only if it grows very large and becomes uncomfortable or unsightly, presses on other body organs, takes up space inside the skull, or releases hormones that cause trouble.
Malignant, or cancerous, tumors usually grow more quickly than benign tumors. They break through the basement membrane and invade surrounding tissue. They can become life threatening when cells break away and spread to other parts of the body, in a process called metastasis. There are two ways cancer cells spread through the body. They can travel through the bloodstream to distant parts of the body. Since the blood vessels in tumors are weaker and more leaky than normal blood vessels, the tumor cells can escape and circulate in the blood. Escaping tumor cells can also be carried in the lymphatic fluid that bathes the body tissues. Then they can establish metastatic colonies in the lymph nodes that drain all of the tissues of the body. Once cancer cells invade another body organ, they can grow new malignant tumors, called secondary tumors.
No matter where it spreads or how much it spreads, a cancer is always classified by the primary site where it started. This is because wherever they are in the body, cancer cells still possess many of the characteristics of the original tumor cells. If a cancer starts in the breast and spreads to the lungs, it is still classified as breast cancer. Or if a cancer starts in the stomach and spreads to the liver, it is still classified as stomach cancer. It is important to doctors treating the cancer to identify the origin of the cancer so they can decide how to treat it. This is because the organs of the body are made up of different types of cells that behave in different ways. They may grow at different speeds and be more or less likely to spread in the blood. They also may respond differently to drugs or radiation.
In order to survive, all the cells in the body need oxygen and other nutrients and a way to dispose of waste material. Blood brings these essential supplies to cells and also removes waste. This is true not only for normal body cells but also for cancerous cells in tumors. As a tumor grows bigger, the cells in its center get farther and farther from the blood vessels surrounding it. The cancer cells are in danger of starving to death unless they can develop their own blood supply. They need a clever strategy to meet this challenge, and, unfortunately, they have found it.
Normal cells stimulate new blood vessels to grow when they need to repair damaged tissue. They have genes that can switch the growth of blood vessels on and off. The formation of new blood vessels is called angiogenesis. Aggressive cancer cells send out a flood of SOS signals, by means of signaling proteins that call for more blood. New capillaries (tiny blood vessels) begin to sprout from the tissues surrounding the tumor, and tumor angiogenesis takes place. When the tumor is nourished by the new blood vessels, it begins to expand and spread. Wherever it spreads in the body, the metastatic tumor carries cells like those in the original, or primary, tumor.
Since there are so many different cancers, they can be classified in different ways. Most cancers, however, fit into one of three main types: carcinoma, sarcoma, and leukemia/lymphoma. About 85 to 90 percent of all cancers are carcinomas. These cancers begin in covering tissues of the body, such as the outer layer of the skin. They also occur in the delicate tissue that lines the mouth. Carcinomas may also arise in tissues lining the internal organs, the chest cavity, the abdominal cavity, and the organs of the digestive and reproductive systems.
Sarcomas originate in connective and supporting tissues, such as bones, cartilage, nerves, and fat. In addition to bone cancer, sarcomas include cancer of skeletal muscles and Ka-posi's sarcoma, a skin cancer that sometimes appears in AIDS patients. Sarcomas are usually divided into two main types: bone sarcomas and soft tissue sarcomas. Together, they make up fewer than 1 percent of cancers.
Cancers of the blood cells are called leukemias. Leukemias arise in tissues where blood cells are produced, such as the bone marrow. Large numbers of abnormal white blood cells fill the bone marrow and enter the bloodstream. They interfere with the production of red blood cells and cause problems like bleeding and anemia. Leukemia is one of the most common cancers affecting children.
Lymphomas are cancers that develop in the lymphatic system. This system includes the lymph nodes, tonsils, adenoids, spleen, and bone marrow. The lymphatic system fights off germs that cause infection and illness. Lester, the Red Sox pitcher, was diagnosed and treated for a form of lymphoma called non-Hodgkin's lymphoma.
It has been established by experts that cancers can be familial, or inherited, or sporadic, occurring at random in the population. Although most tumors arise in people with no family history of the disease, there is good reason for researchers to study both causes.
Damaged or mutated genes can be inherited. When doctors establish a patient's medical history, they ask about close relatives who have had cancer and what kind of cancer they had. Doctors know that some types of mutated genes can be passed from parents to children. They are aware that patients with a family history of certain cancers are more vulnerable to that type of cancer. But they also know that although patients with a family history of cancer are at higher risk, they are not destined to develop cancer.
Sometimes cancers that occur in several family members are just coincidental. They might also be caused by something in the shared environment. Compared to tumors that occur sporadically, the actual number of cases of familial cancer is quite small. There are, however, certain types of cancer that occur more often in families. These include some cancers of the breast, prostate cancer, ovarian cancer, cancer of the colon, skin cancers, and an eye cancer called retinoblastoma.
