[harrie]

DCIS by itself, contained within the ducts, is non-invasive. DCIS can become "invasive DCIS" if the part of the DCIS escapes the ducts and goes into the tissue.
It was my understanding, that the actual cells within the DCIS are technically "cancer" cells, same as the "cancer" cells found in invasive carcinoma.
The difference between DCIS and invasive CA is that if it is just DCIS and the breast tissue is clear of cancer cells, then there is no concern of cancer cells spreading. Thus there is no reason for the chemo.
The cells within the DCIS can be tested for hormonal positivity, even the HER2, as you would test the cells of the stage I breast cancer. Theoretically, the results can possibly predict the risks of future invasion.