Question: What is the difference between ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS)?
Answer: Ductal carcinoma in situ refers to the cells that arise in the ducts of the breast that are malignant in their own right. In other words, if you were to look at them under a microscope, you could say these are malignant cells, although ductal carcinoma in situ does not act yet like other malignancies; it does not spread past the breast.
Now lobular carcinoma situ is a horse of a different color altogether. It's probably not really a cancer and it's been given the name by accident. Some of us even call that lobular neoplasia so that we separate that from the word carcinoma, which everyone understands to mean cancer.
But lobular carcinoma situ is a marker of a patient who is at risk for developing a real cancer sometime in the future. We don't really believe that the cells are malignant in their own right, but they just predict that that patient has a higher chance than her next door neighbor, for example, of developing a real cancer at some time in the future.