Cancer cells in the breast form a malignant (cancerous) tumor that appears as a lump. Most breast cancers are carcinomas or cancers that begin in the lining of organs.

Excluding skin cancer, breast cancer is the most common type of cancer among women in the U.S. (American Cancer Society 2015b).

The normal breast is composed of 15 to 20 sections called lobes. Each lobe is made up of smaller sections called lobules. Lobules contain small groups of glands that produce breast milk. Thin tubes, called ducts, carry the milk from the lobules to the nipple. The breast also contains blood vessels and lymph vessels. Lymph vessels are similar to blood vessels, but instead of circulating blood throughout the body, they circulate a clear fluid called lymph. The lobes, lobules, ducts, blood vessels, and lymph vessels are surrounded by fatty and connective tissue called stroma.

Cancer cells in the breast form a malignant (cancerous) tumor that appears as a lump. However, not all breast lumps are cancerous. In fact, most breast lumps are benign (noncancerous). Benign breast lumps are abnormal growths that do not spread outside the breast and are generally not life threatening. They can be caused by fibrocystic changes, a combination of cysts (fluid-filled sacs) and fibrosis (the formation of scar-like tissue) in the breast. Benign lumps can be painful and may cause swelling, particularly when a woman is just about to start her menstrual cycle. These changes can also make the breast feel lumpy and cause a clear or slightly cloudy discharge from the nipple.

Most breast cancers are carcinomas, or cancers that begin in the lining of organs. There are several types of breast cancer. "In situ" means the cancer is confined to the tissue where it began. "Invasive" refers to cancer that has spread from the layer of tissue where it began into surrounding healthy tissue.

Symptoms of Breast Cancer

The most common symptom of breast cancer is a lump or mass in the breast. Cancerous lumps tend to be painless and hard and have uneven edges, though this isn’t always the case. It is best to have any new changes in the breast looked at by a physician.

Other symptoms include:

  • Swelling in the breast
  • Skin irritation or dimpling
  • Breast or nipple pain
  • The nipple inverting (turning inward on itself)
  • Scaliness, redness, or thickening of the breast or nipple
  • Nipple discharge that isn’t breast milk

With invasive cancers, it is possible for the cancer to spread to nearby lymph nodes under the arm or around the collarbone. A tumor may form there before the breast tumor is large enough to be felt.

Causes of Breast Cancer

Although the exact causes of breast cancer remain unknown, certain risk factors—things that increase an individual's chances of developing a cancer—have been identified. While risk factors may be useful in identifying high-risk individuals, they do not determine whether a person develops a disease. Some risk factors, such diet, are within an person’s control, while others, such as age, are not.

Some risk factors breast cancer include:

  • Gender: Women are 100 times more likely to develop breast cancer than men (one percent of men get breast cancer) (American Cancer Society 2015a).
  • Age: As women get older, they are more likely to develop breast cancer.
  • Genetic mutations: Certain genetic mutations (changes in the DNA sequence of a gene), such as mutations in the BRCA1 and BRCA2 genes, can increase a woman’s risk of developing breast cancer.
  • Family history: Women with close blood relatives who have or have had breast cancer are at increased risk of developing the disease.
  • Personal history: Women who have or have had breast cancer in one breast have an increased risk of developing breast cancer in the other breast or another part of the same breast. This is not the same as having a recurrence. A recurrence is when the same cancer returns
  • Race: Overall, Caucasian women are slightly more likely to develop breast cancer than African American women.
  • Dense breast tissue: Women, particularly younger women, with dense breast tissue have an increased risk of developing breast cancer.
  • Certain benign breast problems: Having certain benign breast changes may increase a woman’s chances of developing breast cancer.
  • Lobular carcinoma in situ (LCIS): When a woman is diagnosed with lobular carcinoma in situ, it means cells that look like cancer cells are in the lobules but cannot break through the walls of the duct or spread to other tissues or organs. LCIS can increase a woman’s chances of developing breast cancer later.
  • Menstrual periods: Women who started menstruating early (before age 12) or underwent menopause late (after age 55) have a slightly increased risk of developing breast cancer.
  • Breast radiation early in life
  • Treatment with DES: Women who were treated with the drug DES (diethylstilbestrol) while pregnant have a slightly increased risk of developing breast cancer.
  • Not having children or having children later: There is a slight increase in risk for women who do not have children or begin having them after age 30 (American Cancer Society 2015b).
  • Certain forms of birth control: Breast cancer risk is slightly higher for women actively using birth control pills or depot-medroxyprogesterone acetate (DMPA or Depo-Provera®), an injectable form of birth control.
  • Breastfeeding versus not breastfeeding: Some studies have linked breastfeeding with a slight reduction in breast cancer risk for women, particularly if the woman breastfeeds for 1 ½ to 2 years (American Cancer Society 2015b).
  • Hormone therapy after menopause: Treatment with estrogen and progesterone together (sometimes referred to as combined hormone therapy) after menopause increases a woman’s chances of developing breast cancer. Taking estrogen by itself does not seem to increase a woman’s chances of developing breast cancer, though it can increase her chances of developing other types of cancer.
  • Alcohol consumption: The more alcohol a woman consumes on a daily basis, the higher her chances of developing the disease.
  • Being overweight or obese

Patients with a strong family history of breast cancer may be eligible for genetic counseling. For more information, patients can visit Genetic Counseling or talk to their physician to request a referral.

Meet Maria, who fought breast cancer and is now becoming a doctor.


American Cancer Society. 2015a. Detailed Guide: Breast Cancer. PDF.

Overview Guide: Breast Cancer Overview. PDF.

National Cancer Institute. 2012. What You Need To Know About™ Breast Cancer. PDF. Bethesda: National Cancer Institute, National Institutes of Health, U.S. Department of Health and Human


Departments and Programs Who Treat This Condition


Breast Cancer Program

BMC’s Breast Health Program provides collaborative, multidisciplinary care to patients with breast cancer from the first consultation through treatment and follow-up visits.