What Is Cancer?

Cancer is a disease that occurs when malignant cells grow in the body. Cancerous cells cluster together to form a mass called a tumor and can spread throughout the body to other, more distant areas.

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Jump to: Causes | Symptoms | Types of cancer | Stages | Treatment | Can cancer be cured? | Chances of getting cancer | Occurence

What is cancer?

Cancer is a disease that occurs when malignant (or dangerous) cells grow in the body. These cells can form almost anywhere, including the brain, lungs, pancreas, and more. Cancerous cells cluster together to form a mass called a tumor and can spread throughout the body to other, more distant areas. Although some cancers can be fatal, others can be successfully treated with procedures like surgery and chemotherapy.

What causes cancer?

It’s not always possible to identify the exact reason why someone developed cancer. However, there are certain risk factors that can increase a person’s chance of developing the disease. Generally speaking, these can either be hereditary or environmental—i.e., cancer may either “run in the family” or can be caused by exposure to sunlight, radiation, or tobacco smoke. There are some cancer risk factors that people have some control over (avoiding cigarette smoke, for example) and others that they don’t (like age). Here are some of the factors thought to cause cancer.


Cancer is caused by changes that occur in a person’s genes. First, some background: Humans have an estimated 20,000 to 25,000 genes, all of which are made up of DNA. Think of DNA as a kind of blueprint. It’s the code that tells your genes how to make proteins, the molecules that maintain and support the organs and tissue in the body.

If a person’s DNA “mutates” or changes—a result of, say, the harmful chemicals in tobacco smoke or UV rays from the sun—the information in the gene becomes rearranged or deleted. Called DNA mutations, these errors can then cause the growth of cancerous cells, which multiply throughout the body.

But a person can also be born with genetic mutations. The National Cancer Institute (NCI) estimates that about 5% to 10% of all cancers are caused by gene mutations that were inherited from the person’s mother or father. People who have inherited mutations in the BRCA1 and BRCA2 genes, for example, are more likely to develop breast and ovarian cancer than those whose DNA does not contain these mutations. That’s why some people may want to be tested for these inherited gene mutations, particularly if a certain type of cancer runs in the family.


Although people can develop cancer at any age, 87% of all cancers in the United States are diagnosed in people who are at least 50 years old, according to the American Cancer Society (ACS). The median age at which breast cancer is diagnosed is age 61; for prostate cancer, it’s age 66; for colorectal cancer, it’s age 68; and for lung cancer, it’s age 70, according to the NCI.


The energy from the sun is called ultraviolet or UV radiation, and it reaches Earth in two forms: in UVA and UVB rays. Both can damage the DNA in a person’s skin cells and is a major risk factor for skin cancer. Sunlamps and tanning beds are other sources of UV rays.


X-rays and gamma rays are two other types of radiation–both of which are found naturally and in man-made devices like imaging tests, scanners, and certain power plants–that can cause DNA mutations, which may lead to cancer in the future.


Tobacco smoke contains at least 69 cancer-causing chemicals, including arsenic and formaldehyde. Not only is smoking the leading cause of lung cancer—about 80% to 90% of deaths from this disease are linked to smoking, according to the American Lung Association—but it’s also linked to cancers of the lung, esophagus, larynx, mouth, throat, kidney, liver, pancreas, stomach, and more.

Other carcinogens

Like tobacco smoke, radiation, and UV rays, other chemicals, like asbestos and soot, can cause mutations in a person’s DNA. Those DNA mutations can eventually trigger the growth of cancerous cells. Where you live and what you do for work may contribute to your exposure to certain carcinogens.

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General signs and symptoms of cancer

Cancer can cause almost any type of symptom—everything from fatigue to pain to shortness of breath and more. In some cases, a person with cancer will notice symptoms during the early stages of the disease, but in other instances, the cancer can go unnoticed until the tumor has either grown in size (putting pressure on an organ, for example) or spread to other areas in the body. Doctors, too, can spot some of the warning signs of cancer: They may notice a lump or lesion on a patient’s body or uncover an abnormal mass of cells on a routine imaging test. Signs and symptoms of cancer include:

A lump

Some cancers can be felt underneath the skin, especially tumors that start in the breast. If you’re wondering what a cancer lump feels like, know that most of the time lumps are not cancer—in fact, normal breast tissue can feel lumpy too.

Breast cancer lumps can feel as if the tissue in or near your breast (or under your arm) is thick or firm. The NCI says that if a person notices these signs, they should check the other breast to see whether it yields a similar feel. If both breasts feel the same, the lumps may be normal. However, if you notice a change in your breast, talk to your doctor.

