Cancer is characterized by uncontrolled growth of cells in the body and the ability of these malignant cells to spread (metastasize) to distant sites within the body. If the spread is not controlled, cancer can result in death. Cancer is not just one disease but a group of almost 100 different types of diseases.
Cancer kills one out of every four Americans. As of 2017, it was the second leading cause of death in the United States, surpassed only by heart disease. More than 1.7 million new cases of cancer are diagnosed every year in the United States (excluding basal and squamous cell skin cancers), and more than 1,500 Americans die every day from the disease. According to the American Cancer Society, one-third of the almost 600,000 deaths from cancer that occur are attributable to smoking and another 20% are due to lifestyle factors such as overweight or obesity, physical inactivity, and/or poor nutritional intake. The National Cancer Institute estimates that approximately 14.5 million Americans alive as of January 2014 had a history of cancer. Some of these people were cured, and others were still affected with the disease and possibly undergoing treatment.
Anyone is at risk for developing cancer. Since the occurrence of cancer increases as a person ages, most cases are seen in adults middle-aged or older. Nearly 86% of cancers are diagnosed in people 50 years of age and older.
Lifetime risk is the term used to refer to the probability that an individual will develop cancer over the course of a lifetime. In the United States, men have a one-in-two lifetime risk of developing cancer. For women, the risk is one in three. Overall, African Americans are more likely to develop cancer than Caucasians and are 33% more likely to die of cancer than Caucasians.
The major risk factors for cancer are tobacco, alcohol, diet, sexual and reproductive behavior, infectious agents, family history, occupation, environment, and pollution.
Cancer, by definition, is a disease of the genes. A gene is a small part of DNA, which is the instruction manual of the cell. Genes hold the instructions to make proteins, which carry out many of the body's functions. These proteins allow the human body to carry out all the basic processes that support life such as enabling people to breathe, think, and move.
Tumors are divided into two general categories: benign or malignant. A benign tumor is slow growing and does not spread or invade surrounding tissue. Once it is removed, it does not usually recur. A malignant tumor, by contrast, invades surrounding tissue and can spread to other parts of the body. Malignant tumors can be removed, but if the cancer cells have spread to the surrounding tissues, the tumor is likely to recur.
Some cancers are caused by changes in the cell's DNA that are due to the environment. Environmental factors that are responsible for causing the initial mutation in the DNA are called carcinogens. Internal factors can cause cancer as well. Certain hormones have been shown to have an effect on the growth or control of a particular cell line. Hormones are substances made by one organ and passed through the bloodstream to perform a function in another organ.
Although there is scientific evidence that both environmental and genetic factors play a role in most cancers, approximately 5%–10% of all cancers are classified as hereditary (genetic), which means a faulty gene that leads to a cancer is passed from parent to child. Having such a genetic factor poses a greater risk for that particular type of cancer in certain descendants of the family. Having a cancer-causing gene does not necessarily mean that a person will automatically get cancer. Rather, it means that person is predisposed to a type of cancer or more likely to get this cancer when compared to the general population. Cancers known to have a hereditary tendency in some cases include breast cancer, colon cancer, ovarian cancer, skin cancer, and prostate cancer.
Aside from genes, certain inherited physiological traits can contribute to cancers. For example, inheriting fair skin makes individuals more likely to develop skin cancer, but only if they also have prolonged exposure to sunlight.
There are many different types of cancers. Some of the most common types are:
The most common cancers are skin cancer, lung cancer, colon and rectal (colorectal) cancer, breast cancer (in women), and prostate cancer (in men). In addition, cancer of the kidneys, ovaries, uterus, pancreas, bladder, and blood and lymph-node cancer (leukemias and lymphomas) are included among the 12 major cancers that affect most Americans.
Almost every tissue can give rise to cells that cause cancer, and each of these cancers has its own symptoms and prognosis. There are, however, basic and similar genetic processes that lead to tumor growth in the human body. Genes are responsible for producing proteins that regulate cell growth and division. When these genes do not function properly, the proteins are abnormal and cells can grow uncontrollably. This action results in the formation of a tumor. As more genetic mutations (changes) occur in this tumor, it becomes more life-threatening and has a greater chance of spreading to other parts of the body.
Three classes of genes appear to play a role in the development of cancer:
Some cancers have a hereditary component in which a child does not inherit cancer from the parents.
