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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Breast cancer is the uncontrolled growth of abnormal cells that can develop in one of several areas of the breast, including the

  • ducts that carry milk to the nipple

  • small sacs that produce milk (lobules)

  • non-glandular tissue.

Breast cancer is considered invasive when the cancer cells have penetrated the lining of the ducts or lobules. That means the cancer cells can be found in the surrounding tissues, such as fatty and connective tissues or the skin. Noninvasive breast cancer (in situ) occurs when cancer cells fill the ducts but haven't spread into surrounding tissue.

These are the main forms of invasive breast cancer:

  • Invasive ductal carcinoma This type of breast cancer, which accounts for three-quarters of cases, develops in the milk ducts. It can break through the duct wall and invade the fatty tissue of the breast. It can then spread (metastasize) to other parts of the body through the bloodstream or lymphatic system.

  • Invasive lobular carcinoma This type of breast cancer accounts for about 15% of cases. It originates in the breast's milk-producing lobules. It can spread to the breast's fatty tissue and other places in the body.

  • Medullary, mucinous, and tubular carcinomas These slow-growing breast cancers account for about 8% of breast cancers.

  • Paget's disease This is a rare form of breast cancer. It starts in the milk ducts of the nipple and can spread to the dark circle around the nipple (areola). Women who get Paget's disease usually have a history of nipple crusting, scaling, itching, or inflammation.

  • Inflammatory carcinoma This is another rare form of breast cancer. It can seem like an infection, because there is usually no lump or tumor. The skin is red, warm, and looks pitted like an orange peel. Because it spreads quickly, inflammatory carcinoma is the most aggressive and difficult to treat of all breast cancers.

As more women have regular mammograms, doctors are detecting many noninvasive or precancerous conditions before they become cancer. These conditions include

  • ductal carcinoma in situ (DCIS) This occurs when cancer cells fill the ducts but haven't spread through the walls into fatty tissue. Nearly all women diagnosed at this early stage can be cured. Without treatment, about 25% of DCIS cases will lead to invasive breast cancer within 10 years.

  • lobular carcinoma in situ (LCIS) This is less of a threat than DCIS. It develops in the breast's milk-producing lobules. LCIS doesn't require treatment, but it does increase a woman's risk of developing cancer in other areas of both breasts.

A woman's risk of developing breast cancer increases with age; more than three out of four breast cancer cases occur in women over age 50. Other risk factors for breast cancer include

  • having close relatives, such as a mother, sister, or grandmother, who have had the disease

  • being of Ashkenazi Jewish descent

  • having had chest radiation for another cancer, such as Hodgkin disease

  • having already had the disease or certain other abnormalities of breast tissue

  • increased exposure to the female hormone estrogen by having a first menstrual period before age 13, entering menopause after age 51, or using estrogen replacement therapy for more than 5 years

  • never having been pregnant, or having a first pregnancy after age 30

  • being overweight, especially after menopause

  • drinking alcohol (cancer risk doubles with three or more drinks per day)

  • having a sedentary lifestyle with little regular exercise.

Although breast cancer is about 100 times more common in women than in men, men can develop the disease.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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