Definition:

stage I breast cancerlisten (... brest KAN-ser)

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

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Stage I breast cancer. Drawing shows stage IA on the left; the tumor is 2 cm or smaller and has not spread outside the breast. Drawings in the middle and on the right show stage IB. In the middle, no tumor is found in the breast, but small clusters of cancer cells are found in the lymph nodes. In the drawing on the right, the tumor is 2 cm or smaller and small clusters of cancer cells are found in the lymph nodes.