How does dcis become harmfully invasive?

Unknown. If we knew the answer to this question, women with dcis would not all need surgery! about 50% of dcis (ductal carcinoma in situ), if left untreated, will go on to become invasive cancer. Certain genes in the dcis cells get "turned on", making them invasive, but we don't know what makes this switch happen.
Spread. Dcis, if left untreated, has the potential to grow outside the duct and invade the suporting tissue that surrounds the ducts in the breast. The lymphatics and blood vessels are found within this supportive tissue; invasive cancer has the ability to enter into these vessels and travel outside the breast to other organs in the body.