Rather, a predisposition to cancer is inherited. For example, a faulty tumor suppressor gene may be inherited. This gene is not able to control cell growth, but the corresponding gene inherited from the other parent is still functional. Cell growth is under control. As a child grows up, though, radiation, pollution, or any other environmental factor could change the functional gene, making it defective as well. Now, neither of these tumor suppressor genes is functioning, and it is likely that a tumor will develop. Defects in proto-oncogenes and DNA repair genes can be inherited, as well, leaving a person more vulnerable to cancer than the general population.
Some cancers seem to run in families. In these cancers, no specific gene is responsible for the clustering of cancer in a family; however, a particular type of cancer may be seen more often than in the general population. It is suggested that this is due to a combination of genetic and shared environmental factors.
Eighty-to-ninety percent of lung cancer cases occur in smokers. Smoking has also been shown to be a contributory factor in cancers of the mouth, pharynx, larynx, esophagus, pancreas, uterine cervix, kidney, bladder, as well as other types of cancer, including a type of acute leukemia. Smoking accounts for at least 30% of all cancer deaths. Scientists have shown that inhaling secondhand smoke (passive smoking) can increase one's risk of developing cancer.
Excessive consumption of alcohol is a risk factor in certain cancers such as liver and breast cancer. Alcohol, in combination with tobacco, significantly increases the chances that an individual will develop mouth, pharynx, larynx, and esophageal cancers. The combined effect of tobacco and alcohol is greater than the sum of their individual effects. This is called synergy.
The human papilloma virus, a sexually transmitted disease, has been shown to cause cancer of the cervix. Having many sexual partners and becoming sexually active early has been shown to increase one's chances of contracting this disease and, therefore, developing cervical cancer. In addition, it has also been shown that women who do not have children or those who have children late in life have an increased risk for both ovarian and breast cancer.
Certain cancers tend to occur more commonly among members of a family. Much of the time this seems to happen by chance or is due to a common family habit such as cigarette smoking or extended sun exposure. Certain cancers can occur in excess in some families due to a genetic predisposition that is passed from generation to generation. For example, if the BRCA1 gene is defective in a family, members of that family may have an increased risk to develop breast, colon, ovarian, or prostate cancer. Other defective genes can make persons susceptible to other types of cancer. Therefore, inheriting particular genes can increase a person's chance of developing cancer.
Ample evidence has shown that occupational hazards account for 4% of all cancer deaths. For example, asbestos workers have an increased incidence of lung cancer. Similarly, bladder cancer is associated with dye, rubber, and gas workers; skin and lung cancer with people who are smelters, gold miners, and arsenic workers; leukemia is seen more frequently in people who work with glue and varnish; liver cancer is more prevalent in PVC manufacturers; and lung, bone, and bone marrow cancer is associated with radiologists and uranium miners.
High-frequency radiation has been shown to cause human cancer. Ultraviolet radiation from the sun accounts for a majority of melanoma cases. Other sources of radiation are x-rays, radioactive substances, and rays that enter the earth's atmosphere from outer space. Virtually any part of the body can be affected by these types of radiation, especially bone marrow and the thyroid gland.
Additionally, being exposed to substances such as certain chemicals, metals, or pesticides, can increase the risk of cancer. Asbestos is an example of a well-known carcinogen, increasing the risk for lung cancer. This risk is increased even further for a smoker who is exposed to asbestos over a period of time.
Cancer is a progressive disease and goes through several stages. Each stage can produce a number of symptoms. Unfortunately, many types of cancer do not display any obvious symptoms or cause pain until the disease has progressed to an advanced stage. Early signs of cancer are often subtle and are easily mistaken for signs of other less-dangerous diseases.
Despite the fact that several hundred different types of cancers produce very different symptoms, the American Cancer Society (ACS) has established seven symptoms as possible warning signals of cancer:
Other diseases can produce similar symptoms, but it is important to have these symptoms checked as soon as possible, especially if they linger. The earlier a cancer is diagnosed and treated, the better the chance of it being cured. Many cancers, for example breast cancer and prostate cancer, may not have any early symptoms. Therefore, it is important to undergo routine recommended cancer screening tests.
If a person has symptoms of cancer, a physician will begin with a complete medical history and a thorough physical examination. The doctor will examine different parts of the body to identify any variations from the normal size, feel, and texture of the organ or tissue. In addition to the physical exam, the doctor may order various tests.
