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 signs of metastasis.

In the early stages of the disease, any spread of cancer cells to lymph nodes can be detected only by removing the lymph nodes and examining them with a microscope. With time, though, the cancer cells can grow and multiply, causing the lymph nodes to enlarge. They might grow large enough to be detected by a doctor feeling for lumps under the skin or seen on an imaging test. It is important to determine whether breast cancer has spread to any axillary lymph nodes, because metastasis to these lymph nodes means that the cancer is also more likely to have spread to other organs of the body.

Benign Breast Conditions

Most breast lumps are benign (not cancerous). However, tissue samples may need to be taken from the lumps and viewed under a microscope to prove they are not cancerous. Most lumps in the breast are caused by fibrosis and/or cysts (in the past, often referred to as fibrocystic changes). Fibrosis refers to connective tissue or scar tissue formation, and cysts are fluid-filled sacs. Fibrosis and cysts can cause breast swelling and pain, which are often worse just before a woman's menstrual period is about to begin. They may cause the breasts to feel lumpy, and the woman may also notice a clear or slightly cloudy nipple discharge.

Benign breast tumors, such as fibroadenomas and intraductal papillomas, are abnormal growths, but they are not cancerous and do not spread outside of the breast. They are not life-threatening. Still, some benign breast conditions are important because women with these conditions have a higher risk of developing breast cancer. For more information on these breast conditions, see pages 435–440.

Types of Breast Cancer

There are several types of breast cancer, some of which are quite rare. In some cases, a single breast tumor can have a combination of different types of cancerous cells or have a mixture of invasive and in situ cancer. It is important to understand some of the key terms that will be used to describe the type of breast cancer you have, as your type will determine your prognosis and treatment options.

Nearly all breast cancers are carcinomas — either ductal carcinoma or lobular carcinoma. This type of cancer begins in the lining layer — the epithelial cells — of such organs as the breast. Other types of cancer, such as sarcomas and lymphomas, can also start in the breast.

Not only are most breast cancers carcinomas, most are a certain type of carcinoma called adenocarcinoma. An adenocarcinoma is a type of carcinoma that starts in glandular tissue; adeno is the medical term for gland, the tissue that makes and releases a substance. The ducts and lobules of the breast are glandular tissue — they make breast milk. And nearly all breast cancers start in the cells of either the ducts or the lobules. The two main types of breast adenocarcinoma are ductal carcinoma and lobular carcinoma. There are also several subtypes of ductal carcinoma, some of which have important implications for prognosis and treatment.

In Situ Cancer

"In situ" is a term used for an early stage of cancer in which cancer cells are confined to the immediate part of the breast (such as the duct or lobule) where they began. In breast cancer, it means that the cancer cells are only in the ducts (ductal carcinoma in situ) or lobules (lobular carcinoma in situ) where they started. The cells have not invaded the nearby stroma, nor have they spread to other organs in the body. Because the cancer cells have not yet invaded other tissues of the breast, in situ cancer is often described as noninvasive cancer.

Ductal Carcinoma in Situ (DCIS)

Also known as intraductal carcinoma, ductal carcinoma in situ (DCIS) is the most common type of noninvasive breast cancer. In DCIS, cancer cells are found inside the ducts, but they have not spread through the walls of the ducts into the stroma (see illustration, next page). Because DCIS is noninvasive, it has not spread to lymph nodes or distant sites, and it does not cause cancer death. Still, sometimes an area of DCIS will contain an area of invasive cancer. This area can range in size from a small tumor to a tiny spot where cancer cells have grown through the duct wall, called a microinvasion. The chance that DCIS will contain an invasive cancer is higher if the area of DCIS is large or the DCIS cells are considered high grade, meaning they look very abnormal under the microscope. DCIS is sometimes subclassified based on its grade and type to help determine the most appropriate treatment and to help predict the risk for cancer recurrence. There are several types of DCIS, but one important distinction among them is whether there are areas of dead or degenerating cancer cells, known as tumor cell necrosis. The term comedocarcinoma is often used to describe DCIS with necrosis. Because DCIS can continue to grow and become an invasive cancer, it is important that it be treated. Nearly all women with a DCIS diagnosis can be cured. DCIS is often found on a mammogram, an x-ray of the breast. With more women getting mammograms each year, the number of DCIS diagnoses is also increasing.

