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Breast Cancer Resource
What Is Breast Cancer?

What Is Breast Cancer?

Breast cancer is the abnormal growth of cells in the breast.

The body has genes that control the way our cells divide and grow. The body’s cells divide, grow and die. Sometimes theprocesses do not work as they should and the cells begin to grow out of control. They divide, grow, and grow and grow, forming growths called tumors. 

Most breast cancer tumors develop because an error, called a mutation, occurs in the genes of a normal breast cell. The genes are like an instruction booklet that tells cells how to divide, grow and heal themselves. Without the correct instructions, the cell malfunctions and if it divides, it will pass the mutation, or wrong instructions, on to the new cells. Genetic mutations can be spontaneous (they just happen) or they may be inherited (you were born with it).  Scientists have identified genes that are crucial to normal cell functioning, such as BRCA1 and BRCA2. When these genes are mutated, they can cause breast cancer or other types of cancer.

Tumors can be benign (non cancerous) or malignant (cancerous). They key difference between the two is that malignant tumors have cells that invade other tissues. Malignant tumors in the breast or surrounding tissues are breast cancer. Cells from these tumors can metastasize, which means they break away, travel to another part of the body and start a new tumor.  The cells of a benign tumor do not spread to other tissues. However, sometimes benign tumors are removed because they may cause other health risks. They typically do not reappear once removed.

Additional Resources:

National Cancer Institute:  Cancer Cells
American Cancer Society: What Causes Breast Cancer

Susan G. Komen for the Cure®: What is Breast Cancer
(On the home page, under the"Understanding Breast Cancer” drop-down menu, select “Breast Facts.” On the left sidebar, select “What is Breast Cancer?.”)

Susan G. Komen for the Cure®: Inherited Genetic Mutations
(On the home page, under the"Understanding Breast Cancer” drop-down menu, select “Risk Factors and Prevention.” On the left sidebar, select “Family History/Genetic Risks.”)

Types and Stages of Breast Cancer

Types of Breast Cancer

Breast cancer can develop in many different forms and types. Breast cancer can be defined by many characteristics including where it occurs in the breast, the size of the tumor (if present), its spread, or other factors that influence how the cancer grows.

To determine a course of treatment, the doctor needs to know the extent or stage of the disease (see below), as well as other factors that impact the prognosis or how well the tumor will react to different treatments.

For example, the doctor may order tests to determine if the cancer is hormone receptor-positive and/or HER2-positive. The hormone receptor test shows whether the cancer has receptors for certain hormones, estrogen and progesterone, that may help the tumor grow.  The HER2 test shows whether the tissue has a protein called human epidermal growth factor receptor-2 (HER2) or the HER2/neu gene. Having too much HER2 protein or too many copies of the gene in the tissue may increase the chance that the breast cancer will come back after treatment.

Some types of breast cancer, such as Paget’s disease, metaplastic breast cancer and inflammatory breast cancer, are very rare.  Paget’s disease forms in the nipple and in 95% of cases, it occurs with an underlying breast cancer.  Metaplastic tumors begin in the breast ducts, and then invade surrounding non-ductal tissues. Inflammatory breast cancer is discussed below.

Stages of Breast Cancer

The stage is determined by the size of the tumor and whether the cancer has spread. Staging may involve additional tests or may be determined after the initial surgery.

Stage 0 is carcinoma in situ. In situ is latin for “in place.” Ductal and lobular carcinomas that have not spread outside the duct or lobule are called in situ cancers.

Lobular carcinoma in situ (LCIS): Abnormal cells are in the lobules (the milk producing glands) of the breast. LCIS seldom becomes invasive cancer but increases the risk of cancer developing in both breasts.

Ductal carcinoma in situ (DCIS): (also called intraductal carcinoma) Abnormal cells occur in the lining of the milk ducts in the breast and have not spread outside the duct or invaded nearby breast tissue.

Stage I is an early stage of invasive breast cancer. The tumor is no more than 2 cm across. Cancer cells have not spread beyond the breast.

Stage II can include one of the following:

The tumor in the breast is no more than 2 cm across. The cancer had spread to the lymph nodes under the arm.
The tumor is between 2 and 5 cm. The cancer may have spread to the lymph nodes under the arm.
The tumor is larger than 5 cm and the cancer has NOT spread to the lymph nodes under the arm.

Stage III may be a larger tumor but the cancer has not spread beyond the breast and nearby lymph nodes. This is called locally advanced cancer. There are various subcategories to stage III cancer depending on the size and spread and location of the spread.

Inflammatory breast cancer (IBC) is a rare type of stage IIIB breast cancer. The breast may look red and swollen because cancer cells block the lymph vessels in the skin of the breast. Inflammatory breast cancer may also be indicated by an inverted nipple, sudden and unusual increase in breast size, a persistent itch or rash. IBC is rarely found with a routine mammogram or ultrasound unless a lump is present. A needle core biopsy, MRI or PET scan may aid in diagnosis.

Stage IV is distant metastatic breast cancer. The cancer has spread to other parts of the body.

Additional Resources:

American Cancer Society: How is breast cancer diagnosed?

Susan G. Komen for the Cure®: Diagnosis
(On the home page, under the"Understanding Breast Cancer” drop-down menu, select “Diagnosis.”)