MedicalTerminology DB

Meaning of stage I breast cancer: Stage I breast cancer is divided into stages IA and IB.

Definition of stage I breast cancer

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.

Written and maintained by James Murray Last updated July 21, 2026 Educational reference, not medical advice. Read the limitations.

For learning, not diagnosis. This glossary provides general educational information and is not a substitute for advice from a qualified healthcare professional. If you may be experiencing a medical emergency, contact local emergency services.