What is the defining characteristic of invasive breast cancer?

Invasive breast cancer is characterized by the ability of cancer cells to grow beyond the confines of the original ductal or lobular structures into the surrounding breast tissue. This infiltration into adjacent tissues is significant because it indicates that the cancer has the potential to spread further within the body, including to nearby lymph nodes and other distant organs. The designation of "invasive" highlights the cancer's aggressiveness and its ability to disrupt normal tissue architecture.

The other choices focus on aspects that do not align with the definition of invasive breast cancer. The presence of cancer cells being contained within ducts describes non-invasive cancer, such as ductal carcinoma in situ (DCIS), which does not spread beyond the ducts. As for the undetectability by mammograms, while it is possible for some tumors to be missed, this is not a defining characteristic of invasive breast cancer itself. Additionally, saying that cancer cells are only present in lymph nodes overlooks the fact that invasive breast cancer begins in the breast tissue, not solely in lymph nodes. Thus, the defining feature remains that invasive breast cancer involves the growth of cancer cells into surrounding breast tissues.