**1. Introduction**

68 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

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Cancer is a group of diseases that leads to uncontrolled cell division and eventually forms a lump or mass called a tumor. They are classified and named after the part of the body where the tumor originates. Breast cancer begins in breast tissue, which is made up of glands for milk production, called lobules, and the ducts that connect lobules to the nipple. The remainder of the breast is made up of fatty, connective, and lymphatic tissue. On the basis of origin, it is of two types (i) ductal and (ii) lobular. Ductal carcinoma constitutes 80-90% and lobular carcinoma constitutes 10-20% breast cancer cases.

Breast cancer is one of the most frequently diagnosed cancers in women worldwide, comprising 16% of all female cancers cases. It is estimated that this disease will affect one in eight females in America during their lifetime. It is estimated that occurrence of female breast cancer is 28% of cancers from all sites in U.S.A, and the relative risk of ever developing breast cancer is 0.125 (1 in 8) (American Cancer Society, 2009). Although breast cancer is thought to be a disease of the developed world, a majority (69%) of all breast cancer deaths occurs in developing countries (WHO Global Burden of Disease, 2004) and relative survival is poor in underdeveloped and developing countries (Coleman et al., 2008). The relative risk of developing breast cancer in the lifetime of women in the developed and developing countries is 0.048 (1 in 21) and 0.018 (1 in 56) respectively. In India, breast cancer is the leading cancer among women (Fig. 1) and the relative risk is 0.033 (1 in 30) (NCRP, 2008).
