NCI: Cancer that has spread from where it started in the breast into surrounding, healthy tissue. Most invasive breast cancers start in the ducts (tubes that carry milk from the lobules to the nipple). Invasive breast cancer can spread to other parts of the body through the blood and lymph systems.,NCI: A carcinoma that infiltrates the breast parenchyma. The vast majority are adenocarcinomas arising from the terminal ductal lobular unit (TDLU). Often, the invasive adenocarcinoma co-exists with ductal or lobular carcinoma in situ. It is the most common carcinoma affecting women.