Stage I breast cancer is divided into stages IA and IB. In stage IA, the tumor is 2 centimeters or smaller and has not spread outside the breast. In stage IB, (1) no tumor is found in the breast, but small clusters of cancer cells (larger than 0.2 millimeter but not larger than 2 millimeters) are found in the lymph nodes; or (2) the tumor is 2 centimeters or smaller and small clusters of cancer cells (larger than 0.2 millimeter but not larger than 2 millimeters) are found in the lymph nodes.