A woman's breast is made up of milk glands and milk ducts, surrounded by fatty tissue and connective supports. Uncontrolled growth of cells in any of these breast tissues can cause breast cancer.

Breast cancer is the most common cancer among women.. Chances of developing breast cancer rise dramatically as women age. At age 30, the chances of getting the disease are 1 in 209; at age 50, the risk of developing breast cancer goes up to 1 in 42; and over the age of 70, it becomes 1 in 25.

Breast cancer is rare in men, accounting for less than 1% of all cases. Breast cancer is the second most common cause of death from cancer in women.

The body's cells reproduce themselves throughout your lifetime, as tissues wear out and their cells are replaced in a controlled manner. Breast cancer – like all cancers – occurs when that control is lost and cells begin to divide at an unusually high rate.

No single trigger or cause has been identified for breast cancer. Certain risk factors exist, though, that increase a woman's chance of developing it:

• age – it's more common in women over 50
• family history – if a woman's mother or sister had the disease before Menopause, this is occasionally associated with one of two genes linked to breast cancer
• previous breast cancer
• family history of Ovarian Cancer
• age of pregnancy – women who haven't had children, or whose first child was born after age 30
• age of menstruation – starting periods at a young age (under 12 years old)
• entering menopause later (over age 55) increases breast cancer risks
• recent research suggests that women who start smoking regularly within 5 years of the onset of their menstrual periods are 70% more likely to develop breast cancer before the age of 50 than non-smokers
• having dense breast tissue
• radiation treatment to the chest, especially before 30 years of age
• alcohol consumption contributes to the risk of breast cancer
• hormone replacement therapy (HRT; estrogen plus Progesterone) increases the risk of breast cancer slightly after 5 years of therapy
• oral contraceptives increase risks slightly, if used over many years
• Obesity with excess caloric and fat intake
• physical inactivity

The increased risks of getting breast cancer associated with the above factors are often statistically quite small. In fact, for most women, the only risk factor they have is being over 50 years of age. Any concerns should be discussed with your doctor.

9 out of 10 times, women are the first to notice a lump or mass in their breast. It usually isn't painful, but can cause an unusual sensation in the area where the lump is.

When a tumour grows, the size or shape of the breast may change. As well, the nipple might draw in, or some of the skin will pull in, causing what looks like a dimple to appear. Other symptoms may include a lump or swelling in the armpit and redness or swelling of the breast.

A breast that develops a red rash should be evaluated for breast cancer even in the absence of a lump.

While these might be signs of breast cancer, they might also indicate another non-cancerous condition. In fact, about 8 out of 10 breast growths are non-cancerous. However, a doctor should be consulted to determine the exact cause of the lump.

The most serious complication of breast cancer is metastasis. That's when some cells from a tumour break off and move to other areas of the body, either through the blood or the lymphatic vessels – invading the tissue at new, possibly distant sites. When breast cancer cells metastasize, it's most commonly to the lymph nodes, lungs, liver, bones, brain, and skin. It can take years, even after the breast tumour is diagnosed and treated, before cancer that has spread from the original tumour appears.

Once metastatic tumours are discovered, chances are that the cancer has spread to other locations as well, even if they remain undetected.

A suspicious lump in your breast should be checked by your doctor. Aside from an extensive physical examination, the doctor will also want to do a mammogram, which is a low-level X-ray that can show abnormal areas in your breast. An ultrasound can sometimes be performed to see if the lump is a fluid-filled cyst or a solid growth. Even if the lump that can be felt is not visible on the mammogram, some sort of biopsy may need to be done.

Using a needle, fluid can be removed from a cyst to check if it contains cancerous cells. A solid lump must be analyzed under a microscope. This requires either a needle biopsy, a core biopsy, or a surgical biopsy to be performed.

For a needle biopsy, a fine needle is inserted into the lump and cells are drawn out with the needle. With a core biopsy, a sample of breast cells and tissues is removed for examination. A surgical biopsy is an operation that removes the whole lump and some normal tissue around it. It can be done under a local or general anesthetic.

Cancerous cells can be tested for the presence of estrogen or progesterone receptors. Other tests, including X-rays, bone scans, ultrasounds, CT scans, or an MRI, may be done to see if the cancer has spread to other organs and tissues.