National Cancer Institute
Understanding Cancer Series: Cancer
Cancer is a renegade system of growth that originates within a patient's biosystem, more commonly known as the human body. There are many different types of cancers, but all share one hallmark characteristic: unchecked growth that progresses toward limitless expansion.
It is difficult to imagine anyone who has not heard of this illness. Most people have been affected because either they or their loved ones or friends are cancer survivors.
Because cancer is so prevalent, people have many questions about its biology, detection, diagnosis, possible causes, and strategies for prevention.
Slide 2
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Different Kinds of Cancer
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Cancer can originate almost anywhere in the body.
Carcinomas, the most common types of cancer, arise from the cells that cover external and internal body surfaces. Lung, breast, and colon are the most frequent cancers of this type in the United States.
Sarcomas are cancers arising from cells found in the supporting tissues of the body such as bone, cartilage, fat, connective tissue, and muscle.
Lymphomas are cancers that arise in the lymph nodes and tissues of the body's immune system.
Leukemias are cancers of the immature blood cells that grow in the bone marrow and tend to accumulate in large numbers in the bloodstream.
Scientists use a variety of technical names to distinguish the many different types of carcinomas, sarcomas, lymphomas, and leukemias. In general, these names are created by using different Latin prefixes that stand for the location where the cancer began its unchecked growth. For example, the prefix "osteo" means bone, so a cancer arising in bone is called an osteosarcoma. Similarly, the prefix "adeno" means gland, so a cancer of gland cells is called adenocarcinoma--for example, a breast adenocarcinoma.
To diagnose the presence of cancer, a doctor must look at a sample of the affected tissue under the microscope. Hence, when preliminary symptoms, Pap test, mammogram, PSA test, FOBT, or colonoscopy indicate the possible existence of cancer, a doctor must then perform a biopsy, which is the surgical removal of a small piece of tissue for microscopic examination. (For leukemias, a small blood sample serves the same purpose.) This microscopic examination will tell the doctor whether a tumor is actually present and, if so, whether it is malignant (i.e., cancer) or benign. In addition, microarrays may be used to determine which genes are turned on or off in the sample, or proteomic profiles may be collected for an analysis of protein activity. This information will help doctors to make a more accurate diagnosis and may even help to inform treatment planning.
Instead of finding a benign or malignant tumor, microscopic examination of a biopsy specimen will sometimes detect a condition called "hyperplasia." Hyperplasia refers to tissue growth based on an excessive rate of cell division, leading to a larger than usual number of cells. Nonetheless, cell structure and the orderly arrangement of cells within the tissue remain normal, and the process of hyperplasia is potentially reversible. Hyperplasia can be a normal tissue response to an irritating stimulus. An example of hyperplasia is a callus that may form on your hand when you first learn to swing a tennis racket or a golf club.
In addition to hyperplasia, microscopic examination of a biopsy specimen can detect another type of noncancerous condition called "dysplasia." Dysplasia is an abnormal type of excessive cell proliferation characterized by loss of normal tissue arrangement and cell structure. Often such cells revert back to normal behavior, but occasionally they gradually become malignant. Because of their potential for becoming malignant, areas of dysplasia should be closely monitored by a health professional. Sometimes they need treatment.
Microscopic examination also provides information regarding the likely behavior of a tumor and its responsiveness to treatment. Cancers with highly abnormal cell appearance and large numbers of dividing cells tend to grow more quickly, spread to other organs more frequently, and be less responsive to therapy than cancers whose cells have a more normal appearance. Based on these differences in microscopic appearance, doctors assign a numerical "grade" to most cancers. In this grading system, a low number grade (grade I or II) refers to cancers with fewer cell abnormalities than those with higher numbers (grade III, IV).
After cancer has been diagnosed, doctors ask the following three questions to determine how far the disease has progressed:
- How large is the tumor, and how deeply has it invaded surrounding tissues?
- Have cancer cells spread to regional lymph nodes?
- Has the cancer spread (metastasized) to other regions of the body?
Based on the answers to these questions, the cancer is assigned a "stage." A patient's chances for survival are better when cancer is detected at a lower stage.
