Causes

It's not clear what causes atypical hyperplasia.

Atypical hyperplasia forms when breast cells become abnormal in number, size, shape, growth pattern and appearance. The appearance of the abnormal cells determines the type of atypical hyperplasia:

• Atypical ductal hyperplasia describes abnormal cells within the breast ducts.
• Atypical lobular hyperplasia describes abnormal cells within the breast lobules.

Atypical hyperplasia is thought to be part of the complex transition of cells that may accumulate and 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 continue to progress in appearance and multiply, evolving into in situ cancer, in which cancer cells remain confined to milk ducts.
• Invasive cancer. The abnormal cells accumulate within the duct and continue to multiply and transition to become cancer cells. Invasive cancer tends to invade surrounding tissue, blood vessels or lymph channels.