After diagnosis and treatment has begun, breast cancer is then "staged." During this process, specific information about your breast cancer will be used to determine what stage the breast cancer is at (its severity). Staging of the breast cancer is based on two main factors: the size of the tumor and the extent of the disease, or how far the cancer has spread to other parts of the body. The staging of your breast cancer will not be fully known until after surgery has been completed. This information is important because it determines what type of future treatment is best for you and how best to receive that treatment. The higher the stage number, the lower the chance for cure of your breast cancer.
Stage 0 refers to cancer that has not spread to any other tissues and has remained in the place where it started. This is also referred to as carcinoma in situ. Two examples of this type of breast cancer include lobular carcinoma in situ, where cancer cells are found in the lobule of the breast, and ductal carcinoma in situ, where cancer cells are found in the duct (see diagram below).
Stage 1 is diagnosed when the original tumor is 1 inch in size or less and the cancer is still contained in the breast, with no spread to other tissues in the body.
Stage 2 disease is when the tumor either is larger (greater than 1 inch) or is still small but has spread to the nearby lymph nodes located under the arm.
Stage 3 is when the tumor is larger than 1 inch and has spread to the nearby lymph nodes. This stage is referred to as locally advanced or invasive breast cancer. Stage 3 breast cancer can also be further classified in groups A to C, where group 3A disease is still relatively small and confined to the underarm lymph nodes and group 3C disease can be any size and has spread to lymph nodes in the chest.
Stage 4 is diagnosed as metastatic cancer, which means that the cancer has spread to other parts of the body. In breast cancer, common areas that the cancer has been found outside of the breast include the liver, bone, and lung.