What is Cancer?

Normal Cells and Cancer Cells

To understand what cancer is, it is helpful to understand the behavior of a normal cell. A normal cell performs its designated job and divides only when the body signals that more cells are needed to carry out its function. Cancer cells are abnormal cells that divide and multiply without appropriate control or regulation. The genetic material (DNA) of a cell can become damaged or changed, producing mutations that affect normal cell growth and division. When this happens, cells do not die when they should and new cells form when the body does not need them. These extra cells can form a mass of tissue or the tumor. Over 200 types of cancers identified have one characteristic in common: unchecked cell growth. They multiply and can invade other tissues or spread to other parts of the body.

Cancer is a Genetic Disease

Cancer is a genetic disease because it is driven by gene mutations. Gene mutations occur when the DNA of cells is not copied perfectly into new cells. Cancer cells are created when the genetic material
(DNA) that causes the cell to be destroyed is mutated. Possible causes for genetic can be: 
  • Genetics. It is inherited from one or both parents. 
  • The result of every day environmental exposures and lifestyle factors.
  • Exposure to specific cancer-causing substances like ultra-violet (UV) radiation from the sun, tobacco smoke or asbestos. 
  • The result of certain viral infections, like hepatitis, HIV or HPV. 

Typically, multiple mutations are required for an abnormal cell to arise. To prevent cancer from occurring, the human body has many checks and balances aimed at discovering and destroying mutated cells. But, given certain circumstances, even the healthiest person can develop cancer. 

Tumors Can Be Benign or Malignant

The cells of a tumor must go through several stages before they become cancerous. As a tumor grows, it destroys surrounding tissue and takes critical supplies of oxygen and nutrients for its own growth.

Benign tumors are not cancer. Usually, doctors can remove them. Cells from benign tumors do not spread to other parts of the body. In most cases, benign tumors do not come back after they are removed. Most important, benign tumors are rarely a threat to life.

Malignant tumors are cancer. They are generally more serious. Cancer cells can invade and damage nearby tissues and organs. 

Cancer is Not Just One Disease but Many Diseases

There are more than 100 different types of cancer . Most cancers are named for the organ or type of cell in which they start. However, cancer types can be grouped into broader categories.

The main categories of cancer include:

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.

The Spreading of Cancer

There are three ways that cancer spreads in the body:

Through tissue: Cancer invades the surrounding normal tissue.

Through the lymph system: Cancer invades the lymph system and travels through the lymph vessels to other places in the body.

Through the blood: Cancer invades the veins and capillaries and travels through the blood to other places in the body.

When cancer cells break away from the primary tumor and travel through the lymph or blood to other places in the body, another, or secondary tumor, may form. This process is called metastasis. The secondary, or metastatic, tumor is the same type of cancer as the primary tumor. For example, if colorectal cancer spreads to the lungs, the cancer cells in the lungs are actually colorectal cancer cells. The disease is metastatic colorectal cancer, not lung cancer.

Learn about cancer and genetics.
What to do if you are newly diagnosed with cancer.
More about types of cancer.

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