Breast cancer begins in the breast tissue. It is characterized by the uncontrolled growth and spread of abnormal cells, which usually starts in the milk sacs or ducts. The tumour may be hard or soft and may sometimes be described as a thickening of the breast. Breast cancers are almost always painless.

Breast cancer is the most common cancer among women, other than skin cancer. It rarely affects men. One woman in nine will develop breast cancer by age 85. It is the second leading cause of cancer deaths in women, after lung cancer. It is the leading cause of cancer death among women aged 40 to 55.

The National Cancer Institute of Canada estimates that in 2007, an estimated 22,300 women will be diagnosed with breast cancer and 5,300 will die of it.