Starting in a milk passage, or duct, of the breast, this cancer breaks through the wall of the duct and invades the fatty tissue of the breast. At this point, is has the potential to metastasize, or spread, to other parts of the body through the bloodstream and lymphatic system. Infiltrating ductal carcinoma is the most common type of breast cancer, accounting for about 80% of the breast malignancies.

ILC has arisen in the milk-producing glands and is invasive in the breast's fatty tissue. This cancer has the potential to spread (metastasize) elsewhere in the body. About 10% to 15% of invasive breast cancers are invasive lobular carcinomas. ILC is often difficult to detect by physical examination or even by mammography and is usually found serendipitously.

This means that it is not precancer but has "infiltrated outside of the ducts". Most cancers are infiltrating ductal and it does not imply that it is particularly bad or aggressive. If it starts in the lobule rather than the duct it is an infiltrating lobular carcinoma. back to pathology report

Penetrating normal, surrounding tissue.

Refers to a tumor that penetrates the normal, surrounding tissue.