Causes

No one knows precisely what causes the disease. Indeed, about 4 out of 5 women diagnosed with breast cancer have no family history of the illness, and many have no known risk factors. Still, women with certain characteristics are more prone to developing breast cancer. Postmenopausal women, for instance, are more likely than young women to be affected. For more on this, see our section on risk factors.

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But regardless of how a breast cancer arises, it tends to spread (metastasize) by cells breaking away from the original tumor and traveling through the bloodstream or lymphatic system to remote parts of the body, where they grow into new tumors. If not checked by treatment, cancers of the breast, for instance, often invade the brain, liver, and bones.