Breast cancer is divided into five stages. Stages 0-2 are considered "early", stage 3 considered "advanced", and stage 4 "late". Staging categories are important for predicting future prognosis, and determine optimal treatment recommendations.
Stage 0 is DCIS, or ductal carcinoma in situ. Breast cancer arises from the cells that line the milk ducts. When the cancerous cells are still contained inside the duct, it is diagnosed as DCIS. This can only be determined by a pathologist doctor looking at the tissue under a microscope. In general, when the DCIS lesion is small, there is no need to suspect cancer spread outside the breast.
Stage 1 is invasive or infiltrating cancer. Here, the cancer cells have broken through the duct wall and are found outside the ducts as well. In this case, doctors need to determine whether the cancer has spread to the lymph nodes. Stage 1 breast cancer must be equal or smaller than 2 cm in its invasive component, AND have no spread to lymph nodes. Often, the tissue removed at surgery contain DCIS in addition to the invasive cancer. However, only the dimensions of the invasive cancer count. If the patient needs to have multiple surgeries and the invasive cancer is found at more than one operation, usually the dimensions are added together to arrive at the final size.
Stage 2 has two subcategories. In stage 2A, the invasive cancer can be 2 cm or less and has spread to axillary (armpit) lymph node(s), i.e. positive node(s). Also, the invasive cancer can be as large as 5 cm, but has not spread to lymph nodes, i.e. negative nodes. In stage 2B, the invasive cancer is between 2cm and up to 5 cm and has spread to nodes. Here, cancer may measure even larger than 5 cm if it has not spread to nodes.
Stage 3 includes invasive cancer larger than 5 cm that has spread to lymph nodes. Also, cancer of any size that heavily involves the axillary lymph nodes to the point that these nodes are bulky and stuck together or stuck to other structures in the axilla (armpit) are in this stage. Tumor spread to lymph nodes either above or below the clavicle bone, or to nodes underneath the sternum (breast bone), also falls into this category. Furthermore, if the cancer of any size is attached to the chest wall (pectoralis muscle and/or ribs), it qualifies as stage 3. Inflammatory cancer, where the skin of the breast is red and swollen, is classified in this stage, regardless of size.
Stage 4 is invasive cancer found outside the breast and axillary lymph nodes, or "metastatic" to distant sites. At this stage, it does not matter how large the primary cancer in the breast is. Nor does it matter whether axillary/clavicle/breast bone lymph nodes have cancer or not. The most common sites for metastasis for breast cancer are bone and liver, followed by lungs and brain. Standard testing include bone scan and CT scan of the chest, abdomen and pelvis. More recently, PET scan is often done to look for cancer spread. Sometimes, a brain MRI or CT is also useful.