Bleeding or discharge

Bleeding can occur in both the early stages of cancer or the later stages. Depending on the type of cancer, people can notice blood in their stool (a possible sign of colorectal cancer), their urine (a symptom of bladder or kidney cancer) or in the mucus that accompanies a cough (a sign of lung cancer). Abnormal discharge from the nipple may signal breast cancer.

Unusual bowel or bladder habits

Colon cancer can trigger symptoms like rectal bleeding, blood in the stool, cramping in the lower abdomen, or pain when passing urine. Painful urination or other changes in bladder function may also be signs of bladder or prostate cancer.

A lingering cough or hoarseness

One common sign of lung cancer is a cough that won’t go away or continues to get worse. Lung, larynx (voice box), and thyroid cancer can also cause changes to a person’s voice, making it seem raspy.

Skin changes

A mole or skin lesion that has changed size, shape, or color could be a sign of skin cancer. Basal cell carcinoma may appear as a red or pink growth, while squamous cell carcinoma can have a rough surface. The warning signs of melanoma—a particularly dangerous form of skin cancer—include a sore that doesn’t heal, is painful, oozes, or bleeds.

Other cancers can also cause changes to the skin. Jaundice, or the yellowing of the eyes and skin, can be a sign of pancreatic cancer, and excessive hair growth can be a sign of adrenal cancer.

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Types of cancer

Cancer can develop in almost any area of the body. In most cases, the cancer is named after the organs or tissues in which it first develops—for example, breast cancer refers to the growth of cancerous cells in the breast tissue, whereas prostate cancer refers to the growth of cancerous cells in the prostate gland.

More than 852,000 women are estimated to develop cancer each year, according to the ACS, and about half of them will be diagnosed with either breast, colorectal, or lung and bronchus cancer. Although fewer men will develop cancer—more than 836,000 are diagnosed yearly, most with either prostate or lung and bronchus cancer—their diseases tend to be more fatal. An estimated 318,420 men will die of cancer yearly compared to 282,500 women. In both males and females, the deadliest form of cancer is lung cancer.

There are more than 100 types of cancers, some of which—like lip, tongue, and gallbladder cancer—are rare. The most commonly diagnosed types of cancer include:

  • Bladder cancer
  • Breast cancer
  • Colon and rectal cancer
  • Endometrial cancer
  • Kidney cancer
  • Leukemia
  • Liver cancer
  • Lung cancer
  • Melanoma
  • Non-Hodgkin lymphoma
  • Nonmelanoma skin cancer
  • Pancreatic cancer
  • Prostate cancer
  • Thyroid cancer

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Stages of cancer

After a person is diagnosed with cancer, doctors will assign the disease a “stage.” This process (called “staging”) helps doctors quantify how much cancer is in the body and determine which type of treatment a person should receive.

There are five stages of cancer: stage 0 (or, carcinoma in situ), stage 1, stage 2, stage 3, and stage 4. Lower stages indicate that the disease is more localized, or contained, whereas higher stages refer to cancers that have spread into other areas of the body. As a general rule, early-stage cancers are more likely to be successfully treated than later-stage cancers.

The most common method of staging cancers is the TNM system, developed by the American Joint Committee on Cancer. The Tdenotes information about the tumoritself, including the size or whether it has invaded any nearby tissue. The N specifies whether the cancer has spread into the lymph nodes(structures in the body that contain immune cells) and how many lymph nodes are affected. Lastly, the M refers to how far the cancer has metastasized (or spread) to distant areas of the body. Each letter is followed by a number, which further describes how far the cancer has spread or grown. For example, a person with stage 1 colorectal cancer may be assigned a grade of T1, N0, M0, meaning that the tumor may have grown into one of the muscle layers in the gastrointestinal tract but hasn’t spread to nearby lymph nodes or other, more distant areas of the body. Doctors will take all of this information into account and classify the cancer as stage 1, 2, 3, or 4.

One important note: The stage of a person’s cancer does not change, even if the tumor shrinks or the disease has metastasized. Doctors will always refer to the cancer as the stage in which it was first diagnosed and will describe any further changes to the disease by changing the numbers in the TNM system.

Stage 4 cancer

Also known as metastatic cancer, this type of cancer has spread to distant organs and lymph nodes in the body. One example: In stage 4 breast cancer, the tumor may have spread from the breast to the bones, brain, liver, or lungs. Common treatments for stage 4 cancers include chemotherapy, radiation therapy, and surgery. While different types of cancers have different survival rates, in general, it can be challenging to treat the disease if it was detected at this late stage.

Stage 3 cancer

At stage 3, cancer may have spread to the lymph nodes, but it hasn’t metastasized to more distant areas of the body. In women with stage 3 breast cancer, for example, the cancer might have invaded the chest wall and reached the nearby lymph nodes, but it hasn’t spread to other areas of the body, like the brain or bones.

