According to BreastCancer.org, 7 key pieces of information are used for staging:
The extent (size) of the tumor (T): How large is the cancer? Has it grown into nearby areas?
The spread to nearby lymph nodes (N): Has the cancer spread to nearby lymph nodes? If so, how many?
The spread (metastasis) to distant sites (M): Has the cancer spread to distant organs such as the lungs or liver?
Estrogen Receptor (ER) status: Does the cancer have the protein called an estrogen receptor?
Progesterone Receptor (PR) status: Does the cancer have the protein called a progesterone receptor?
HER2 status: Does the cancer make too much of a protein called HER2?
Grade of the cancer (G): How much do the cancer cells look like normal cells?
T followed by a number from 0 to 4 describes the main (primary) tumor's size and if it has spread to the skin or to the chest wall under the breast. Higher T numbers mean a larger tumor and/or wider spread to tissues near the breast.
TX: Primary tumor cannot be assessed.
T0: No evidence of primary tumor.
Tis: Carcinoma in situ (DCIS, or Paget disease of the breast with no associated tumor mass)
T1 (includes T1a, T1b, and T1c): Tumor is 2 cm (3/4 of an inch) or less across.
T2: Tumor is more than 2 cm but not more than 5 cm (2 inches) across.
T3: Tumor is more than 5 cm across.
T4 (includes T4a, T4b, T4c, and T4d): Tumor of any size growing into the chest wall or skin. This includes inflammatory breast cancer.
NX: Nearby lymph nodes cannot be assessed (for example, if they were removed previously).
N0: Cancer has not spread to nearby lymph nodes.
N0(i+): The area of cancer spread contains fewer than 200 cells and is smaller than 0.2 mm. The abbreviation "i+" means that a small number of cancer cells (called isolated tumor cells) were seen in routine stains or when a special type of staining technique, called immunohistochemistry, was used.
N0(mol+): Cancer cells cannot be seen in underarm lymph nodes (even using special stains), but traces of cancer cells were detected using a technique called RT-PCR. RT-PCR is a molecular test that can find very small numbers of cancer cells.
N1: Cancer has spread to 1 to 3 axillary (underarm) lymph node(s), and/or cancer is found in internal mammary lymph nodes (those near the breast bone) on sentinel lymph node biopsy.
N1mi: Micrometastases (tiny areas of cancer spread) in the lymph nodes under the arm. The areas of cancer spread in the lymph nodes are at least 0.2mm across, but not larger than 2mm.
N1a: Cancer has spread to 1 to 3 lymph nodes under the arm with at least one area of cancer spread greater than 2 mm across.
N1b: Cancer has spread to internal mammary lymph nodes on the same side as the cancer, but this spread could only be found on sentinel lymph node biopsy (it did not cause the lymph nodes to become enlarged).
N1c: Both N1a and N1b apply.
N2: Cancer has spread to 4 to 9 lymph nodes under the arm, or cancer has enlarged the internal mammary lymph nodes
N2a: Cancer has spread to 4 to 9 lymph nodes under the arm, with at least one area of cancer spread larger than 2 mm.
N2b: Cancer has spread to one or more internal mammary lymph nodes, causing them to become enlarged.
N3: Any of the following:
N3a: either:
Cancer has spread to 10 or more axillary lymph nodes, with at least one area of cancer spread greater than 2 mm,
OR
Cancer has spread to the lymph nodes under the collarbone (infraclavicular nodes), with at least one area of cancer spread greater than 2 mm.
N3b: either:
Cancer is found in at least one axillary lymph node (with at least one area of cancer spread greater than 2 mm) and has enlarged the internal mammary lymph nodes,
OR
Cancer has spread to 4 or more axillary lymph nodes (with at least one area of cancer spread greater than 2 mm), and to the internal mammary lymph nodes on sentinel lymph node biopsy.
N3c: Cancer has spread to the lymph nodes above the collarbone (supraclavicular nodes) on the same side of the cancer with at least one area of cancer spread greater than 2 mm.
M categories for breast cancer
M followed by a 0 or 1 indicates whether the cancer has spread to distant organs -- for example, the lungs, liver, or bones.
M0: No distant spread is found on x-rays (or other imaging tests) or by physical exam.
cM0(i+): Small numbers of cancer cells are found in blood or bone marrow (found only by special tests), or tiny areas of cancer spread (no larger than 0.2 mm) are found in lymph nodes away from the underarm, collarbone, or internal mammary areas.
M1: Cancer has spread to distant organs (most often to the bones, lungs, brain, or liver) as seen on imaging tests or by physical exam, and/or a biopsy of one of these areas proves cancer has spread and is larger than 0.2mm.
Further details can be found at: Breast Cancer Staging | Susan G. Komen®
