What Is Cancer - Chapter 1 Introduction To Preventing Cancer: Cancer

Content provided by the Faculty of the Harvard Medical School

What is cancer?

Cancer is not a single disease but a collection of more than 100 diseases that share a common underlying problem: cells that behave abnormally, grow out of control, and interfere with the body's vital functions. One of the most lethal diseases, cancer is second only to heart disease as a cause of death in the United States.

But the situation is not all doom and gloom. A lot of good news can also be found in recent cancer statistics. The 2008 American Cancer Society's annual cancer statistics indicate that death rates from cancer in the United States have decreased by 18.4% among men and by 10.5% percent among women. Overall death rates from cancer have been declining since the early 1990s. These encouraging trends reflect a combination of increased efforts at prevention, earlier detection, and improved treatments. As more people undergo recommended screening tests and adopt healthier habits, more cancers will be found early enough to ensure survival, and some may even be prevented in the first place. To understand why, it is first necessary to understand how cells function normally and why they can turn cancerous.

The biology of cancer

Cells are the building blocks of the human body. Life begins as a single cell. That cell, a fertilized egg, divides into two, then four, and so on. As cells multiply, they begin to differentiate, forming specific tissues and organs, such as the brain, lungs, and heart. Even after birth, human beings are, in some respects, constantly being "reborn." Cells continue dividing - duplicating their genetic code, a process known as replication. As new cells emerge, the older ones die out. In this way, your body remains healthy. Orchestrating this process are the genes contained in each cell's nucleus. These genes consist of strands of DNA (deoxyribonucleic acid), which function like detailed molecular blueprints, providing instructions on everything from your hair color to your metabolism. They also produce proteins that provide the "stop" and "go" signals for the processes of growth, development, and renewal.

Your cells are actually programmed to die after a certain time so that healthy new cells can replace them. When an old cell dies, nearby cells come in to clean up the debris. This process of orderly and planned cell death is called apoptosis. When it works well, apoptosis helps your body maintain a delicate balance between old cells and new ones. As old cells die, new ones take their place.

Cancer is a perversion of these normal processes. What was once orderly becomes chaotic and unmanageable. Normal cells replicate at a steady pace; cancerous cells replicate uncontrollably. Normal cells maintain a balance between old and new; cancerous cells never die, and eventually so many accumulate that a tumor, also known as a carcinoma, forms. Normal cells respect boundaries and stay put; cancerous cells migrate elsewhere, in a process known as metastasis.

For many years, researchers wondered why normal cells turn cancerous. Why does the precisely choreographed dance of cellular life disintegrate into a free-for-all? At first, the likely culprits appeared to be carcinogens, substances such as tobacco smoke that are known to produce cancer. Scientists also determined that heredity plays a part, based on their observations that certain types of cancer seem to run in families. But puzzling questions remained. If cigarette smoke is a carcinogen (a substance that causes cancer), why doesn't everyone who smokes develop lung cancer? If cancer can be inherited, why are some members of a family spared while others develop the disease?

Thanks to a host of discoveries, researchers have begun to understand cancer as the end result of a multi-step process that takes place over several years or even decades. Seldom does one factor, such as a defective gene, result in cancer. More often, the disease develops because of the complex interactions among the genes inside your cells and external factors, such as diet and exposure to toxins, which can damage those genes.

Last updated:	May 01, 2008