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In the most basic terms, cancer refers to cells that grow out-of-control and invade other tissues. Cells may become cancerous due to the accumulation of defects, or mutations, in their DNA. Certain inherited genetic defects (for example, BRCA1 and BRCA2 mutations) and infections can increase the risk of cancer. Environmental factors (for example, air pollution) and poor lifestyle choices—such as smoking and heavy alcohol use—can also damage DNA and lead to cancer.
Most of the time, cells are able to detect and repair DNA damage. If a cell is severely damaged and cannot repair itself, it usually undergoes so-called programmed cell death or apoptosis. Cancer occurs when damaged cells grow, divide, and spread abnormally instead of self-destructing as they should.
A tumor is an abnormal mass of cells. Tumors can either be benign (non-cancerous) or malignant (cancerous).
Benign tumors grow locally and do not spread. As a result, benign tumors are not considered cancer. They can still be dangerous, especially if they press against vital organs like the brain.
Malignant tumors have the ability to spread and invade other tissues. This process, known as metastasis, is a key feature of cancer. There are many different types of malignancy based on where a cancer tumor originates.
Metastasis is the process whereby cancer cells break free from a malignant tumor and travel to and invade other tissues in the body. Cancer cells metastasize to other sites via the lymphatic system and the bloodstream. Cancer cells from the original—or primary—tumor can travel to other sites such as the lungs, bones, liver, brain, and other areas. These metastatic tumors are "secondary cancers" because they arise from the primary tumor.
Metastatic cancer retains the name of the primary cancer. For example, bladder cancer that metastasizes to the liver is not liver cancer. It is called metastatic bladder cancer. Metastasis is significant because it helps determine the staging and treatment. Some types of metastatic cancer are curable, but many are not.
Certain genes control the life cycle—the growth, function, division, and death—of a cell. When these genes are damaged, the balance between normal cell growth and death is lost. Cancer cells are caused by DNA damage and out-of-control cell growth. The following is a partial list of factors known to damage DNA and increase the risk of cancer:
Genetic mutations may cause cancer. For example, mutations of genes BRCA1 and BRCA2 (linked to an increased risk of breast and ovarian cancers) can inhibit the body's ability to safe-guard and repair DNA. Copies of these mutated genes can be passed on genetically to future generations, leading to a genetically-inherited increased risk of cancer.
Cancer may be caused by environmental exposure. Sunlight can cause cancer through ultraviolet radiation. So can air pollutants like soot, wood dust, asbestos, and arsenic, to name just a few.
Some microbes are known to increase cancer risks. These include bacteria like H. pylori, which causes stomach ulcers and has been linked to gastric cancer. Viral infections (including Epstein-Barr, HPV, and hepatitis B and C) have also been linked to cancer.
Lifestyle choices can lead to cancer as well. Eating a poor diet, inactivity, obesity, heavy alcohol use, tobacco use including smoking, and exposure to chemicals and toxins are all associated with greater cancer risk.
Medical treatment with chemotherapy, radiation, targeted treatments (drugs designed to target a specific type of cancer cell) or immunosuppressive drugs used to decrease the spread of cancer throughout the body can also cause damage to healthy cells. Some “second cancers”, completely separate from the initial cancer, have been known to occur following aggressive cancer treatments; however, researchers are producing drugs that cause less damage to healthy cells (for example, targeted therapy).
There are more than 100 different types of cancer. Every cancer and every individual is unique. Cancer symptoms and signs depend on the size and location of the cancer as well as the presence or absence of metastasis.
Symptoms and signs of cancer may include:
Other more obvious signs of cancer may include:
Cancer can occur anywhere in the body. Broadly, cancers are classified as either solid (for example breast, lung, or prostate cancers) or liquid (blood cancers). Cancer is further classified according to the tissue in which it arises.
Carcinomas are cancers that occur in epithelial tissues in the body. They comprise 80% to 90% of all cancers. Most breast, lung, colon, skin, and prostate cancers are carcinomas. This class includes the two most common skin cancers, basal cell carcinoma and squamous cell carcinoma. Also in this class is the glandular cancer adenocarcinoma.
Sarcomas occur in connective tissue like the bones, cartilage, fat, blood vessels, and muscles. This class of cancers includes the bone cancers osteosarcoma and Ewing sarcoma, Kaposi sarcoma (which causes skin lesions), and the muscle cancers rhabdomyosarcoma and leiomyosarcoma.
