Breast cancer is staged using the American Joint Committee on Cancer (AJCC) TNM system, which is based on 3 key pieces of information:
The size of the breast tumor (T) and if it has grown into nearby areas whether the cancer has reached nearby lymph nodes (N).
Whether the cancer has metastasized (spread to other parts of the body) (M).
Numbers or letters after T, N, and M provide more details about each of these factors.
Stage 0
Tis, N0, M0
This is ductal carcinoma in situ (DCIS), a pre-cancer of the breast.
Paget disease of the nipple (without an underlying tumor mass) is also stage 0.
In all cases the cancer has not spread to lymph nodes or distant sites. Stage IA
T1, N0, M0 The tumor is 2 cm or less across (T1) and has not spread to lymph nodes (N0) or distant sites (M0).
Stage IB
T0 or T1, N1mi, M0 The tumor is 2 cm or less across (T0 or T1) with micrometastases in 1 to 3 axillary lymph nodes (and is greater than 0.2mm across and/or more than 200 cells but is not larger than 2 mm)(N1mi). The cancer has not spread to distant sites (M0).
Stage IIA T0 or T1, N1 (but not N1mi), M0:
The tumor is 2 cm or less across (or is not found) (T1 or T0) and either:
It has spread to 1 to 3 axillary lymph nodes, with the cancer in the lymph nodes larger than 2 mm across (N1a),
OR
Tiny amounts of cancer are found in internal mammary lymph nodes on sentinel lymph node biopsy (N1b),
OR
It has spread to 1 to 3 axillary lymph nodes and to internal mammary lymph nodes (found on sentinel lymph node biopsy) (N1c).
The cancer has not spread to distant sites (M0).
T2, N0, M0 The tumor is larger than 2 cm but less than 5 cm across (T2) but hasn’t spread to the lymph nodes (N0). The cancer has not spread to distant sites (M0).
Stage IIB
T2, N1, M0 The tumor is larger than 2 cm but less than 5 cm across (T2). It has spread to 1 to 3 axillary lymph nodes and/or tiny amounts of cancer are found in internal mammary lymph nodes on sentinel lymph node biopsy (N1). The cancer hasn’t spread to distant sites (M0).
T3, N0, M0
The tumor is larger than 5 cm across but does not grow into the chest wall or skin (T3). The cancer has not spread to the lymph nodes (N0) or to distant sites (M0).
Stage IIIA
T0 to T2, N2, M0 The tumor is not more than 5 cm across (or cannot be found) (T0 to T2). It has spread to 4 to 9 axillary lymph nodes, or it has enlarged the internal mammary lymph nodes (N2). The cancer hasn’t spread to distant sites (M0).
T3, N1 or N2, M0
The tumor is larger than 5 cm across but does not grow into the chest wall or skin (T3). It has spread to 1 to 9 axillary nodes, or to internal mammary nodes (N1 or N2). The cancer hasn’t spread to distant sites (M0).
Stage IIIB
T4, N0 to N2, M0 The tumor has grown into the chest wall or skin (T4), and one of the following applies:
It has not spread to the lymph nodes (N0).
It has spread to 1 to 3 axillary lymph nodes and/or tiny amounts of cancer are found in internal mammary lymph nodes on sentinel lymph node biopsy (N1).
It has spread to 4 to 9 axillary lymph nodes, or it has enlarged the internal mammary lymph nodes (N2).
The cancer hasn’t spread to distant sites (M0).
Inflammatory breast cancer is classified as T4d and is at least stage IIIB. If it has spread to many nearby lymph nodes (N3) it could be stage IIIC, and if it has spread to distant lymph nodes or organs (M1) it would be stage IV.
Stage IIIC any T, N3, M0 The tumor is any size (or can’t be found), and one of the following applies:
Cancer has spread to 10 or more axillary lymph nodes (N3). Cancer has spread to the lymph nodes under the collar bone (infraclavicular nodes) (N3). Cancer has spread to the lymph nodes above the collar bone (supraclavicular nodes) (N3). Cancer involves axillary lymph nodes and has enlarged the internal mammary lymph nodes (N3). Cancer has spread to 4 or more axillary lymph nodes, and tiny amounts of cancer are found in internal mammary lymph nodes on sentinel lymph node biopsy (N3). The cancer hasn’t spread to distant sites (M0).
Stage IV any T, any N, M1 The cancer can be any size (any T) and may or may not have spread to nearby lymph nodes (any N). It has spread to distant organs or to lymph nodes far from the breast (M1). The most common sites of spread are the bones, liver, brain, or lungs.
Details of the TNM staging system
The TNM staging system classifies cancers based on 3 areas called the T, N, and M categories:
T categories for breast cancer
The letter T followed by a number from 0 to 4 describes the main (primary) tumor’s size and 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, LCIS, or Paget disease of the nipple 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.
N categories for breast cancer
The letter N followed by a number from 0 to 3 indicates whether the cancer has spread to lymph nodes near the breast and, if so, how many lymph nodes are involved.
Lymph node staging for breast cancer is based on how the nodes look under the microscope, and has changed as technology has improved. Newer methods have made it possible to find smaller and smaller collections of cancer cell.
If the area of cancer spread is at least 0.2 mm (or 200 cells), but still not larger than 2 mm, it is called a micrometastasis. Micrometastases are counted only if there aren’t any larger areas of cancer spread. Areas of cancer spread larger than 2 mm are known to affect outlook and do change the N stage. These larger areas are sometimes called macrometastases, but are more often just called metastases.
NX: Nearby lymph nodes cannot be assessed.
N0: Cancer has not spread to nearby lymph nodes.
N0(i+): The area of cancer spread contains less 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, 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 tiny amounts of cancer are found in internal mammary lymph nodes (those near the breast bone) on sentinel lymph node biopsy.
N1mi: Micrometastases (tiny areas of cancer spread) in 1 to 3 lymph nodes under the arm. The areas of cancer spread in the lymph nodes are 2 mm or less across (but at least 200 cancer cells or 0.2mm across).
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, but this spread could only be found on sentinel lymph node biopsy.
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 2mm, OR
Cancer has spread to the lymph nodes under the collar bone (infraclavicular nodes), with at least one area of cancer spread greater than 2mm.
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 tiny amounts of cancer are found in internal mammary lymph nodes on sentinel lymph node biopsy.
N3c: Cancer has spread to the lymph nodes above the collar bone (supraclavicular nodes) with at least one area of cancer spread greater than 2mm.
M categories for breast cancer
The letter M followed by a 0 or 1 indicates whether the cancer has spread to distant organs — for example, the lungs or bones.
MX: Distant spread (metastasis) cannot be assessed.
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, collar bone, or internal mammary areas.
M1: Cancer has spread to distant organs (most often to the bones, lungs, brain, or liver).
A significant limitation of TNM staging is that it does not account for biological factors known to have predictive and prognostic value, including tumor grade, hormonal receptor status, and HER2 status.