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 - A pathologist will categorize each tumor based on its histological (microscopic anatomy) appearance and other criteria. The most common pathologic types of breast cancer are invasive ductal carcinoma, malignant cancer in the breast's ducts, and invasive lobular carcinoma, malignant cancer in the breast's lobules.
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 tumour - The currently accepted staging scheme for breast cancer is the TNM classification :
Tumor - There are five tumor classification values (Tis, T1, T2, T3 or T4) which depend on the presence or absence of invasive cancer, the dimensions of the invasive cancer, and the presence or absence of invasion outside of the breast (e.g. to the skin of the breast, to the muscle or to the rib cage underneath).
Lymph Node - There are four lymph node classification values (N0, N1, N2 or N3) which depend on the number, size and location of breast cancer cell deposits in lymph nodes.
Metastases - There are two metastatic classification values (M0 or M1) which depend on the presence or absence of breast cancer cells in locations other than the breast and lymph nodes (so-called distant metastases, e.g. to bone, brain, lung).