Stage 2 cancer

Broadly speaking, stage 2 cancers may have penetrated the walls of the surrounding muscle tissue and infiltrated a small number of very nearby lymph nodes, but they haven’t reached more distant lymph nodes or other areas of the body. Doctors may refer to some stage 2 cancers as “localized” cancer, in which the cancerous cells are only found in the tissue or organ where the disease began. In women with stage 2 breast cancer, for example, the tumor may be less than five centimeters in length but it hasn’t reached any lymph nodes or other parts of the body.

Stage 1 cancer

Often referred to as early-stage cancer, stage 1 cancers haven’t spread beyond the area of the body in which they were first detected. For women with stage 1 breast cancer, the tumor hasn’t spread out of the breast, although it might have spread to the close-by lymph nodes near the armpit. In general, it’s easier to treat earlier-stage cancers than the tumors that are more advanced; options can include surgery and chemotherapy, among others.

Stage 0 cancer

Also known as carcinoma in situ, stage 0 cancer is defined as a group of abnormal cells that hasn’t spread to other surrounding areas of the body. Stage 0 cells are sometimes called pre-cancerous cells. These cells may or may not become cancerous in the future; they can be removed early with treatments like surgery or radiation therapy.

For example, women with stage 0 breast cancer may have ductal carcinoma in situ or DCIS, in which abnormal cells have developed in the lining of a breast duct. In this case, the cells have not spread to the surrounding breast tissue but may do so at a later time.

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Cancer treatment

Doctors will determine which treatment a person should receive based on the type and stage of the cancer. Some people may only need one treatment, whereas others may need multiple forms of therapy.

Surgery is one of the most common types of cancer treatments and is often performed on localized tumors that haven’t spread to other areas of the body. The surgery can be “open”—meaning, the doctor will make a large cut to remove the tumor, surrounding healthy tissue, and nearby lymph nodes all at once—or “minimally invasive,” in which a surgeon can use special tools to remove the growths by making a few smaller cuts.

Radiation therapy is another type of cancer treatment that’s used to kill or shrink cancer cells. Radiation may be used by itself or in combination with surgery or chemotherapy. Because radiation therapy can also damage nearby healthy cells, many people experience side effects like fatigue, hair loss, nausea, and more.

Doctors can also kill cancerous cells with drugs. Known as chemotherapy (or, colloquially, chemo), this treatment can be given in a number of ways, including intravenously, topically, or orally, in the form of pills or liquids. Like radiation therapy, this treatment can also cause side effects like nausea and hair loss.

Other cancer treatment options include hormone therapy (used to treat some prostate and breast cancers) and immunotherapy (which helps bolster a person’s immune system so they can better fight the disease). Lastly, some people may be eligible to join clinical trials, or studies in which experts are conducting cancer research and testing new treatments.

Although there is currently no cure for cancer, per se, these treatments can help put some cancers into remission and possibly eradicate them for good.

Is cancer ever really “cured”?

In general, doctors can’t say for sure that a person’s cancer is cured. That’s because there’s no guarantee that the disease will never return.

However, most cancers that do return will come back within five years. Therefore, if a person’s cancer has remained in complete remission—meaning, there have been no signs and symptoms of cancer—for longer than that time period, the cancer may never return.

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Chances of getting cancer

It’s not always clear why some people develop cancer and others do not. Although the odds of getting certain cancers can be higher in certain populations—for example, those who smoke are approximately 20 times more likely to develop lung cancer as those who don’t smoke—the ACS estimates that the average woman has a one in three chance of developing cancer and a one in five chance of dying from cancer.

About one out of every eight women will develop breast cancer, and one in 37 may die from it. Likewise, one in every 17 women will develop lung (or bronchus) cancer, and one in 20 may die of the disease. The odds of a woman getting colorectal cancer are one in 23, whereas the odds of dying from it are one in 55.

Roughly one in two men will develop cancer, and one in four will die from the disease. About one in seven men will develop prostate cancer, which may be fatal in about one out of every 39 men. Lung cancer will develop in about one in 14 men, claiming the lives of one in 16. Lastly, about one in 21 men will be diagnosed with colorectal cancer, and one in 50 are estimated to die of it.

How many people die from cancer each year?

Cancer is the second leading cause of death in the United States, claiming the lives of nearly one in four Americans. (Heart disease is currently the most fatal condition.) The ACS estimates that about 600,920 Americans die from cancer every year. That’s almost 1,650 per day.

Throughout most of the 20th century, the rate of cancer deaths continued to climb, largely due to the popularity of smoking. The good news: Thanks to new treatments, advancements in early detection and screening, and the flurry of anti-smoking campaigns, the number of deaths due to cancer have been declining since they peaked in 1991. Then, the disease claimed the lives of about one in 465 people; by 2014, that number had fallen to one out of every 621 people.

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