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Cancer is a group of diseases where abnormal cells grow out of control and crowd out normal cells. It affects 1 in 3 people in the United States. Chances are that you or someone you know has been affected by cancer. Here is some information to help you better understand what cancer is.

What makes something cancer?

There are many different kinds of cancer, but they all involve abnormal cells. Cancers often have these features:

• Gene changes (mutations): Cancer cells have changes in their genes that make them abnormal. Some of these gene changes may be passed down from a parent (inherited mutations), while others may happen later in life (acquired mutations).
• Uncontrolled cell growth: Most abnormal cells die off or are unable to reproduce. But cancer cells can keep growing and dividing to make more abnormal cells. Cancer cells can crowd out normal cells.
• Tumor formation: Not all cancer cells form tumors, and not all tumors are cancer. But many types of cancer cells do clump together to form tumors.
• Cancer spread (metastasis): Cancer cells can invade nearby tissues, and many can even spread to other parts of the body.

Many types of abnormal cell growths have some of these features, but not all of them are cancer. For example:

• Tumors are lumps or masses of abnormal cells (neoplasms) that can be malignant (cancer) or benign (not cancer).
• Precancers are abnormal cells that are not cancer but could become cancer over time.
• Cysts are abnormal sac-like growths filled with air, fluid, pus, or tissue. Most cysts do not have abnormal cells in them and are not cancer.

What causes cancer?

Cancer starts when something goes wrong in the normal process of cells growing and dividing to make new cells. A cell’s genes (pieces of DNA that tell the cell how to function) change and make the cell abnormal. Most cells die if they become abnormal, but some gene changes allow cells to survive, grow, and divide to make more abnormal cells.

Gene changes that lead to cancer can have many possible causes. Lifestyle habits, genes you get from your parents, and being exposed to certain chemicals or radiation can all play a role. Many times, there is no clear cause.

Cancer is more than just one disease

There are many types of cancer. Most types also have subtypes based on features like what the cells look like under a microscope.

Cancer can develop anywhere in the body. It’s usually named for the part of the body where it starts, even if it spreads. For example, if cancer starts in the breast and spreads to the lungs, it’s still called breast cancer. It’s not considered lung cancer, but metastatic breast cancer. Metastatic means it has spread to another part of the body.

Some cancers are also named for the type of cell they start in. For example, carcinomas start in the skin or the lining of organs, while sarcomas begin in bone, muscle, or connective tissue.

Two main types of cancer

Cancers are often grouped into two main categories:

• Blood (hematologic) cancers start in blood cells or blood-forming tissues. These include leukemia, lymphoma, and multiple myeloma.
• Solid tumor cancers develop in organs or tissues. The most common solid tumors are breast, prostate, lung, and colorectal cancers.

Why cancer types matter

While all cancers involve uncontrolled cell growth, different types can behave in different ways. For example:

• Some grow and spread fast, while others are slower
• Some depend on hormones to grow
• Some are more likely to stay in one place, while others spread more easily.
• Some are treated with surgery; others respond better to radiation therapy or drugs such as chemotherapy, targeted therapy, or immunotherapy. For many cancers, more than one treatment is often used to get the best treatment outcome.

It’s very important to know the type (and subtype) of cancer before starting treatment, if possible. Knowing the exact type helps doctors know which treatment will work best.

What is the cancer stage?

When cancer is found, tests are done to see how big it is and whether it has spread. This is called staging the cancer.

A lower stage (like stage 1 or 2) means the cancer has either not spread or has spread to lymph nodes or just outside the primary site (the place it started). A higher stage (like stage 3 or 4) means it has spread farther.

The stage of a cancer is very important to know. Along with other testing on the tumor, doctors use the stage to figure out the best treatment options for a person.

How does cancer spread (metastasize)?

Cancer cells have the unique ability to:

• Grow quickly and out of control
• Spread to other parts of the body
• Invade other organs and tissues

When cancer cells break away from the original tumor, they can travel through the bloodstream or the lymphatic (lymph) system to other parts of the body. Most of these cells die. But some might survive, settle in a new area, and form new tumors. Learn more in How Does Cancer Spread?