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In the United States, breast cancer is the most common cancer in
women (excluding skin cancer). Men can also develop breast cancer, but
breast cancer in men is rare, accounting for less than 1% of all breast
cancers.
About the breast
The breast is mostly made up of fatty tissue. Within this tissue is a
network of lobes, which are made up of tiny, tube-like structures
called lobules that contain milk glands. Tiny ducts connect the glands,
lobules, and lobes, carrying the milk from the lobes to the nipple,
located in the middle of the areola (darker area that surrounds the
nipple). Blood and lymph vessels also run throughout the breast; blood
nourishes the cells, and 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 normal cells in the breast change and grow
uncontrollably, forming a mass called a tumor. A tumor can be benign
(noncancerous) or malignant (cancerous, meaning it can spread to other
parts of the body).
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.
Breast cancer most commonly spreads to the regional lymph nodes. The
lymph nodes can be axillary (located under the arm), cervical (located
in the neck), internal mammary (located under the chest bone), or
supraclavicular (located just above the collarbone). When it spreads
further through the body, it most commonly spreads to the bones, lungs,
and liver. Less commonly, breast cancer may spread to the brain. The
cancer can also recur (come back after treatment) locally in the skin,
in the same breast (if it was not removed as part of treatment), other
tissues of the chest, or elsewhere in the body.
Types of breast cancer
Most breast cancers start in the ducts or lobes. Almost 75% of all
breast cancers begin in the cells lining the milk ducts and are called
ductal carcinomas. Cancer that begins in the lobules is called lobular
carcinoma. The difference between ductal and lobular cancer is
determined by a pathologist (a doctor who specializes in interpreting
laboratory tests and evaluating cells, tissues, and organs to diagnose
disease), who examines a tumor sample removed during a biopsy (see Diagnosis).
If the disease has spread outside of the duct or lobule and into the
surrounding tissue, it is called invasive or infiltrating ductal or
lobular carcinoma. Cancer that is located only in the duct or lobule is
called in situ, meaning “in place.” How in situ disease grows and
spreads, as well as how it is treated, depends on whether it is ductal
carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS).
Most in situ breast cancers are DCIS. Currently, oncologists
recommend surgery to remove DCIS to help prevent the cancer from
becoming an invasive breast cancer and spreading to other parts of the
breast or the body. Radiation therapy and hormonal therapy may also be
recommended for DCIS (see Treatment for more information).
LCIS is not considered cancer and is usually monitored by the doctor.
LCIS in one breast is a risk factor for developing invasive breast
cancer in both breasts (see the Risk Factors section for more information).
Other, less common types of breast cancer include medullary, mucinous,
tubular, metaplastic, and papillary breast cancer, as well as other even
less common types. Inflammatory breast cancer
is a faster-growing type of cancer that accounts for about 1% to 5% of
all breast cancers. It may be misdiagnosed as a breast infection because
there is often swelling of the breast and redness of the breast skin
that starts suddenly. Paget’s disease is a type of cancer that begins in
the ducts of the nipple. The skin often appears scaly and may be itchy.
Although it is usually in situ, it can also be an invasive cancer.
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