Breast cancer occurs when the cells in the breast divide and grow out of control. Cancerous cells can invade and damage the surrounding tissues. If they continue to grow and spread, they could become life-threatening. Breast cancer usually begins in the ducts that carry milk to the nipple (called ductal carcinoma), or within the lobules (called lobular carcinoma). In rare cases, other cancer types can also occur in the breast, such as lymphomas (cancer of the lymphatic system) or sarcomas (cancer of the soft tissues).
Although a lump in the breast is a common symptom of breast cancer, not all breast cancers have obvious symptoms. For example, some lumps may be too small to be felt but can be detected with a screening mammogram or other tests. There are also some benign (non-cancerous) conditions that can cause lumps in the breast, such as cysts. If you notice any new lumps or other unusual breast changes, please consult with your doctor.