After diagnosis, breast cancer is staged. Staging is a process that gathers information about the cancer to be used to determine its severity.
Staging of breast cancer is based on 2 main factors: the size of the tumour and the extent of the disease (how far the cancer has spread to other parts of the body). The staging is not fully known until after surgery has been completed. This information is important because it, along with other information about the tumour (such as growth rate, sensitivity to hormones, and HER2 status), it helps to determine what type of future treatment is best and how that treatment is best received. The higher the stage number, the lower the chance for a cure.
Stage 0 refers to cancer that has remained in the place where it started has not spread to any other tissues. 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 milk duct.
Stage I is diagnosed when the original tumour is 2 centimetres in size or less and the cancer is still contained in the breast, with no spread to other tissues in the body.
Stage II disease is separated into 2 stages, depending on the size of the tumour and whether the lymph nodes under the arm are involved.
In Stage III cancer, which is separated into 3 main stages, tumour cells are found in the nearby lymph nodes (under the arm or near the breast bone) and the nodes are attached to each other or other structures, or it is has spread to the chest wall or skin of the breast.
If it spreads to the skin, it is called inflammatory breast cancer, a less common form of breast cancer.
Stage IV 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.