Breast Cancer: Introduction

Approved by the Cancer.Net Editorial Board, 04/2017

ON THIS PAGE: You will find some basic information about this disease and the parts of the body it may affect. This is the first page of Cancer.Net’s Guide to Breast Cancer. Use the menu to see other pages. Think of that menu as a roadmap for this complete guide.

In the United States, breast cancer is the most common cancer in women (excluding skin cancer). Men can also develop breast cancer. However, breast cancer in men is rare, accounting for less than 1% of all breast cancers.

About the breast

The breast is made up of different tissue, ranging from very fatty tissue to very dense tissue. Within this tissue is a network of lobes. Each lobe is made up of tiny, tube-like structures called lobules that contain milk glands. Tiny ducts connect the glands, lobules, and lobes, carrying milk from the lobes to the nipple. The nipple is located in the middle of the areola, which is the darker area that surrounds the nipple. Blood and lymph vessels also run throughout the breast. Blood nourishes the cells. The lymph system drains bodily waste products. The lymph vessels connect to lymph nodes, the tiny, bean-shaped organs that help fight infection.

About breast cancer

Cancer begins when healthy cells in the breast change and grow out of control, forming a mass or sheet of cells called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor means the tumor can grow but will not spread.

Breast cancer spreads when the cancer grows into other parts of the body or when breast cancer cells move to other parts of the body through the blood vessels and/or lymph vessels. This is called metastasis.

This guide covers early-stage and locally advanced breast cancer, which includes stages I, II, and III. The stage of breast cancer describes where the cancer is located, how much the cancer has grown, and if or where it has spread.

Although breast cancer most commonly spreads to nearby lymph nodes, it can also spread further through the body to areas such as the bones, lungs, liver, and brain. This is called metastatic or stage IV breast cancer. For more information on this disease, see our Metastatic Breast Cancer guide.

If breast cancer comes back after initial treatment, it can recur locally, meaning in the breast and/or regional lymph nodes. It can also recur elsewhere in the body, called a distant recurrence or metastatic recurrence.

Types of breast cancer

Breast cancer can be invasive or noninvasive. Invasive breast cancer is cancer that spreads into surrounding tissues. Noninvasive breast cancer does not go beyond the milk ducts or lobules in the breast. Most breast cancers start in the ducts or lobes and are called ductal carcinoma or lobular carcinoma:

  • Ductal carcinoma. These cancers starts in the cells lining the milk ducts and make up the majority of breast cancers.

    • Ductal carcinoma in situ (DCIS). This is cancer that is located only in the duct.

    • Invasive or infiltrating ductal carcinoma. This is cancer that has spread outside of the duct.

  • Lobular carcinoma. This is cancer that starts in the lobules.

    • Lobular carcinoma in situ (LCIS). LCIS is located only in the lobules. LCIS is not considered cancer. However, LCIS is a risk factor for developing invasive breast cancer in both breasts (see the Risk Factors and Prevention section for more information.)

Less common types of breast cancer include:

  • Medullary

  • Mucinous

  • Tubular

  • Metaplastic

  • Papillary breast cancer

  • Inflammatory breast cancer is a faster-growing type of cancer that accounts for about 1% to 5% of all breast cancers.

  • Paget’s disease is a type of cancer that begins in the ducts of the nipple. Although it is usually in situ, it can also be an invasive cancer.

Breast cancer subtypes

Breast cancer is not a single disease, even among the same type of breast cancer. There are 3 main subtypes of breast cancer that are determined by doing specific tests on a sample of the tumor. These tests will help your doctor learn more about your cancer and recommend the most effective treatment plan.

Testing the tumor sample can find out if the cancer is:

  • Hormone receptor-positive. Breast cancers expressing estrogen receptors (ER) and/or progesterone receptors (PR) are called “hormone receptor-positive.” These receptors are proteins found in and on cells. Tumors that have estrogen receptors are called “ER-positive.” Tumors that have progesterone receptors are called “PR-positive.” Only 1 of these receptors needs to be positive for a cancer to be called hormone receptor positive. This type of cancer may depend on the hormones estrogen and/or progesterone to grow. Hormone receptor-positive cancers can occur at any age, but may be more frequent in women who have gone through menopause. About 60% to 75% of breast cancers have estrogen and/or progesterone receptors. Cancers without these receptors are called “hormone receptor-negative.”

  • HER2-positive. About 20% to 25% of breast cancers depend on the gene called human epidermal growth factor receptor 2 (HER2) to grow. These cancers are called “HER2-positive” and have excessive numbers of HER2 receptors or copies of the HER2 gene. The HER2 gene makes a protein that is found on the cancer cell and is important for tumor cell growth. This type of cancer may grow more quickly. HER2-positive cancers can be either hormone receptor-positive or hormone receptor-negative. Cancers that do not express HER2 are called “HER2-negative.”

  • Triple-negative. If a  tumor does not express ER, PR, and/or HER2, the tumor is called “triple-negative.” Triple-negative breast cancer makes up about 15% of invasive breast cancers. Triple-negative breast cancer seems to be more common among younger women, particularly younger black women. Triple-negative cancer is also more common in women with a mutation in the breast cancer genes 1 and 2, commonly called BRCA1 and BRCA2 genes. Experts recommend that all people with triple-negative breast cancer be tested for BRCA gene mutations. See the Risk Factors and Prevention section for more information on these genetic mutations.

Looking for More of an Introduction?

If you would like more of an introduction, explore these related items. Please note that these links will take you to other sections on Cancer.Net:

The next section in this guide is Statistics. It helps explain the number of people who are diagnosed with this disease and general survival rates. You may use the menu to choose a different section to read in this guide.