Laboratory tests on blood and urine are often used to obtain information about a person's health. If cancer is suspected, a special test can be done that measures the amount of certain substances, called tumor markers, in the blood, urine, or particular tissues. These proteins are released from some types of cancer cells. Thus, the levels of these substances may be abnormal when certain cancers are present. Laboratory tests alone cannot be used to make a definitive diagnosis of cancer. Blood tests are generally more useful in monitoring the effectiveness of the treatment or in following the course of the disease and detecting any signs of recurrence.
A doctor may look for tumors by examining images of areas inside the body. The most common way to obtain these images is by using x-rays. Other techniques used to examine the internal organs and tissues include computed tomography (CT scan), magnetic resonance imaging (MRI), and ultrasonography.
The most definitive diagnostic test for most cancers is a biopsy. In this technique a piece of tissue is surgically removed for examination under a microscope. A biopsy provides information about the cellular nature of an abnormality: the stage it has reached, the aggressiveness of the cancer, and the extent of its spread. Further analysis of the tissue obtained by biopsy defines the cause of the abnormality. Because a biopsy provides the most accurate analysis, it is considered the gold standard of diagnostic tests for cancer.
Regular screening examinations conducted by healthcare professionals can result in the early detection of various types of cancer. Early detection means treatment is more likely to succeed. For example, the American Cancer Society recommends an annual mammogram (x-ray of the breast) for women 45 to 54 years, to screen for breast cancer. It also recommends a sigmoidoscopy, in which a thin, lighted tube with a tiny camera is used to view the inside of the colon, every five years, or a colonoscopy every ten years for people over the age of 50. This technique can assess the presence of colorectal cancer. Self-examinations for cancers of the breast, testes, mouth, and skin may also help in detecting tumors before the symptoms become serious.
Evolutions in molecular biology and the genetics of cancer have led to the development of several tests designed to assess one's risk of getting certain types of cancer. Genetic testing involves looking closely at certain genes that have been linked to particular cancers, including cancers of the breast, ovary, and colon.
The aim of cancer treatment is to remove all or as much of the tumor as possible and to prevent the recurrence or spread of the primary tumor. While devising a treatment plan for cancer, the likelihood of curing the cancer must be weighed against the side effects of the treatment. If the cancer is aggressive and a cure is not possible, then treatment should be aimed at relieving the symptoms and controlling the cancer for as long as possible.
Surgery can be used for many purposes:
Radiation uses high-energy rays to kill cancer cells. This technique may be used instead of surgery. It also may be used before surgery to shrink a tumor or after surgery to destroy any remaining cancer cells.
Radiation can be either external or internal. In the external form, the radiation comes from a machine that aims the rays at the tumor. In internal radiation (also known as brachytherapy), radioactive material is sealed in needles, seeds, or wires and placed directly in or near the tumor. Radiation may lead to various side effects, such as fatigue, hair loss, and a susceptibility to infections. Usually, these side effects can be controlled.
Chemotherapy is the use of drugs to kill cancer cells. The entire body is exposed to the drugs (systemic therapy) in an effort to destroy the hard-to-detect cancer cells that have spread and are circulating in the body. The cancer cells are affected more dramatically than normal cells because they are rapidly dividing. Chemotherapeutic drugs can be injected into a vein, the muscle, or the skin, or they may be taken by mouth.
When chemotherapy is used before surgery, it is known as primary, or neoadjuvant chemotherapy. Its purpose is usually to reduce the size of the tumor. The more common use of chemotherapy is in adjuvant therapy. In this technique, chemotherapy is given after surgery to destroy any remaining cancer cells and to help prevent cancer from recurring. Chemotherapy can also be used in conjunction with radiation.
Side effects of chemotherapy vary according to the drugs administered but can include susceptibility to infections, fatigue, poor appetite, weight loss, nausea, vomiting, diarrhea, and hair loss. Decreased fertility can be a long-term side effect in some instances.
