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What is Cancer?

Cancer is the medical term we use to talk about an array of diseases that all started the same way: a normal cell went haywire and began growing and dividing uncontrollably.

What causes a cell to stop acting the way it should? Every time a cell divides there is the potential for errors to develop in the letter code that makes up the cell’s genetic material (DNA). Sometimes the checkpoints in the cell that make sure DNA are copied correctly catch these errors, and send a message for the cell to self-destruct. Other times, the immune system notices something is wrong, and kills the cell. Or, the cell may just keep on dividing, passing on that mistake to its daughter cells.

A cell can function normally with a certain number of mutations. But at some point, the mutations take over and make the cell goes haywire. This is part of the reason why your risk of getting cancer increases as you get older—your cells have had more time to collect mutations and pass them along to their daughter cells.

As these abnormal cells grow and divide they can form into a tumor. This is what happens when cancer begins in, for example, the breast, colon, or lung. Blood cancers, like leukemia, typically don’t form this type of tumor. In the process of cancerous, a cell also can develop the ability to push into and invade other parts of the body where it doesn’t belong. This invasion can be limited to the organ where the cancer developed. Or, the cancer cells may spread—metastasize—to other parts to the body.

Learn More About a Specific Type of Cancer

Who Gets Cancer?

We all have cells in our body that have some errors, or mutations. These errors can occur simply because the cell makes a mistake—and that can happen to anyone’s cells. Many things scientists refer to as “environmental risk factors for cancer” also can cause these mutations to develop. In cancer, “environmental” refers to anything that is not genetically inherited that increases cancer risk, such as obesity, smoking and exposure to certain chemicals. The environmental factors have a genetic impact because they cause genetic damage that increases cancer risk. You can also be born with a genetic mutation that increases your risk for developing cancer. In this situation, a cell typically needs to acquire fewer environmental mutations before it begins to go haywire.

There are known cancer risk factors. But the surprising thing about cancer is that many people who get cancer do not have any of these risk factors and other people who have many of them don’t. Scientists want to know not only why some people with few risk factors get cancer but also why others with many risk factors do not.

Cancer Categories

Cancer can start almost anywhere in the body, but where a cancer begins is only one thing you need to know about a tumor. Scientists now know that not all tumors that grow in the same organ are fueled by the same mutations. And as more cancer treatments are developed that target these specific mutations, it will be increasingly important to know the specific type of mutations a tumor has. This is why your doctor may tell you that you have a certain “subtype” of tumor.

Even so, doctors still typically group cancers together into five broad categories that provide information about where the cancer started. These categories are important because these are words you may hear:

Carcinoma - Cancer that begins in the skin or in tissues that line or cover internal organs.

Sarcoma - Cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.

Leukemia - Cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.

Lymphoma and Myeloma - Cancers that begin in the cells of the immune system.

Central Nervous System Cancers - Cancers that begin in the tissues of the brain and spinal cord.

Since cancers can start from different types of cells—and can be fueled on by different types of mutations—not all cancers respond to the same cancer treatments. Your doctor will tell you what treatments are best for your specific type of cancer.

Learn More About a Specific Type of Cancer

Metastatic Cancer

The mutations cancer cells acquire give them the ability to travel to areas of the body where they don't belong. To get to these other areas, cancer cells can move through and invade normal tissue; travel through the lymph vessels that make up the lymph system; or travel through the blood, via veins and capillaries.

When cancer cells break away from the primary tumor and travel through the lymph or blood to other places in the body, they may form another tumor. This process is called metastasis. The most common places for cancer to metastasize are (in no particular order) the bone, liver, and lung. However, some tumors are more likely to metastasize to certain areas than others.

For example, if your breast cancer spreads to your liver you do not now have liver cancer. That’s because the cancer cells in your liver originated as breast cells, not liver cells. Instead, you would call it breast cancer metastases in the liver.

Talk to a Helpline Counselor About Metastatic Cancer