Cancer: What It Is

Note: Links to specific cancers can be found at the end of this article.

Every day, without our even being aware of it, the cells that make up our body are growing, dividing and/or producing more cells as needed in order to keep us healthy. But sometimes, this process goes haywire; certain cells lose their normal control mechanism and start growing out of control. This is what happens with cancer.

Some - but not all - cancers are tumors. And some - but not all - tumors are cancer. The National Cancer Institute (NCI) says a tumor is a mass of extra tissue. Some tumors can be felt or seen externally on the body. Some tumors are internal and can be picked up via an imaging scan. NCI says there are two types of tumors - benign and malignant.

• Benign tumors - Benign tumors are not cancer. According to NCI, they often can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. As a result, benign tumors are usually not considered life threatening. However, all tumors should be monitored and treated correctly. Some benign tumors may also be more dangerous, because they press against and/or damage adjoining tissue.
• Malignant tumors - Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them. They can also break away from the original tumor, and enter the bloodstream or lymphatic system (the tissues and organs that produce and store cells that fight infection and disease). This process, called metastasis, is how cancer spreads from the original (primary) tumor to form new (secondary) tumors in other parts of the body.

According to the American Cancer Society (ACS), most cancers form as a tumor. But some do not. For example, the cancer cells of leukemia involve the blood and blood-forming organs, but do not actually form a mass or tumor.

Types of cancer

Cancer can begin in any part of the body, and is categorized by the type of tissue where it starts.

• Carcinomas - Carcinomas, according to NCI, are the most common types of cancer. Carcinomas start in cells that cover external and internal body surfaces. Lung, breast and colon cancer are the most frequent cancers of this type in the United States. (Read about "Lung Cancer" "Breast Cancer" "Colorectal Cancer")
• Sarcomas - Sarcomas start in cells found in the supporting tissues of the body such as bone, cartilage, fat, connective tissue and muscle. (Read about "Bone Cancer")
• Lymphomas - Lymphomas begin in the lymph nodes and tissues of the body's immune system. (Read about "Lymphoma")
• Leukemias - Leukemias are cancers of the immature blood cells that grow in the bone marrow and can accumulate in large numbers in the bloodstream. (Read about "Leukemia")

Cancers are usually identified by their site of origin. That means that the type or name of the cancer is the original site where the cancer developed, even if the cancer has spread to other parts of the body. For example, even if breast cancer has spread to the liver, it would still be considered breast cancer, not liver cancer. (Read about "Liver Cancer")

In some cases, ACS says the cancer first appears in one or more of the sites to which it has spread, and the original site where the cancer developed is not known. These cancers are called Cancers of Unknown Primary. Sometimes, additional tests can determine where the cancer started, and the cancer would be renamed for its site of origin. Other times, the primary site may never be known. ACS says that even if the source is never discovered, treatment can still be successful.

Causes and risk factors

In many cases, the exact cause of the cancer is unknown. However, we do know that there are some things that can increase the risk of different types of cancer. (Read about "Reduce Cancer Risks") Among the things that can affect a person's risk of developing cancer, according to the American Cancer Society are:

• Family and personal medical history can affect your risk. (Read about "Family Health History")
• Exposure to carcinogens in the environment, such as chemicals or radiation, can increase your risk. A carcinogen is a substance that can cause cancer. Some people are more susceptible to carcinogens than others.
• Lifestyle, including such factors as diet, exercise, and whether or not you smoke, also impacts your risk. (Read about "Dietary Guidelines" "Quit Smoking")

In addition, cancer can be more likely to develop when the immune system isn't functioning properly. (Read about "Primary Immunodeficiency" "Immune System Glossary" "The Immune System")

Finding and staging cancer

The American Cancer Society says many forms of cancer are treated most successfully when caught early. This is why regular screenings, such as Pap tests, mammograms, PSA tests, etc., are so important. (Read about "Cancer Check-ups")

If a screening produces a suspicious result, or if a doctor suspects cancer for some other reason, additional tests can be done, for example, blood or urine tests or imaging scans.

However, for most cancers, the way to make a definitive diagnosis is with a biopsy. Biopsy is the removal of cells or tissues for examination by a specialist physician called a pathologist. The pathologist may study the tissue under a microscope or perform other tests on the cells or tissue. Examination of suspicious cells through a biopsy helps the doctor determine if the cells are malignant (cancerous) or benign. (Read about "Biopsy")

If cancer is present, a doctor must determine what type of cancer it is, how fast it is growing, and whether or not it has invaded nearby healthy tissue or spread to other parts of the body. This helps the doctor determine at what stage the cancer is, and how best to treat it.

Staging is the term used to describe the extent or severity of an individual's cancer. A cancer's stage depends on different factors, including its location, its size, whether lymph nodes have been affected and whether or not the cancer has spread. (Read about "The Lymph System")

NCI says different tests can be used to help determine a cancer's stage. These include blood and urine tests; imaging tests such as x-rays, CT scans, MRI, PET and ultrasound (Read about "X-rays" "CT Scan - Computerized Tomography" "MRI - Magnetic Resonance Imaging" "PET - Positron Emission Tomography" "Ultrasound Imaging"); pathology reports from the biopsy; and surgical reports. Knowing a cancer's stage helps to determine the best way to treat it. (Read about "Cancer Treatments")

Metastasis and angiogenesis

When a cancer spreads to another part of the body, it is called metastasis. Researchers say one of the most important things required for metastasis to occur is the growth of a new network of blood vessels. This process of forming new blood vessels is called angiogenesis. Angiogenesis is a normal biological process. For example, angiogenesis is necessary for the repair or regeneration of tissue during wound healing. Tumor angiogenesis, however, is different. NCI says tumor angiogenesis is the growth of a network of blood vessels that penetrates into cancerous growths, supplies them with nutrients and oxygen and removes waste products. NCI says tumor angiogenesis starts with cancerous tumor cells releasing molecules that send signals to activate genes in the host tissue that, in turn, make proteins to encourage growth of new blood vessels. Certain molecules can either activate or inhibit angiogenesis. Studies are now examining the effectiveness of drugs aimed at inhibiting angiogenesis in specific cancers.

Below, in alphabetical order, is a list of individual cancers and tumors. You can learn more about each of them by following the appropriate link.