Cancer is a group of more than 100 different diseases characterized by the uncontrolled, abnormal growth of cells. These cells form a lump or mass called a tumor. Some cancers, however, such as blood cancers, do not form tumors. Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors may grow, but they do not spread to other parts of the body and are usually not life threatening. Malignant tumors grow and invade other tissues in the body.

Sometimes cancer will spread to the lymph nodes. Lymph nodes are tiny, bean-shaped structures that filter the flow of lymph, the clear fluid that plays a role in the body's immune system. Lymph nodes are located in clusters in different parts of the body, such as the neck, groin area, and under the arms. Cells from malignant tumors can also break away and travel to other parts of the body, where they can continue to grow. This process is called metastasis. Metastatic cancer is named for the part of the body where it started. For example, if breast cancer spreads to the lungs, it is called metastatic breast cancer, not lung cancer.

Cancer can begin almost anywhere in the body. Tumors are named for the type of cell where the cancer started. For example, carcinomas begin in the skin or tissue that covers the surface of internal organs and glands. Sarcomas begin in the connective tissue, such as muscle, fat, cartilage, or bone.

Hippocrates, the ancient Greek physician, is credited with being the first to recognize the difference between benign and malignant tumors. The invasion of tumors so reminded him of crab claws that he called the disease karkinos, the Greek name for crab. In English this term survives as carcinoma. The English language also adopted the word cancer, which is the Latin word for crab.

A single cancerous cell eventually becomes a microscopic collection of cells and ultimately begins to invade surrounding tissue.

Each cancer runs its own distinctive course. In leukemia, the abnormal cells disperse throughout the body in the blood and bone marrow. With most cancers, a mass of cancer cells called a tumor begins to grow.

Some tumors (particularly in children and young adults) may double their size in a month. Colon and lung tumors may require two months to double, and some cancers take a year or more.

Most tumors are diagnosed when they are well advanced, with a mass of ten grams (approximately one-third of an ounce,) or more than one billion cancer cells.

These cells may invade tissues and spread throughout the body through the blood and lymph systems. A total body tumor burden of more than one kilogram (approximately two pounds) is usually not compatible with human life.

There are two broad categories of tumors: Benign and Malignant.

Benign tumors remain localized to the tissue in which they arise; they may grow large but will not spread to other parts of the body. If they grow into openings such as the trachea (wind pipe) or a major blood vessel, they can be fatal. But if found early, they can be cured by surgical removal, or in some cases by radiation therapy.

Malignant ("cancerous") tumors are a more serious matter. Some of their cells might break off, invading and destroying surrounding tissue or traveling through the blood or lymph streams to distant parts of the body, where new tumors might form. From these new tumors, malignant cells could break off again and establish even more colonies.

This process of invasion and spread is called "metastasis." Common sites for metastases,or secondary tumors, include the lymph glands, bones, lungs, liver and brain.

The body has its own defense mechanisms against tumor cells including processes to repair mutations of DNA, and certain cells and substances of the immune system which can destroy cancer cells. A diagnosis of cancer means that these "natural" defenses are no longer effective against the cancer.

* One in three people will develop cancer.

* One in four people will die of cancer.

* In 2005, about 1.4 million new cases of cancer were diagnosed.

* More than 1500 Americans died each day of cancer this year.

* Over 1,000,000 cases of skin cancer will be diagnosed this year.

* Cancer is the leading cause of death among Americans under the age of 85.