A term for diseases in which abnormal cells divide without control.  There are many different types of cancers, but all share one hallmark characteristic: unchecked growth that progresses toward limitless expansion.

Cancer cells can invade nearby tissues and can spread through the bloodstream and lymphatic system to other parts of the body.

Different Kinds of Cancer

Carcinomas, the most common types of cancer, arise from the cells that cover external and internal body surfaces. Lung,  breast, and colon are the most frequent cancers of this type in the United States.

Sarcomas are cancers arising from cells found in the supporting tissues of the body such as bone, cartilage, fat, connective tissue, and muscle.

Lymphomas are cancers that arise in the lymph nodes and tissues of the body's immune system.

Leukemias are cancers of the immature blood cells that grow in the bone marrow and tend to accumulate in large numbers in the bloodstream.

Loss of Normal Growth Control

Cancer arises from a loss of normal growth control. In normal tissues, the rates of new cell growth and old cell death are kept in balance. In cancer, this balance is disrupted. This disruption can result from uncontrolled cell growth or loss of a cell's ability to undergo cell suicide by a process called"apoptosis." Apoptosis, or "cell suicide," is the mechanism by which old or damaged cells normally self-destruct.

This gradual increase in the number of dividing cells creates a growing mass of tissue called a "tumor" or "neoplasm." If the rate of cell division is relatively rapid, and no "suicide" signals are in place to trigger cell death, the tumor will grow quickly in size; if the cells divide more slowly, tumor growth will be slower. But regardless of the growth rate, tumors ultimately increase in size because new cells are being produced in greater numbers than needed. As more and more of these dividing cells accumulate, the normal organization of the tissue gradually becomes disrupted.

Malignant versus Benign Tumors

Depending on whether or not they can spread by invasion and metastasis, tumors are classified as being either benign or malignant. Benign tumors are tumors that cannot spread by invasion or metastasis; hence, they only grow locally. Malignant tumors are tumors that are capable of spreading by invasion and metastasis. By definition, the term "cancer" applies only to malignant tumors.

Why Cancer is Potentially Dangerous

A malignant tumor, a "cancer," is a more serious health problem than a benign tumor because cancer cells can spread to distant parts of the body. For example, a melanoma (a cancer of pigmented cells) arising in the skin can have cells that enter the bloodstream and spread to distant organs such as the liver or brain. Cancer cells in the liver would be called metastatic melanoma, not liver cancer. Metastases share the name of the original ("primary") tumor. Melanoma cells growing in the brain or liver can disrupt the functions of these vital organs and so are potentially life threatening.

Cancer Detection and Diagnosis

Detecting cancer early can affect the outcome of the disease for some cancers. When cancer is found, a doctor will determine what type it is and how fast it is growing. He or she will also determine whether cancer cells have invaded nearby healthy tissue or spread (metastasized) to other parts of the body. In some cases, finding cancer early may decrease a person's risk of dying from the cancer. For this reason, improving our methods for early detection is currently a high priority for cancer researchers.

Early Cancer May Not Have Any Symptoms

Some people visit the doctor only when they feel pain or when they notice changes like a lump in the breast or unusual bleeding or discharge. But don't wait until then to be checked because early cancer may not have any symptoms. That is why screening for some cancers is important, particularly as you get older. Screening methods are designed to check for cancer in people with no symptoms.

Biopsy

To diagnose the presence of cancer, a doctor must look at a sample of the affected tissue under the microscope. Hence, when preliminary symptoms, Pap test, mammogram, PSA test, FOBT, or colonoscopy indicate the possible existence of cancer, a doctor must then perform a biopsy, which is the surgical removal of a small piece of tissue for microscopic examination. (For leukemias, a small blood sample serves the same purpose.) This microscopic examination will tell the doctor whether a tumor is actually present and, if so, whether it is malignant (i.e., cancer) or benign. In addition, microarrays may be used to determine which genes are turned on or off in the sample, or proteomic profiles may be collected for an analysis of protein activity. This information will help doctors to make a more accurate diagnosis and may even help to inform treatment planning.

Cancer tissue has a distinctive appearance under the microscope. Among the traits the doctor looks for are a large number of irregularly shaped dividing cells, variation in nuclear size and shape, variation in cell size and shape, loss of specialized cell features, loss of normal tissue organization, and a poorly defined tumor boundary.

Is There a Cancer "Epidemic"?

A common misconception arises from news stories suggesting we are experiencing a cancer "epidemic." This only appears to be the case because the number of new cancer cases reported is rising as the population is both expanding and aging. Older people are more likely to develop cancer; however, this trend is offset by new births, which are also increasing, and cancer is rare among the young. So as more and more members of a 75-million-strong "baby-boomer" cohort begin shifting en masse to older, more cancer-prone ages, the number of new cancer cases is expected to increase in the next several decades. But since the birth rate is also expected to increase, the cancer rate may either stay the same or, perhaps, decline.

Source:  National Cancer Institute