What Is Cancer?

Cancer is not a single disease but rather a group of diseases characterized by their ability to cause cells to change in abnormal ways and grow out of control. Most types of cancer form a tumor, a lump or mass of cancerous cells. Cells from a tumor may break away and travel to other parts of the body, where they can settle and multiply. This spreading process is called metastasis, and new cancers that have broken off and spread from the original tumor are metastases. Not all tumors are malignant (cancerous); some are benign (noncancerous) and do not spread or become life-threatening. A few cancers don't form masses, or lumps, such as those that affect the blood, like leukemia.

Not all lumps are cancerous. Lumps found in the breast are most often benign cysts (small, fluid-filled sacs) or fibroadenomas (nonmalignant fibrous tumors commonly found in young women). If you notice such a lump, have your doctor evaluate it to make certain it is not a carcinoma, or cancer.

How and why cancer spreads
Most people visualize a malignant tumor as steadily growing, and they may assume that if a lump is small, it's an early cancer. This may not be true. Tumor size alone doesn't indicate a cancer's severity. Although most of the time smaller tumors are less likely to have spread, the aggressiveness of a particular cancer is a major factor in how much it spreads. Some small tumors may quickly travel to the lymph nodes, and some large ones may not spread at all.

Another factor is the patient's immune system. By the time a tumor is found, cancer cells may have been present for some years, but the immune system has controlled their proliferation. Experts believe that at some point, in some women, the malignant cells become too numerous for the body's immune system to handle; the cancer then begins to metastasize, or spread.

Cancer treatment today is based on the premise that reducing the number of cancer cells - by removing the tumor and killing cancer cells that have spread to other organs - can turn the tide in favor of the body and its natural defenses.

Next >> Types of Breast Cancer

In This Article:
Understanding Breast Cancer
What Is Cancer?
Types of Breast Cancer
Less Common Breast Cancers

From the Harvard Health Publications Special Health Report, Breast Cancer: Strategies for Living. Copyright 2004 by the President and Fellows of Harvard College. Illustrations by Harriet Greenfield, M.A., and Jesse Tarantino. All rights reserved. Used with permission of StayWell. Harvard Medical School does not endorse products.
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