Bone marrow failure is a complication of chemotherapy. When high-dose chemotherapy is used, bone marrow failure is anticipated. Bone marrow transplantation (BMT) or peripheral stem cell transplantation (PSCT) are techniques used to treat this complication. Both techniques provide healthy stem cells for an affected person. Stem cells are immature cells that mature into blood cells. Transplanted stem cells replace the patient's stem cells that have been damaged or destroyed by chemotherapy or radiation. This procedure allows an individual to undergo aggressive treatment for cancer. Those who receive BMT or PSCT have an increased risk of infection, bleeding, and other side effects due to the chemotherapy and radiation. Graft-versus-host disease may also occur. This complication develops when the donated marrow reacts against the recipient's tissues. It can occur any time after the transplant. Drugs may be given to reduce the risk of graft-versus-host disease and to treat the problem if it occurs.
The development of targeted therapies used in the treatment of cancer has experienced tremendous growth. Targeted therapies are drugs or other substances that are designed specifically to work at the cellular level by blocking the actions of specific molecules in or on the cancer cell. Many targeted therapies focus on blocking the actions of specific proteins that signal cancer cells to proliferate and grow. Examples of targeted therapies are monoclonal antibodies and small-molecule drugs. Because the action of the drug or substance is targeted to a specific site in or on the cancer cell, normal cells in the same area are less likely to be harmed, thereby potentially reducing side effects. In addition, the action of the targeted therapy agent is more likely to be effective in damaging or killing the cancer cell.
Hormone therapy is used to fight certain cancers that depend on hormones for their growth. Drugs can be used to block the production of hormones or change the way they work. Additionally, organs that produce hormones may be removed. As a result of this therapy, the growth of the tumor slows, and survival may be extended for several months or years. Hormone therapy may be used in treating some breast and prostate cancers.
Alternative therapies have not been scientifically tested and approved. When they are used along with standard therapy, they are known as complementary therapy. Alternative therapy is considered dangerous because some of these unproven treatments might have life-threatening side effects. Additionally, persons who use alternative therapy may lose the opportunity to benefit from standard, proven therapy. Some complementary therapies, however, may help to relieve symptoms of cancer, decrease the magnitude of side effects from treatment, or improve a patient's sense of wellbeing. The ACS recommends that anyone considering alternative or complementary therapy consult a healthcare team before doing so.
Hospice care provides the environment in which palliative care is given to terminally ill patients. Hospice care can be provided either at home or in a homelike facility called a hospice. Hospice care focuses on providing the best possible palliative care for the patient until the patient dies.
One-third of all cancer deaths are due to a poor adult diet. High-fat diets have been associated with cancers of the colon and rectum, prostate, endometrium, and possibly breast. Consumption of meat, especially red meat, has been associated with increased cancer at various sites such as the colon and prostate. Additionally, a high-calorie diet and low level of physical activity (high sedentary behaviour) can lead to obesity, which increases the risk for cancer at various sites, including the breast, colon and rectum, prostate, kidney, and endometrium. Getting exercise, eating fruits and vegetables, and reducing salt intake all seem to be beneficial in avoiding cancer.
Most cancers are curable if detected and treated in their early stages. The prognosis for a person with cancer is affected by many factors, particularly the type and stage of the cancer, the extent to which it has metastasized, and its aggressiveness. In addition, a person's age, general health status, and effectiveness of the treatment being pursued are important factors.
To help predict the outcome of cancer and the likelihood of recovery from the disease, five-year survival rates are used. In the United States, as of 2016, the five-year survival rate for all cancers combined was 69% for people diagnosed between 2005 and 2011. This means that almost 70% of people with cancer are expected to be alive at least five years after they are diagnosed. These people may be free of cancer, or they may be undergoing treatment. This is an increase of almost 50% as compared to the five-year survival rate for people who were diagnosed with cancer between 1975 and 1977.
It is important to note that, although this statistic can give some information about the average survival of people with cancer in a given population, it cannot be used to predict the course of cancer for an individual. No two people are exactly alike, and the survival rate can vary dramatically depending on cancer type and stage at diagnosis. The five-year survival rate does not account for differences in detection methods, types of treatments, additional illnesses, and personal behavior of the individual.
According to experts from leading universities in the United States, a person can reduce the chances of getting cancer by following these guidelines:
See also Adult nutrition ; Alcohol consumption ; Antioxidants ; Cancer diet ; Cancer-fighting foods ; Omega-3 and omega-6 fatty acids ; Prebiotics and probiotics ; Vitamins .
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L. Fleming Fallon, Jr., MD, DrPH
Revised by L. Lee Culvert