What is cancer?

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on August 18, 2022.

Cancer is the uncontrolled growth of abnormal cells in the body. Cancer develops when the body’s normal control mechanism stops working. Old cells do not die and instead grow out of control, forming new, abnormal cells. These extra cells may form a mass of tissue, called a tumor. Some cancers, such as leukemia, do not form tumors.

How common is cancer?

For decades, the overall rate of new cases in the United States has been trending slowly downward, but that doesn’t mean that cancer is now uncommon. In fact, according to the National Cancer Institute, about 39.5 percent of all men and women in the United States will be diagnosed with cancer at some point in their lives.

Some cancers are more common than others, with the following cited as the most prevalent nationwide currently:

  • Breast cancer
  • Lung cancer
  • Prostate cancer

While the rate of most cancers is trending downward, the rate of certain types, such as melanoma, has actually increased in the past few decades.

What are the most common forms of cancer?

Cancer may occur anywhere in the body. In women, breast cancer is one of the most common. In men, it’s prostate cancer. Lung cancer and colorectal cancer affect both men and women in high numbers.

There are five main categories of cancer:

  • Carcinomas begin in the skin or tissues that line the internal organs.
  • Sarcomas develop in the bone, cartilage, fat, muscle or other connective tissues.
  • Leukemia begins in the blood and bone marrow.
  • Lymphomas start in the immune system.
  • Central nervous system cancers develop in the brain and spinal cord.

How do you get cancer?

Experts don’t yet have all the answers about what causes cancer. There are, however, certain characteristics—called risk factors—that may increase your likelihood of developing cancer.

You can control some risk factors (smoking, for example), but not others (like your race or age). Moreover, some cancers can be caused by factors in your environment, such as exposure to the ultraviolet (UV) radiation in sunlight. And a small number may be linked to an initial infection with a virus or bacteria.

Beyond those risk factors, some cancers are also inherited, meaning they are passed down through your family. According to the American Cancer Society, about 5 percent to 10 percent of all cancers are inherited from gene defects (mutations).

In some cases, researchers have figured out the exact genes that may cause cancer, and they can sometimes provide genetic counseling to help you understand your risk. But other cancers are caused by mutations that happen to an individual over time.

An important way for you to possibly detect cancer early, when it’s easier to treat, is to undergo a cancer screening. Through screenings, doctors check people who don’t necessarily have any cancer symptoms. Depending on your age, gender or other risk factors, your care team may recommend you get checked for certain cancers.

How do you know whether you’re a good candidate for a cancer screening? You may need to get screened for certain types of cancer if you:

  • Have had cancer before
  • Have a family member who’s been diagnosed with cancer
  • Have used tobacco or worked with certain chemicals
  • Have a certain gene mutation linked to cancer
  • Have had a blood clot develop without a known reason
  • Are older

What is a cancer stage, and what does it mean?

Your doctor will likely describe your cancer by referencing its stage. Your cancer’s stage can tell you and your care team a lot about your disease, including:

  • Severity of the cancer
  • Appropriate treatments, including any clinical trial options
  • Likelihood of recovery after treatment
  • Chances of the cancer returning (recurrence)

Your care team may need to conduct certain tests—such as a computed tomography (CT) scan or a biopsy—to determine your cancer stage.

Not all cancers are classified with the same staging system. However, the one most commonly used includes the following stages:

  • Stage 0: Cancer cells remain in the same place where they started. This may also be called cancer in situ, meaning that it hasn’t grown or spread.
  • Stage 1: Cancer hasn’t grown into nearby tissues or lymph nodes.
  • Stage 2: Cancer has grown into nearby tissues and possibly nearby lymph nodes.
  • Stage 3: Cancer has grown deeper into nearby tissues and may have spread to nearby lymph nodes, but not to distant parts of the body.
  • Stage 4: Cancer has spread to other parts of the body, or other organs. This is also called metastatic, or advanced, cancer.

Some cancers may also be described using a grade, which describes how different cancerous cells look under a microscope compared with healthy cells. Cells in a low-grade tumor don’t look much different from healthy cells, while cells in a high-grade tumor look very different and may grow and spread more quickly.

How is cancer treated?

Treatment options depend on the type of cancer, its stage, if the cancer has spread and your general health. The goal of treatment is to kill as many cancerous cells while reducing damage to normal cells nearby. Advances in technology make this possible.

The three main treatments are:

The same cancer type in one individual is very different from that cancer in another individual. Within a single type of cancer, such as breast cancer, researchers are discovering subtypes that each requires a different treatment approach.

What is oncology?

The branch of medicine dedicated to diagnosing, treating and researching cancer is known as oncology, while a physician who works in the field is called an oncologist. Some oncologists focus solely on particular cancer types or treatments. Depending on the type, stage and location of a cancer, multiple oncology specialists may be involved in a patient’s care. The field of oncology has three main specialties—medical, surgical and radiation—and numerous sub-specialties.

A medical oncologist is a licensed physician (typically in internal medicine) trained in diagnosing, staging and treating cancer. This specialist also leads the development of the cancer patient’s treatment plan, which may include surgery, chemotherapy, immunotherapy, targeted therapy or hormone therapy, while also coordinating with other oncology specialists and clinicians who may have a role in the patient’s care. A medical oncologist is also the doctor a cancer patient will continue to see after treatment, for checkups over the long-term.

A surgical oncologist is a surgeon who specializes in performing biopsies and removing cancerous tumors and surrounding tissue, as well as other cancer-related operations.

A radiation oncologist specializes in treating cancer with radiation therapy to shrink or destroy cancer cells or to ease cancer-related symptoms.

Many cancer types are treated by an oncology sub-specialty. Gynecologic oncologists, for example, are trained to treat cancers of the female reproductive system such as those affecting the uterus, cervix, or ovaries, while hematologic oncologists specialize in diagnosing and treating blood cancers (leukemia, lymphoma and multiple myeloma). A neuro-oncologist treats cancers of the brain, spine and peripheral nerves.

What can you do to manage the side effects of cancer treatment?

Supportive care services describe a broad range of therapies designed to combat side effects and maintain well-being. Treating cancer requires focusing on more than the disease alone; it must also address the pain, fatigue, depression and other side effects that come with it.

Supportive care services include:

What does the future hold for cancer treatment?

The future of cancer treatment lies in providing patients with an even greater level of personalization. Doctors are beginning to offer treatment options based on the genetic changes occurring in a specific tumor.

An innovative new diagnostic tool, the genomic tumor assessment, examines a patient’s tumor genetically to identify the mechanism that caused the cancer. Genomic tumor assessment may result in a more personalized approach to cancer treatment.

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