Breast cancer is a disease in which cancer cells grow in the breast. Normally, the cells of the breast divide in a regulated manner. If cells keep dividing when new cells are not needed, a mass of tissue forms. This mass is called a tumour. A tumour can be benign or malignant.

A benign tumour is not cancer and will not spread to other parts of the body. A malignant tumour is cancer. Cancer cells divide and damage tissue around them. They can enter the bloodstream and spread to other parts of the body. This can be life threatening.

Breast cancer is the most common form of cancer found in women—it is estimated that in Canada over 18,000 women are diagnosed with breast cancer every year. Although the majority of breast cancer cases occur in women, men can develop it as well; approximately 180 men develop breast cancer in Canada each year.

In Canada, Caucasian and African-Canadian women have the highest rates of breast cancer. The lowest rates occur among Korean, Native, and Vietnamese women.

Breast Anatomy and How Breast Cancer Develops

The breast consists of lobes, lobules, and bulbs that are connected by ducts. The breast also contains blood and lymph vessels. These lymph vessels lead to structures that are called lymph nodes. Clusters of lymph nodes are found under the arm, above the collarbone, in the chest, and in other parts of the body. Together, the lymph vessels and lymph nodes make up the lymphatic system, which circulates a fluid called lymphatic fluid throughout the body.


When breast cancer spreads outside the breast, cancer cells are most often found under the arm in the lymph nodes. In many cases, if the cancer has reached the lymph nodes, cancer cells may have also spread to other parts of the body via the lymphatic system or through the bloodstream.


Breast cancer can develop in different ways, and may affect different parts of the breast. The location of cancer will affect the progression of cancer and the treatment.

Most breast cancers are carcinomas—malignant tumours that grow out of the surface or lining of the glandular tissues of the breast. Other very rare types of breast cancer are formed in the surrounding and supporting tissues, and your doctor may call these sarcomas, acinar tumours, or lymphomas.

Breast cancer is divided mainly into the pre-invasive or “in-situ” form or the invasive or infiltrating form. The pre-invasive form is restricted to the breast itself and has not yet invaded any of the lymphatics or blood vessels that surround the breast tissue. Therefore, it does not spread to lymph nodes or other organs in the body. Treatments are generally local only and cure almost all patients.


Ductal carcinoma in situ (DCIS) - the most common pre-invasive breast cancer. More commonly seen now because this form is generally seen on a mammogram, and is identified by unusual calcium deposits or puckering of the breast tissue (called stellate appearance). If left untreated, DCIS will progress to invasive breast cancer.

Lobular carcinoma in situ (LCIS) - unlike DCIS, LCIS is not really cancer at all. Most physicians consider the finding of LCIS to be accidental, and it is thought to be a marker for breast cancer risk. That is, women with LCIS seem to have a 7-10 times increased risk of developing some form of breast cancer (usually invasive lobular carcinoma) over the next 20 years. LCIS does not warrant treatment by surgery or radiation therapy. Close follow up is most commonly indicated and LCIS is not easily seen on mammogram.


Ductal carcinoma - This is the most common form of breast cancer and accounts for 70% of breast cancer cases. This form develops in the milk ducts.
Lobular carcinoma – Originates in the milk-producing lobules of the breast. Can spread to the fatty tissue and other parts of the body.
Medullary, mucinous, and tubular carcinomas – Three relatively slower-growing types of breast cancer.
Inflammatory carcinoma – The fastest-growing and most difficult type of breast cancer to treat. This cancer invades the lymphatic vessels of the skin and can be very extensive. It is very likely to spread to the local lymph nodes.
Paget’s Disease – Cancer of the areola and nipple. It is very rare (about 1% of all breast cancers). In general, women who develop this type of cancer have had a history of nipple crusting, scaling, itching, or inflammation.

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