What Is Cancer?

Cancer is not just one disease; it is more than one hundred different diseases with one thing in common: out-of-control growth and spread of abnormal cells that occur because of gene mutations, or changes.

Normal cells grow, divide, and die in an orderly fashion. During the early years of a person's life, normal cells divide more rapidly to allow the person to grow. After that, in most tissues, normal cells divide only to replace worn-out or dying cells and to repair injuries.

Cancer cell growth is different from normal cell growth. Instead of dying, cancer cells continue to grow and form new, abnormal cells. Cancer cells can also invade other tissues, something that normal cells cannot do. Growing out of control and invading other tissues are what makes a cell a cancer cell.

In most cases, cancer cells form a tumor. But not all tumors are cancerous. A cancerous tumor is called malignant. Tumors that are not cancerous are called benign. Benign tumors can cause problems — they can grow very large and press on healthy organs and tissues — but they cannot invade, or grow into, other tissues. Because they cannot invade, they also cannot metastasize, or spread, to other parts of the body. These tumors are almost never life-threatening.

Cells become cancer cells because of damage to DNA. DNA is the genetic "blueprint" found in the nucleus of every cell and directs all its actions. In a normal cell, when DNA gets damaged, either the cell repairs the damage or the cell dies. In cancer cells, the damaged DNA is not repaired, but the cell does not die as it should. Instead, the damaged cell goes on making new cells that the body does not need. These new cells will all have the same damaged DNA as the first cell.

People can inherit damaged DNA, but most DNA damage is caused by mistakes that happen while the normal cell is reproducing. Sometimes the cause of the DNA damage is something that can be identified, such as cigarette smoking. But often no clear cause is found.

If cancer cells travel through the bloodstream or lymph vessels, they can spread to other parts of the body, where they can continue to grow and form new tumors. This process is called metastasis.

When cancer spreads, however, it is still named after the part of the body where it started. If breast cancer spreads to the lungs, it is still called breast cancer, not lung cancer. Different types of cancer behave very differently. For example, lung cancer and breast cancer are very different diseases. They grow at different rates and respond to different treatments. That is why people with cancer need treatment that is aimed at their particular type of cancer.

What Is Breast Cancer?

Breast cancer is a malignant tumor that has developed from cells of the breast. The disease occurs almost entirely in women, but men can have breast cancer, too. Although breast cancer can develop from any of the cells in the breast, doctors use the term "breast cancer" most often to mean cancers that start in the cells lining the ducts or lobules of the breast. These cancers are a type of cancer called carcinoma. Other types of cancer, such as sarcomas and lymphomas, can start in the breast. However, because these types of cancer occur much less frequently and have different causes and treatments, they are considered different diseases and are not grouped with breast carcinomas. In these cases, a doctor would say that a woman has lymphoma of the breast or sarcoma of the breast.

To understand breast cancer, it helps to have some basic knowledge about the normal structure and function of the breasts.

Normal Breast Tissue

There are three main components of the female breast:

• lobules, the glands that produce milk;

• ducts, the passages that carry the milk from the lobules to the nipple; and

• stroma, the fatty and connective tissues surrounding the ducts and lobules, blood vessels, and lymph vessels.

Most types of breast cancer begin in the cells that line the ducts (called ductal cancer). Some types begin in the cells that line the lobules (called lobular cancer), but a small number of breast cancers start in the cells of the stroma of the breast.

The Lymphatic System

The lymphatic system is important to understand because it is one of the pathways by which breast cancer can spread. The lymphatic system has two main parts: lymph nodes and lymph vessels. Lymph nodes are small, bean-shaped collections of immune system cells, which are important in fighting infections. Lymph nodes are connected by lymph vessels. Lymph vessels are similar to veins, except they carry lymph instead of blood. Lymph is a clear fluid that contains tissue fluids, waste products, and immune system cells. Cancer cells can invade lymph vessels and spread to lymph nodes, where they can settle and grow.

Most lymph vessels of the breast drain to the lymph nodes under the arms, called the axillary lymph nodes. Lymph nodes in this area are often removed to check for cancer cells. Lymph vessels of the breast can also drain to lymph nodes within the chest, called internal mammary lymph nodes, or to the lymph nodes near the collarbone, called the supraclavicular and infraclavicular lymph nodes. These other sets of lymph nodes are checked less often for...