Genetic testing for the more common oncogenes is possible today. But this kind of testing is controversial. Although genetic tests can predict whether or not a person is at risk for certain familial cancers, many people prefer not to know. And even those who discover that they do not carry an abnormal gene mutation are still at risk for developing cancers that occur sporadically.
Those whose genetic tests indicate they have inherited an abnormal gene can choose to have more frequent tests for cancer, or they can opt for more drastic procedures. Experts have found that those who carry the breast cancer gene BRCA have an 80 percent chance of developing breast cancer. Consequently, some women who have tested positive for the BRCA gene have more frequent mammograms. Still others choose to have their breasts removed surgically to eliminate the risk, even if they do not have cancer.
Genetic testing can cost thousands of dollars; however, some cancer centers offer free testing in connection with research studies. Some insurance plans will pay the costs of genetic testing that is recommended by a person's doctor. But some people fear that those who have a known risk of cancer may have difficulty being hired for jobs or obtaining health or life insurance.
Most cancers are not the result of inherited abnormal genes. They are the result of environmental factors. Some carcinogens have been known and understood for a long time. Others are being discovered through current research. The research is difficult because many cancers develop from the combined effects of more than one carcinogen on the DNA. Carcinogens enter the body through the skin or through the nose, mouth, or other openings.
Sometimes it is apparent that people who work at particular jobs tend to get certain types of cancer. Experts study chemicals or other substances these people encounter in their work. The first step is to test the suspect substance on laboratory animals such as mice or rats. If a large percentage of the animals develop cancer, the researchers go on to evaluate the effect of the substance on people. They do this by comparing a group of people who have been exposed to the agent to a group of people who have not been exposed. If the exposed group has a higher rate of a certain cancer, this is strong evidence that the substance is a carcinogen.
Pets Get Cancer, Too
Veterinarian Karen Halligan says cancer is one of the leading causes of death in pets and accounts for nearly half of all deaths in cats and dogs over the age of ten. She says:
Dogs suffer from more kinds of cancer (at least 100) than any other domestic animal, and one in four will develop the disease in its lifetime. Just like people, pets can develop cancer from exposure to sunlight, smoke, asbestos, chemicals, hormones, radiation and viruses, and from immune system failures. And some breeds have hereditary risk factors, among them golden retrievers, boxers and bulldogs.
Like humans, pets can be treated with surgery, chemotherapy, radiation, immunotherapy, or a combination of treatments.
Karen Halligan, “How to Protect Your Pet from Cancer,” Parade, May 11, 2008, p. 13.
Asbestos is an example of a cancer-causing agent that took years to discover. Asbestos was once commonly used as a building material. Workers in the construction business inhaled large amounts of asbestos fibers in the course of their work. Years later, many of them developed lung cancer. Although asbestos is no longer used in buildings, many older buildings still contain this substance. If these structures undergo renovation or are torn down and the asbestos is disturbed, the fibers float in the air and cause cancer if they are breathed in.
Through experiments, scientists have identified hundreds of chemicals that can cause cancer in animals. Some of these may also be hazardous to humans. One of the deadliest is cigarette smoke, which contains dozens of chemical carcinogens. In addition to lung cancer, smoking causes many cancers of the mouth, larynx, esophagus, pancreas, kidney, bladder, cervix, and some forms of leukemia. It can also cause cancer in non-smokers who live or work closely with smokers.
Radiation is another well-known carcinogen. It comes from different sources, but the most common source of cancer-causing radiation is the sun. Its invisible ultraviolet rays cause most cases of skin cancer, including deadly melanoma. Nuclear radiation can also cause different kinds of cancer, which sometimes do not appear until years after exposure.
At this time, experts have not determined that viruses are a major cause of human cancers. It has been shown, however, that the human papillomavirus (HPV) causes most cases of cervical cancer. Other viruses may cause cancer of the liver and adult T-cell leukemia. One of the viruses that spreads mononucleosis (commonly called the “kissing disease”) is the Epstein-Barr virus (EBV). Nasopharyngeal cancer, which involves the area at the back of the nose, has been linked to EBV. There also seems to be a link with Hodgkin's disease, since people who have had EBV-related illnesses, including mono, are more likely to contract Hodgkin's disease than those who have not.
Cancer is a very complicated disease. It appears in many shapes and forms and has many tricks and ways of surviving. All the problems involved in preventing, diagnosing, and treating it will take many more years to solve. However, as Barbara Basler says in “Good News About Cancer”: “Cancer is still a formidable foe, but in the last few years alone, scientists have gained intimate knowledge of this enemy and are using it to outmaneuver these deadly cells—prolonging life and improving cure rates for thousands of patients.”3