The size of a breast cancer and evidence of its spread are the criteria by which it is classified into stages. The current staging scheme is the American Joint Committee on Cancer TNM system. The letter T stands for tumor, N for lymph nodes, and M for metastasis. In most cases, cells from the primary tumor spread first to the regional lymph nodes, and then to distant sites in the body. The tumor is graded from 0 (no tumor found) to 4 (involvement of the skin or chest wall).
TNM criteria are:
T0: no evidence of primary tumor
Tis: carcinoma in situ
T1: < 2cm
T1a: < 0.5cm
T1b: 0.5-1cm
T1c: 1-2cm
T2: 2-5cm
T3: >5cm
T4: any size, extension to skin or chest wall (excluding pectoralis muscle)
T4a: extension to chest wall
T4b: skin edema, ulceration or satellite nodules
T4c: both a and b
T4d: inflammatory carcinoma
N0: no regional lymph node metastasis
N1: cancer in movable nodes, same side
N2: cancer in fixed nodes
N3: cancer in internal mammary nodes, same side
M0: no metastasis
M1: distant metastasis (including supraclavicular nodes, same side)