If the first site of a cancerous growth or tumour is in the breast, the condition is called breast cancer.
Inherited causes can be thought of as internal factors. They are part of the genetic makeup we received from our parents and do not change through life. Examples of inherited causes include genetic mutations and differences in the way our bodies repair cell DNA after it has been damaged.
The breasts consist mainly of fatty tissue that contains thousands of tiny glands—or lobules—whose function is to produce milk if you carry a pregnancy to term. It's quite normal for your breasts to differ slightly in shape and size and change during your menstrual cycle, in pregnancy and as you age.
Many of the most common breast changes are not signs of cancer, but it's important to be breast aware, so that you know how your breasts normally look and feel and report any changes to your health care provider.
Breast tissue covers a larger area than just the breast: it extends up to the collarbone and from the armpit to the breastbone. Various kinds of breast cancer can occur in different tissues in the breast.
Breast cancer metastasis
Breast cancer can also spread beyond the breast area if cells break off from the growth or tumour and travel through the blood stream or lymphatic system to other parts of the body (this condition is called metastasis).
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Source
American Cancer Society. What is Breast Cancer? Accessed July 31, 2011.