What is Breast Cancer

Breast cancer is a malignant cancerous growth that begins in the tissues of the breast. Breast cancer is the most common cancer in women, but it can also appear in men.  Breast cancer affects one in eight women in the U.S. Cancer occurs as a result of abnormal changes in the genes responsible for regulating the growth of cells and keeping them healthy.  Normally, the cells in our bodies replace themselves through an orderly process of cell growth: healthy new cells take over as old ones die out.   A changed cell gains the ability to keep dividing without control or order, producing more cells just like it and forming a tumor.  Breast cancer is always caused by a genetic abnormality.  However, only 5-10% of cancers are due to an abnormality inherited from your mother or father. 
About 90% of breast cancers are due to genetic abnormalities that happen as a result of the aging process.   Breast cancer is a malignant tumor that has developed from cells in the breast.  Usually breast cancer either begins in the cells of the lobules, which are the milk-producing glands, or the ducts, the passages that drain milk from the lobules to the nipple.  In situ is a Latin phrase meaning in place, or in this instance, it means the abnormal cells are contained in one place (not spread into other tissue.)  DCIS does not always progress to invasive cancer, where the cells invade breast tissue outside the ducts. DCIS accounts for 85-95% of breast cancers.