How does dcis turn into stage 4 breast cancer?

Unknown: By definition, dcis is cancer confined to the breast ducts, without the ability to spread outside the breast. Nevertheless, rarely, a women with a history of previously-treated dcis will develop metastasis. A possible explanation is that a small invasive cancer was present but missed at the initial diagnosis. More likely, is that we just don't understand everything about dcis yet.

Pre-malignant : Dcis is a pre-malignant condition. Meaning the cells have the potential to become cancer if left in the breast, but at their current state they are not invasive. Dcis can turn into stage 4 breast cancer if they progress to invasive cancer.

Long process: While not all dcis will become invasive cancer, many cases will and to date we can not differentiate between those that will and those that won't. Those cases that become invasive, can then spread to lymph nodes or other organs. The latter, breast cancer that spreads to other organs, is stage IV breast cancer.