What Is Cancer?

Overview

Cancer is not one disease but a large group of diseases. In each of them, abnormal cells grow without control and push out the normal cells around them. About 1 in 3 people in the United States are affected by cancer, so most people know someone who has had it.

What Makes Something Cancer

Every kind of cancer involves abnormal cells, and cancers usually share these features:

• Gene changes (mutations). The cells carry changes in their genes that make them abnormal. Some of these changes are inherited from a parent; others are acquired later in life.
• Growth without control. Most abnormal cells die or cannot reproduce. Cancer cells keep growing and dividing, and they can crowd out normal cells.
• Tumors. Many cancer cells clump together into tumors. But not every cancer forms a tumor, and not every tumor is cancer.
• Spread (metastasis). Cancer cells can grow into nearby tissue, and many can travel to other parts of the body.

Some other growths share a few of these features but are not cancer. A tumor (neoplasm) is a lump of abnormal cells that can be malignant (cancer) or benign (not cancer). A precancer is a group of abnormal cells that is not cancer but could turn into cancer over time. A cyst is a sac filled with air, fluid, pus or tissue; most cysts contain no abnormal cells and are not cancer.

What Causes Cancer

Cells normally grow and divide in a controlled way to replace old cells. Cancer starts when the genes that direct this process change and make a cell abnormal. Most abnormal cells die, but some gene changes let a cell survive and keep dividing into more abnormal cells. Lifestyle habits, genes inherited from parents, and exposure to some chemicals or to radiation can all contribute to these gene changes. Often, no single clear cause is found.

Cancer Is Many Diseases

There are many types of cancer, and most types have subtypes based on features such as how the cells look under a microscope. A cancer is usually named for the part of the body where it started, even after it spreads. Breast cancer that spreads to the lungs is still breast cancer — called metastatic breast cancer — not lung cancer. Some cancers are also named for the kind of cell they start in: carcinomas start in the skin or in the lining of organs, and sarcomas start in bone, muscle or connective tissue.

Cancers are often sorted into two main groups:

• Blood (hematologic) cancers start in blood cells or in the tissues that make blood. Examples are leukemia, lymphoma and multiple myeloma.
• Solid tumors grow in organs or tissues. The most common are breast, prostate, lung and colorectal cancers.

Different types behave differently. Some grow and spread fast, others slowly. Some need hormones to grow. Some tend to stay in one place, while others spread easily. Some are best treated with surgery, others with radiation or with drugs such as chemotherapy, targeted therapy or immunotherapy — and many cancers are treated with more than one of these. That is why doctors try to know the exact type and subtype of a cancer before treatment starts.

Cancer Stage

When cancer is found, tests show how large it is and whether it has spread. This is called staging. A lower stage, such as stage 1 or 2, means the cancer has not spread, or has spread only to nearby lymph nodes or just outside where it started. A higher stage, such as stage 3 or 4, means it has spread farther. Doctors use the stage, together with other tests on the tumor, to choose the best treatment.

How Cancer Spreads

Cancer cells can grow quickly and without control, invade other organs and tissues, and spread to other parts of the body. Cells that break off the first tumor can travel through the bloodstream or the lymph system. Most of these cells die, but some survive, settle in a new place and form new tumors.

Questions to Ask Your Doctor

• What exact type and subtype of cancer do I have, and how was that decided?
• What stage is my cancer, and what does that stage mean for my treatment choices?
• Is this type of cancer usually fast-growing or slow-growing?
• Does my cancer depend on hormones to grow, and does that change my treatment?
• Is there anything known about what may have caused my cancer, or is there no clear cause?

Learn More

Related topics in this category explain tumors and neoplasms, cysts, precancers, how cancer spreads, advanced and metastatic cancer and what happens when blood cells don't form normally. For definitions of cancer terms, with pronunciations, see the Cancer Glossary. To see the body parts where cancers start, see its interactive Anatomy Gallery, with 3D tours of 15 body systems.

Source: https://www.cancer.org/cancer/understanding-cancer/what-is-cancer.html