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Breast cancer is a malignant (cancerous) tumor that starts from cells
of the breast. The disease occurs mostly in women, but men can get
breast cancer too. The information here refers only to breast cancer in
women. There is separate information about breast cancer in men
available in the American Cancer Society's document, Breast Cancer in Men.
Normal Breast Structure
In order to understand breast cancer, it is helpful to have
some basic knowledge about the normal structure of the breasts, as
shown in the picture below.
A woman’s breast is made up of glands that make breast milk
(lobules), ducts (small tubes that carry milk from the lobules to the
nipple), fatty and connective tissue, blood vessels, and lymph
(pronounced limf)
vessels. Most breast cancers begin in the cells that
line the ducts (ductal cancer), some begin in the lobules (lobular
cancer), and the rest in other tissues.

The Lymph System
The lymph system is important because it is one of the ways in
which breast cancers can spread. This system has several parts.
Lymph nodes are small, bean-shaped collections of immune
system cells that are connected by lymphatic vessels. Lymphatic vessels
are like small veins, except that they carry a clear fluid called lymph
(instead of blood) away from the breast. Breast cancer cells can enter
lymphatic vessels and begin to grow in lymph nodes.
Most lymph vessels of the breast lead to lymph nodes under the
arm. These are called axillary (ax-uh-lair-ee)
nodes. If breast cancer
cells reach the underarm lymph nodes and continue to grow, they cause
the nodes to swell. Once cancer cells have reached these nodes they are
more likely to spread to other organs of the body too. Knowing whether
cancer cells have spread to lymph nodes is important because if they
have, there is a higher chance that the cells have also gotten into the
bloodstream and spread to other places in the body. This could affect
the treatment plan.
Benign Breast Lumps
Most breast lumps are benign (be-nine); that is, they
are not
cancer. Benign breast tumors are abnormal growths, but they do not
spread outside of the breast and they are not life threatening. But
some benign breast lumps can increase a woman’s risk of getting breast
cancer.
Most lumps turn out to be caused by fibrocystic
(fi-bro-sis-tik)
changes. Cysts are fluid-filled sacs. Fibrosis is the
formation of scar-like tissue. Such changes can cause breast swelling
and pain. The breasts may feel lumpy, and sometimes there is a clear or
slightly cloudy nipple discharge. For more detail, please see the
document,
Non-cancerous Breast Conditions.
Breast Cancer Terms
Understanding key words as they relate to breast cancer can be
a challenge. Here are the key words used to describe breast cancer:
Carcinoma (car-sin-o-muh)
This is a term used to describe a cancer that begins in the
lining layer of organs such as the breast. Nearly all breast cancers
are carcinomas (either ductal carcinomas or lobular carcinomas).
Adenocarcinoma
(ad-uh-no-car-sin-O-muh)
An adenocarcinoma is a type of cancer that starts in glandular
tissue (tissue that makes and secretes a substance). The ducts and
lobules of the breast are glandular tissue (they make breast milk), so
cancers starting in these areas are sometimes called adenocarcinomas.
Carcinoma In Situ (in si-too)
This term is used for the early stage of cancer, when it is
still only in the layer of cells where it began. In breast cancer, in
situ means that the cancer cells are only in the ducts
(ductal
carcinoma in situ) or lobules (lobular carcinoma in situ). They have
not spread into deeper tissues in the breast or to other organs in the
body. They are sometimes referred to as non-invasive breast cancers.
Invasive (Infiltrating) Carcinoma
An invasive cancer is one that has already gone beyond the
layer of cells where it started (unlike carcinoma in situ). Most breast
cancers are invasive carcinomas -- either invasive ductal carcinoma or
invasive lobular carcinoma.
Sarcoma
Sarcomas are cancers that start from connective tissues such
as fat tissue or blood vessels. Sarcomas of the breast are rare.
Types of Breast Cancers
There are many types of breast cancer, though some of them are
very rare. Sometimes a breast tumor can be a combination of these types
and to have a mixture of invasive and in situ cancer.
Ductal carcinoma in situ (DCIS): This is
the most common type of non-invasive breast cancer. DCIS means that the
cancer is only in the ducts. It has not spread through the walls of the
ducts into the tissue of the breast. Nearly all women with cancer at
this stage can be cured. Often the best way to find DCIS early is with
a mammogram.
Lobular carcinoma in situ (LCIS): This
condition begins in the milk-making glands but does not go through the
wall of the lobules. Although not a true cancer, having LCIS increases
a woman's risk of getting cancer later. For this reason, it's important
that women with LCIS to follow the screening guidelines for breast
cancer (these are discussed later in this document).
Invasive (infiltrating) ductal carcinoma (IDC):
This is the most common breast cancer. It starts in a milk passage or
duct, breaks through the wall of the duct, and invades the tissue of
the breast. From there it can spread to other parts of the body. It
accounts for about 8 out of 10 invasive breast cancers.
Invasive (infiltrating) lobular carcinoma (ILC): This
cancer starts in the milk glands or lobules. It can spread to other
parts of the body. About 1 out of 10 invasive breast cancers are of
this type.
Inflammatory breast cancer (IBC): This
uncommon type of invasive breast cancer accounts for about 1% to 3% of
all breast cancers. Usually there is no single lump or tumor. Instead,
inflammatory breast cancer (IBC) makes the skin of the breast look red
and feel warm. It also gives the skin a thick, pitted appearance that
looks a lot like an orange peel. Doctors now know that these changes
are not caused by inflammation or infection, but by cancer cells
blocking lymph vessels in the skin. The breast may become larger,
firmer, tender, or itchy. IBC is often mistaken for an infection in its
early stages. Because there is no defined lump, it may not appear on a
mammogram, which may make it even harder to catch it early. It usually
has a higher chance of spreading and a worse outlook than invasive
ductal or lobular cancer.
There are also many other less common types of breast cancer.
You can get information about these through our toll-free number or on
our Web site.
Revised: 09/26/2007
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