Breast cancer is a cancer that starts in the cells of the breast in women and men. Worldwide, breast cancer is the second most common type of cancer after lung cancer (10.4% of all cancer incidence, both sexes counted) and the fifth most common cause of cancer death. In 2005, breast cancer caused 502,000 deaths worldwide (7% of cancer deaths; almost 1% of all deaths).

Because the breast is composed of identical tissues in males and females, breast cancer also occurs in males. Incidences of breast cancer in men are approximately 100 times less common than in women, but men with breast cancer are considered to have the same statistical survival rates as women.

Classification

Main: Breast cancer classification

Breast cancers are described along four different classification schemes, or groups, each based on different criteria and serving a different purpose:

• Pathology - Each tumor is classified by its histological (microscopic anatomy) appearance and other criteria.
• Grade of tumor - The histological grade of a tumor is determined by a pathologist under a microscope. A well-differentiated (low grade) tumor resembles normal tissue. A poorly differentiated (high grade) tumor is composed of disorganized cells and, therefore, does not look like normal tissue. Moderately differentiated (intermediate grade) tumors are somewhere in between.
• Protein & gene expression status - Currently, all breast cancers should be tested for expression, or detectable effect, of the estrogen receptor (ER), progesterone receptor (PR) and HER2/neu proteins. These tests are usually done by immunohistochemistry and are presented in a pathologist's report. The profile of expression of a given tumor helps predict its prognosis, or outlook, and helps an oncologist choose the most appropriate treatment. More genes and/or proteins may be tested in the future.
• Stage of a tumor - The currently accepted staging scheme for breast cancer is the TNM classification. This considers the Tumor itself, whether it has spread to lymph Nodes, and whether there are any Metastases to locations other than the breast and lymph nodes.

Breast cancer is usually, but not always, primarily classified by its histological appearance. Rare variants are defined on the basis of physical exam findings. For example, Inflammatory breast cancer (IBC), a form of ductal carcinoma or malignant cancer in the ducts, is distinguished from other carcinomas by the inflamed appearance of the affected breast. In the future, some pathologic classifications may be changed. For example, a subset of ductal carcinomas may be re-named basal-like carcinoma (part of the "triple-negative" tumors).Fact: date=February 2008