Breast cancer is staged from 0 to IV

Stage 0 is noninvasive breast cancer, that is, carcinoma in situ with no affected lymph nodes or metastasis. This is the most favorable stage of breast cancer.

Stage I is breast cancer that is less than 2 cm (3/4 in) in diameter and has not spread from the breast.

Stage II is breast cancer that is fairly small in size but has spread to lymph nodes in the armpit OR cancer that is somewhat larger but has not spread to the lymph nodes.

Stage III is breast cancer of a larger size, greater than 5 cm (2 in), with greater lymph-node involvement, or of the inflammatory type.

Stage IV is metastatic breast cancer: a tumor of any size or type that has metastasized to another part of the body. This is the least favorable stage.