What Is Cancer?

Cancer can develop anywhere in the body. It starts when cells grow out of control and crowd out normal cells. This makes it hard for your body to work the way it should.

For many people, cancer can be treated successfully. In fact, more people than ever before lead full lives after cancer treatment.

Cancer is more than just one disease

There are many types of cancer. It’s not just one disease. Cancer can start in the lungs, the breast, the colon, or even in the blood. Cancers are alike in some ways, but they are different in the ways they grow and spread.

How are cancers alike?

The cells in our bodies all have certain jobs to do. Normal cells divide in an orderly way. They die when they are worn out or damaged, and new cells take their place. In cancer, the cells keep on growing and making new cells. They crowd out normal cells. This causes problems in the part of the body where the cancer started. They can also spread to other parts of the body.

How are cancers different?

Some cancers grow and spread fast. Others grow more slowly. They also respond to treatment in different ways. Some types of cancer are best treated with surgery; others respond better to drugs called chemotherapy. Often more than one treatment is used to get the best results.

People with cancer need treatment that works best for their type of cancer.

What is a tumor?

Most cancers form a lump called a tumor or a growth. But not all lumps are cancer. Doctors take out a piece of the lump and look at it to find out if it’s cancer (this is called a biopsy).

  • Lumps that are not cancer are called benign
  • Lumps that are cancer are called malignant

Some cancers, like leukemia (cancer of the blood), don’t form tumors. They grow in the blood cells or other cells of the body.

What causes cancer?

Because cancer is a complex group of diseases, it can have many possible causes, such as lifestyle habits, genetics, carcinogens, and environmental factors. Sometimes there is no obvious cause.

What stage is the cancer?

You should be told if and how far the cancer has spread from where it started. This is called the cancer's stage. You may have heard other people say that their cancer was stage 1 or stage 2. Knowing the stage of the cancer helps the doctor decide what type of treatment is best.

For each type of cancer there are tests that can be done to figure out the stage of the cancer. As a rule, a lower stage (such as a stage 1 or 2) means that the cancer has not spread very much. A higher number (such as a stage 3 or 4) means it has spread more. Stage 4 is the highest stage.

Ask your doctor your cancer's stage and to explain what it means for you.

How is cancer treated?

Common treatments for cancer involve using surgery, medicines (drugs), and radiation. Sometimes more than one type of treatment is used, depending on the type and stage of cancer.

Surgery can be used to remove the cancer. The doctor might also take out some or all of the body part the cancer affects. For breast cancer, part (or all) of the breast might be removed. For prostate cancer, the prostate gland might be taken out. Surgery is not used for all types of cancer. For example, blood cancers like leukemia are best treated with drugs because they don't have a tumor that can be removed.

Many times, drugs are used to kill cancer cells or slow their growth. Some of these drugs are given by IV (into a vein through a needle), and others are a pill you swallow. Each drug works differently and sometimes one or more drugs are used together to treat a cancer. The different kinds of drugs used to treat cancer are:

  • Chemotherapy (chemo)
  • Targeted therapy
  • Immunotherapy
  • Hormone therapy

Radiation is also used to kill or slow the growth of cancer cells. It can be used alone or with surgery or chemo. Radiation treatment is like getting an x-ray. Sometimes it’s given by putting a “seed” inside the cancer to give off the radiation.

What is the best treatment for me?

Some cancers respond better to surgery; others respond better to chemo or radiation. Knowing the type of cancer you have is the first step toward knowing which treatments will work best for you.

The stage of your cancer will also help the doctor decide on the best treatment for you. A stage 3 or 4 cancer is likely to respond better to treatments that treat the whole body, like chemo.

Your health and the treatment you prefer will also play a part in deciding about cancer treatment. Not all types of treatment will work for your cancer, so ask what options you have. And treatments do have side effects, so ask about what to expect with each treatment.

Don’t be afraid to ask questions. It’s your right to know what treatments are most likely to help and what their side effects may be.

How can I learn more about my cancer?

If you have questions about cancer or need help finding resources in your area, please call the American Cancer Society at 1-800-227-2345. We're there when you need us – 24 hours a day, 7 days a week.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Revised: December 8, 2015

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