Myelomas are cancers that occur in plasma cells in the bone marrow. This class of cancer includes multiple myeloma, also known as Kahler disease.
Leukemias are a group of different blood cancers of the bone marrow. They cause large numbers of abnormal blood cells to enter the bloodstream.
Lymphomas are cancers of the immune system cells. These include the rare but serious Hodgkin lymphoma (Hodgkin's lymphoma, also Hodgkin's disease) and a large group of white blood cell cancers known collectively as non-Hodgkin lymphoma (non-Hodgkin's lymphoma).
Mixed cancers arise from more than one type of tissue.
Cancer is the second leading cause of death in the United States. The most common cancers diagnosed in the U.S. are those of the breast, prostate, lung, colon and rectum, and bladder. Cancers of the lung, colon and rectum, breast, and pancreas are responsible for the most deaths. The prognosis of different cancers is highly variable. Many cancers are curable with early detection and treatment. Cancers that are aggressive or diagnosed at a later stage may be more difficult to treat, and can even be life threatening.
Breast cancer is the most common cancer in the United States, and one of the deadliest. About one in eight women will develop invasive breast cancer at some point in her life. Though death rates have decreased since 1989, more than 40,000 U.S. women are thought to have died from breast cancer in 2015 alone.
Lung cancer is the second-most-common cancer in the United States, and it is the deadliest for both men and women. In 2012, more than 210,000 Americans were diagnosed with lung cancer, and in the same year more than 150,000 Americans died from lung cancer. Worldwide, lung cancers are the most common cancers.
Prostate cancer is the most common cancer found in men. In 2013, more than 177,000 Americans were diagnosed with prostate cancer, and more than 27,000 American men died from prostate cancer.
Of the cancers that can impact both men and women, colorectal cancer is the second-greatest killer in the United States.
Liver cancer develops in about 20,000 men and 8,000 women each year. Hepatitis B and C and heavy drinking increase one's risk of developing liver cancer.
About 20,000 American women are diagnosed with ovarian cancer each year. For American women, ovarian cancer is the eighth most common cancer and the fifth leading cause of cancer death.
Pancreatic cancer has the highest mortality rate of all major cancers. Of the roughly 53,000 Americans diagnosed with pancreatic cancer each year, only 8 percent will survive more than five years.
Doctors use the stages of cancer to classify cancer according to its size, location, and extent of spread. Staging helps doctors determine the prognosis and treatment for cancer. The TNM staging system classifies cancers according to:
Some cancers, including those of the brain, spinal cord, bone marrow (lymphoma), blood (leukemia), and female reproductive system, do not receive a TNM classification. Instead, these cancers are classified according to a different staging systems.
The TNM classification of a cancer usually correlates to one of the following five stages.
Various tests may be performed in order to confirm a cancer diagnosis. Positron Emission Tomography and Computed Tomography (PET-CT) Scans and other similar tests can highlight “hot spots” of cancer cells with high metabolic rates.
The most common test and procedures used to diagnose cancer include:
Cancer that originates in the lymph nodes or other area of the lymphatic system is called lymphoma. Cancer that originates elsewhere in the body can spread to lymph nodes. The presence of metastasized cancer in the lymph nodes is may mean the cancer is growing quickly and/or is more likely to spread to other sites. The presence of cancer in lymph nodes often affects prognosis and treatment decisions. Many diagnostic tests look at the lymph nodes as an indicator.
The treatment is highly variable depending on the type and stage of a cancer as well as the overall health of the patient. The most common treatments are surgery, radiation, and chemotherapy. Other treatments include targeted/biological therapies, hematopoietic stem cell transplants, angiogenesis inhibitors, cryosurgery, and photodynamic therapy.
Every treatment has potential risks, benefits, and side effects. The patient and his or her care team, which may include an internist or other specialist, surgeon, oncologist, radiation oncologist, and others, will help determine the best and most appropriate course of treatment.
Despite enormous effort and funding, no one cure has been found yet to eliminate cancer. In 2016, the United States announced a $1 billion investment into creating such a cure, named the “National Cancer Moonshot” by President Barack Obama.
Until a cure can be found, prevention through a healthy lifestyle is the best way to stop cancer. Some ways to help protect yourself from cancer include eating plenty of fruits and vegetables, maintaining a healthy weight, abstaining from tobacco, drinking only in moderation, exercising, avoiding sun damage, getting immunizations, and getting regular health screenings.
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