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Tobacco Use and Cancer
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Among the various factors that can cause cancer, tobacco smoking is the greatest public health hazard. Cigarette smoke contains more than two dozen different chemicals capable of causing cancer. Cigarette smoking is the main cause of lung cancer and contributes to many other kinds of cancer as well, including cancer of the mouth, larynx, esophagus, stomach, pancreas, kidney, and bladder. Current estimates suggest that smoking cigarettes is responsible for at least one out of every three cancer deaths, making it the largest single cause of death from cancer. Other forms of tobacco use also can cause cancer. For example, cigars, pipe smoke, and smokeless tobacco can cause cancers of the mouth.
Chemicals and radiation that are capable of triggering the development of cancer are called "carcinogens." Carcinogens act through a multistep process that initiates a series of genetic alterations ("mutations") and stimulates cells to proliferate. A prolonged period of time is usually required for these multiple steps. There can be a delay of several decades between exposure to a carcinogen and the onset of cancer. For example, young people exposed to carcinogens from smoking cigarettes generally do not develop cancer for 20 to 30 years. This period between exposure and onset of disease is the lag time.
In addition to chemicals and radiation, a few viruses also can trigger the development of cancer. In general, viruses are small infectious agents that cannot reproduce on their own, but instead enter into living cells and cause the infected cell to produce more copies of the virus. Like cells, viruses store their genetic instructions in large molecules called nucleic acids. In the case of cancer viruses, some of the viral genetic information carried in these nucleic acids is inserted into the chromosomes of the infected cell, and this causes the cell to become malignant.
Laboratory tests can determine whether a person carries some of the genetic alterations that can increase the risk of developing certain cancers. For example, women who inherit certain forms of a gene called BRCA1 or BRCA2 have an elevated risk of developing breast cancer. For women with a family history of breast cancer, taking such a test may relieve uncertainty about their future risk. However, the information obtained from genetic tests is often complex and difficult to interpret. The decision to undergo genetic testing should therefore be a personal, voluntary one and should only be made in conjunction with appropriate genetic counseling.
Genes reside within chromosomes, the large DNA molecules, which are composed of two chemical strands twisted around each other to form a "double helix." Each strand is constructed from millions of chemical building blocks called "bases." DNA contains only four different bases: adenine, thymine, cytosine, and guanine (abbreviated A, T, G, and C), but they can be arranged in any sequence. The sequential order of the bases in any given gene determines the message the gene contains, just as the letters of the alphabet can be combined in different ways to form distinct words and sentences.
Genes can be mutated in several different ways. The simplest type of mutation involves a change in a single base along the base sequence of a particular gene--much like a typographical error in a word that has been misspelled. In other cases, one or more bases may be added or deleted. And sometimes, large segments of a DNA molecule are accidentally repeated, deleted, or moved.
One group of genes implicated in the development of cancer are damaged genes, called "oncogenes." Oncogenes are genes whose PRESENCE in certain forms and/or overactivity can stimulate the development of cancer. When oncogenes arise in normal cells, they can contribute to the development of cancer by instructing cells to make proteins that stimulate excessive cell growth and division.
As the single largest cause of cancer death, the use of tobacco products is implicated in roughly one out of every three cancer deaths. Cigarette smoking is responsible for nearly all cases of lung cancer, and has also been implicated in cancer of the mouth, larynx, esophagus, stomach, pancreas, kidney, and bladder. Pipe smoke, cigars, and smokeless tobacco are risky as well. Avoiding tobacco is therefore the single most effective lifestyle decision any person can make in attempting to prevent cancer.
Slide 61
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Is There a Cancer "Epidemic"?
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A common misconception arises from news stories suggesting we are experiencing a cancer "epidemic." This only
appears to be the case because the
number of new cancer cases reported is rising as the population is both expanding and aging. Older people are more likely to develop cancer; however, this trend is offset by new births, which are also increasing, and cancer is rare among the young. So as more and more members of a 75-million-strong "baby-boomer" cohort begin shifting
en masse to older, more cancer-prone ages, the
number of new cancer cases is expected to increase in the next several decades. But since the birth rate is also expected to increase, the cancer
rate may either stay the same or, perhaps, decline.