DCIS stands for ductal carcinoma in situ. This means that breast cancer cells that started in the duct (tubes that carry milk to the nipple) are still maintained by the wall of duct.
When cancer cells are maintained by the wall of duct, they have no ability to leave the breast and go to other parts of the body. This is why DCIS is considered curable.
DCIS is considered Stage 0, which is the earliest stage of breast cancer. No matter how large the area of DCIS, it is always considered Stage 0.
However, if DCIS goes untreated for too long, the cancer cells can eventually break through the duct and invade into the breast tissue. This could then change the staging and curability of the cancer.
As always, talk with your doctor about any specific concerns you have.