Breast cancer starts when cells in the breast begin to grow out of control. These cells usually form a tumor that can often be seen on an x-ray or felt as a lump. The tumor is malignant (cancer) if the cells can grow into surrounding tissues or spread (metastasize) to distant areas of the body. Breast cancer occurs almost entirely in women, but men can get breast cancer too.
Although many types of breast cancers can cause a lump in the breast, not all do. Many breast cancers are found on screening mammograms which can detect cancers at an earlier stage, often before they can be felt, and before the symptoms develop.
What is Breast Cancer? American Cancer Society (updated Sept. 21, 2017), https://www.cancer.org/cancer/breast-cancer/about/what-is-breast-cancer.html