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Cancer stage is based on the size of the tumor, whether the cancer is invasive or non-invasive, whether lymph nodes are involved, and whether the cancer has spread beyond the breast.

The purpose of the staging system is to help organize the different factors and some of the personality features of the cancer into categories, in order to:

• best understand your prognosis (the most likely outcome of the disease)
• guide treatment decisions (together with other parts of your pathology report), since clinical studies of breast cancer treatments that you and your doctor will consider are partly organized by the staging system
• provide a common way to describe the extent of breast cancer for doctors and nurses all over the world, so that results of your treatment can be compared and understood

Stage 0

Stage 0 is used to describe non-invasive breast cancers, such as DCIS and LCIS. In stage 0, there is no evidence of cancer cells or non-cancerous abnormal cells breaking out of the part of the breast in which they started, or of getting through to or invading neighboring normal tissue.

Stage I

Stage I describes invasive breast cancer (cancer cells are breaking through to or invading neighboring normal tissue) in which:

• the tumor measures up to 2 centimeters, AND
• no lymph nodes are involved

Stage II

Stage II is divided into subcategories known as IIA and IIB.

Stage IIA describes invasive breast cancer in which:

• no tumor can be found in the breast, but cancer cells are found in the axillary lymph nodes (the lymph nodes under the arm), OR
• the tumor measures 2 centimeters or less and has spread to the axillary lymph nodes, OR
• the tumor is larger than 2 centimeters but not larger than 5 centimeters and has not spread to the axillary lymph nodes

Stage IIB describes invasive breast cancer in which:

• the tumor is larger than 2 but no larger than 5 centimeters and has spread to the axillary lymph nodes, OR
• the tumor is larger than 5 centimeters but has not spread to the axillary lymph nodes

Stage III

Stage III is divided into subcategories known as IIIA, IIIB, and IIIC.

Stage IIIA describes invasive breast cancer in which either:

• no tumor is found in the breast. Cancer is found in axillary lymph nodes that are clumped together or sticking to other structures, or cancer may have spread to lymph nodes near the breastbone, OR
• the tumor is 5 centimeters or smaller and has spread to axillary lymph nodes that are clumped together or sticking to other structures, OR
• the tumor is larger than 5 centimeters and has spread to axillary lymph nodes that are clumped together or sticking to other structures

Stage IIIB describes invasive breast cancer in which:

• the tumor may be any size and has spread to the chest wall and/or skin of the breast AND
• may have spread to axillary lymph nodes that are clumped together or sticking to other structures, or cancer may have spread to lymph nodes near the breastbone
• Inflammatory breast cancer is considered at least stage IIIB.

Stage IIIC describes invasive breast cancer in which:

• there may be no sign of cancer in the breast or, if there is a tumor, it may be any size and may have spread to the chest wall and/or the skin of the breast, AND
• the cancer has spread to lymph nodes above or below the collarbone, AND
• the cancer may have spread to axillary lymph nodes or to lymph nodes near the breastbone

Stage IV

Stage IV describes invasive breast cancer in which:

• the cancer has spread to other organs of the body -- usually the lungs, liver, bone, or brain

"Metastatic at presentation" means that the breast cancer has spread beyond the breast and nearby lymph nodes, even though this is the first diagnosis of breast cancer. The reason for this is that the primary breast cancer was not found when it was only inside the breast. Metastatic cancer is considered stage IV.