One of the most influential characteristics of your breast cancer is whether or not it is invasive (also called infiltrating).
If a breast cancer is invasive (i.e., infiltrating), this means that it has spread beyond the lobular wall and the duct of the breast and has invaded the surrounding connective and fatty tissue of the breast. Invasive (i.e., infiltrating) cancers are considerably more serious, because they can pass into the bloodstream or lymphatic system and spread to other parts of the body.
Breast cancers that are termed non-invasive or in situ are those that are contained in the lobules and ducts of the breast. At the time of diagnosis, non-invasive and in situ breast cancers have not spread to the surrounding connective and fatty tissue of the breast.
However, in situ breast cancers can develop into a more invasive (i.e., infiltrating) and, therefore, more serious form of breast cancer.