Lobular Carcinoma in Situ (LCIS)

Lobular carcinoma in situ (LCIS) is not a true cancer or precancer. Also called lobular neoplasia, LCIS is sometimes grouped with ductal carcinoma in situ as a noninvasive breast cancer, but it differs from DCIS in that it does not seem to become an invasive cancer if it is not treated. That is why it is not considered a true cancer. However, women with LCIS have a higher risk of invasive breast cancer developing in either breast.

Infiltrating (or Invasive) Carcinoma

Infiltrating (or invasive) carcinoma starts in the cells lining a duct (milk passage) or lobule (milk-producing gland) of the breast, and then breaks through this layer of cells to grow into the stroma of the breast. At this point, the cancer can spread to other parts of the body through the lymphatic system and bloodstream. There are two main types of invasive breast carcinoma: infiltrating ductal carcinoma and infiltrating lobular carcinoma.

Infiltrating Ductal Carcinoma

Infiltrating ductal carcinoma (IDC) is the most common type of breast cancer. It represents about 80 percent of invasive breast carcinomas. This type of cancer starts in the cells lining a duct and grows into the stroma.

Some invasive carcinomas have features that make them different from the typical infiltrating ductal carcinoma. These subtypes of invasive carcinoma are often named after features seen when they are viewed under the microscope, such as the way the cells are arranged. They are rare. In general, these subtypes are still treated like standard infiltrating ductal carcinoma.

The following subtypes tend to have a better prognosis than standard infiltrating ductal carcinoma:

• adenoid cystic (or adenocystic) carcinoma

• low-grade adenosquamous carcinoma (a type of metaplastic carcinoma)

• medullary carcinoma

• mucinous (or colloid) carcinoma

• papillary carcinoma

• tubular carcinoma

Some subtypes have the same or possibly a worse prognosis than standard infiltrating ductal carcinoma:

• metaplastic carcinoma (most types, including spindle cell and squamous)

• micropapillary carcinoma

• mixed carcinoma (has features of both invasive ductal and lobular)

More information about these kinds of breast cancer can be found in the glossary, starting on page 499.

Triple-negative breast cancer: The term triple-negative breast cancer is used to describe breast cancers (usually invasive ductal carcinomas) whose cells lack estrogen receptors and progesterone receptors and do not have an excess of the HER2 protein on their surfaces. Because the tumor cells lack the necessary receptors, common treatments such as hormone therapy and drugs that target estrogen, progesterone, and HER2 are ineffective in treating triple-negative breast cancer. Chemotherapy is the most effective treatment for this type of cancer. For more information on triple-negative breast cancer, see page 50.

Infiltrating Lobular Carcinoma

Infiltrating lobular carcinoma (ILC) starts in a milk-producing gland of the breast. Like infiltrating ductal carcinoma, ILC has the potential to spread elsewhere in the body. About 10 to 15 percent of invasive breast cancers are invasive lobular carcinomas. Most patients with ILC present with physical symptoms, and it can be harder to detect on screening mammograms than infiltrating ductal carcinoma.

Inflammatory Carcinoma

Inflammatory carcinoma, or inflammatory breast cancer, is a rare and very aggressive form of breast cancer. It accounts for about 1 to 3 percent of all breast cancers. Inflammatory breast cancer does not present as a breast lump. Instead, it makes the skin of the breast look red and feel warm, as if it were infected and inflamed. The entire breast can swell, and the skin has a thick, pitted appearance that doctors often describe as resembling an orange peel, called p'eau d'orange. Sometimes the skin develops ridges and small bumps that look like hives. Although this type of breast cancer is called "inflammatory," the changes associated with it are not due to inflammation or infection. The breast becomes red and swollen, or "inflamed," because cancer cells are blocking the lymph vessels in the breast. Inflammatory carcinoma tends to grow and spread very quickly, and it has a worse prognosis than more common breast cancers. This type of breast cancer requires aggressive treatment, often with a combination of chemotherapy, radiation therapy, and surgery.

Paget Disease of the Nipple

Paget disease of the nipple (also known as Paget disease of the breast) is a type of breast cancer that starts in the ducts of the breast and spreads to the skin of the nipple and the areola (the dark circle around the nipple). It is rare, accounting for only 1 percent of all breast cancer cases. The skin of the nipple and areola often appears crusty, scaly, and red, with areas of bleeding or oozing. The woman might notice burning or itching. Paget disease of the nipple can be associated with in situ carcinoma or with infiltrating breast carcinoma. If no lump can be felt in the breast tissue and the biopsy shows DCIS but no invasive cancer, the woman's prognosis is excellent. If invasive cancer is present, the prognosis is not as good, and the cancer will need to be staged and treated like any other invasive cancer.