What Is Cancer?

Table of Contents

Cancer is a condition in which cells grow and divide without stopping. Cancer interferes with normal, healthy cell growth. Instead of dying, old or damaged cells survive. New cells form when they are not needed. In some cases, they form growths called tumors. Scientists study cancers to understand why they start, how they spread, and how to stop them.


Why Does Cancer Occur?

There is no known one cause of the abnormal cell growth that leads to cancer. It can start with a cell’s genetic code, called DNA. Errors in the DNA can be passed down through families. Or, more often, they develop during a person’s lifetime. Each time a cell divides, an error can develop. The body has checkpoints to catch these errors and tell the cell to self-destruct. Or the immune system notices something is wrong and kills the cell. Sometimes neither of these happens. The cell keeps dividing. It passes the error on to new cells it creates.

These errors are called mutations. A cell can function normally with some mutations. But, at some point, the mutations may lead to diseases, such as cancer. This is one reason why your risk of getting cancer increases as you get older. Your cells have had more time to form mutations and pass them along to new abnormal cells. These abnormal cells can:

  • Grow, divide, and form tumors. This is what happens when cancer begins in, for example, the breast, the colon, or the lung. Doctors call these solid tumor cancers. Blood cancers, like leukemia, typically don’t form this type of tumor.
  • Spread (or metastasize). A cell can develop the ability to travel and invade other areas. Sometimes the invasion stays within the same area of the body. Other times, the cancer cells spread to other parts to the body. This process of spreading is called metastasis.


Who Gets Cancer?

Anyone can get cancer. We all have cells in our body that have some errors. Sometimes they develop on their own, by mistake. Other times, something outside of the body causes the error.

Doctors describe anything outside of the body that may lead to cancer as an “environmental risk factor for cancer.” Examples include smoking, obesity, or exposure to certain chemicals. They can damage genes and increase cancer risk. Some people are born with genetic mutations that are linked to cancer. This means that the change in the gene has been passed down through a family. This is called “inherited risk.”

Many people who get cancer do not have any known risk factors. Other people who have many risk factors don’t get cancer. Scientists are trying to understand this better. They want to know why some people with few risk factors get cancer yet others with many risk factors do not.

A helpline counselor wearing a headset

Is Cancer in Your Life?

Our Cancer Support Helpline offers free, personalized navigation to cancer patients or their loved ones at 888-793-9355 and online.

Reach Out Today

What Are Cancer Categories, Types & Subtypes?

Doctors group cancers into 6 broad categories. You may hear these words:

  • 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 the blood
  • lymphoma – cancer that begins in the cells of the immune system
  • myeloma – cancer that starts in plasma of the bone marrow
  • central nervous system cancers – cancers that begin in the tissues of the brain and spinal cord

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.

But there is more to a cancer than where it starts. Two cancers that start in the same place may be very different, and two that start in different parts of the body may have more in common than it appears. Scientists have found that:

  • Not all cancers of the same type (in the same organ) are caused by the same mutations.
  • One mutation can cause different cancers in different people.
  • Different changes in cells, genes, and processes can lead to cancer. They have discovered some and keep looking for more.

Some new cancer treatments are designed to target certain changes or mutations. They aim to stop cancer where it starts — in the cell. This is why biomarker testing is important. Biomarkers describe the biology of the cancer. They provide details about what is going on in the cells that allows cancer to grow. They help your doctor understand the “subtype” of your cancer.

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

two seated men reading on a phone

Learn About Types of Cancer

Looking for details about a specific cancer, such as risk factors, symptoms, treatment options, and side effects?

Find More Information

What Is Metastatic Cancer?

The mutations cancer cells acquire give them the ability to travel to areas of the body where they don't belong. They do this by:

  • Moving through and invading normal tissue
  • Traveling through the lymph vessels that make up the lymph system
  • Traveling through the blood, via veins and capillaries

When cancer cells break away from the primary tumor and travel through 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, the liver, and the lungs. However, some tumors are more likely to metastasize to certain areas than others.

Cancers are named and treated based on where they started, not where they spread. For example, breast cancer that spreads to the liver is still breast cancer. This is because the cancer cells in your liver started as breast cells, not liver cells. In this case, the cancer would be called breast cancer metastases in the liver. The breast is still the primary site of the cancer.

Duration: 88 min

Coping with Metastatic Cancer

Watch the webinar, Coping with Metastatic Cancer, which took place August 14, 2015. The webinar features three experienced presenters, followed by a Q&A with participants.