When someone is diagnosed with breast cancer, tests will be done to find out if the cancer has spread from the breast to other parts of the body. This is called staging and is an important step toward planning a treatment program.
Carcinoma in situ is Stage 0, the earliest stage of breast cancer and includes ductal carcinoma in situ or DCIS. In situ refers to cancerous cells that are confined within the duct where they originated.
The stages of invasive breast cancer range from 1 to 4 and are defined by the size of the tumor (T), and whether cancer has spread to any lymph nodes (N) or distant organs (metastasis) (M). Invasive, or infiltrating, refers to the movement of cancerous cells beyond the wall of the tissue where they originated (e.g., duct or lobule). This can still be diagnosed at a very early stage.