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Quick Facts

Overview of Cancer

By The Manual's Editorial Staff,

What is cancer?

Cancer is the out-of-control growth of cells in your body. Cells are the tiny building blocks of your body. Cells specialize in what they do. For example, your intestines have muscle cells to make them contract, nerve cells to control the muscle cells, and other cells to absorb food. There are many more types of cells in other parts of your body.

Normally, new cells in your body grow only to replace cells that have died or gotten too old. Each new cell looks and functions just like its parent cell. Cancerous (malignant) cells are different from normal cells in many ways because they:

  • Multiply very quickly

  • Keep on multiplying even though the organ they're in doesn't need more cells

  • Look abnormal and usually don't function properly

  • Don't stay where they belong—they invade nearby organs or spread to distant parts of your body (metastasize)

Some kinds of cancer cells group together to form a solid mass called a tumor. However, not all tumors are cancerous. Tumors that aren't cancerous are called benign (harmless).

When doctors talk about the “stage” of cancer (stage I, stage II, stage III, or stage IV), they're describing how big the cancer is and if and where it has spread. Some cancers grow and spread faster than others. These are called aggressive cancers.

Why does cancer kill you?

If you're healthy and gain 15 or 20 pounds of weight, you'll probably feel fine. However, even a few pounds of cancer cells can be enough to kill you if they:

  • Block an important body function

  • Give off substances that interfere with other organs

Dangerous places for cancer to grow include:

  • Lungs: Trouble breathing, pneumonia

  • Belly: Blocking your intestines

  • Brain: High pressure inside your skull

  • Major blood vessels: Heavy or fatal bleeding

  • Bone marrow (the hollow insides of your bones, where blood cells are made): Dangerously low blood count (anemia), excessive bleeding, trouble fighting off infections, weak bones that break easily

The substances cancers give off can cause problems even when the cancer isn't growing in a dangerous place. Some of the substances do the following:

  • Take away your appetite: severe weight loss, trouble fighting off infections

  • Interfere with making blood cells: low blood count (anemia), excessive bleeding, trouble fighting off infections

  • Interfere with your body's chemical balance: abnormal heart function, coma

The problems caused by the substances are called paraneoplastic syndromes.

Where in your body can you get cancer?

Cancer can develop from almost any cell, including those in your blood, bones, and organs. Each type of cancer is different depending on what type of cell it started from. Cancers are named by the organ they started in. For example, "lung cancer."

What are the most common cancers?

Cancer isn't one disease, but many.

The 5 most common cancers in men, from most to least common:

  • Prostate

  • Lung

  • Colon and rectum

  • Bladder

  • Kidney

The 5 most common cancers in women, from most to least common:

  • Breast

  • Lung

  • Colon and rectum

  • Uterus

  • Thyroid

That said, skin cancer is probably the most common. However, because accurate counts aren't available for all types of skin cancer, skin cancer isn't included on these lists.

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