It's not clear what causes atypical hyperplasia.

Atypical hyperplasia forms when breast cells become abnormal in number, size, shape, growth pattern and appearance. Location of the abnormal cells within the breast tissue — the lobules or the milk ducts — determines whether the cells are atypical lobular hyperplasia or atypical ductal hyperplasia.

Atypical hyperplasia is thought to be part of the complex transition of cells that may evolve into breast cancer. The progression to breast cancer typically involves:

  • Hyperplasia. The process begins when normal cell development and growth become disrupted, causing an overproduction of normal-looking cells (hyperplasia).
  • Atypical hyperplasia. The excess cells stack upon one another and begin to take on an abnormal appearance.
  • Noninvasive (in situ) cancer. The abnormal cells can continue to change in appearance and multiply, evolving into in situ cancer, in which cancer cells remain confined to the area where they start growing.
  • Invasive cancer. Left untreated, the cancer cells may eventually become invasive cancer, invading surrounding tissue, blood vessels or lymph channels.
Oct. 18, 2014

You Are ... The Campaign for Mayo Clinic

Mayo Clinic is a not-for-profit organization. Make a difference today.