Breast cancer is a malignant tumor occurring in the gland epithelial tissue of the breast. Breast cancer is caused by abnormal growth of breast cells. These abnormal cells divide faster than normal cells and multiply, forming a bump or bulge. Cancer cells spread from the breast to the lymph nodes or other parts of the body.

The most common form of breast cancer is a cytopathic lesion of the lactation duct, also known as infiltrating ductal carcinoma. Breast cancer that originates from glandular tissue called lobules is called invasive lobular carcinoma. Breast cytopathies may also start in other cells and tissues within the breast.