Think of a cancer microenvironment as a complete ecosystem that sustains cancer cells in a tumor. The environment includes not only biological material, such as the cancer cells and the extracellular matrix, but also response triggers, such as mechanical cues and biochemical signals.
One reason researchers are so keenly interested in studying cancer microenvironments is because of their utility in understanding cancer progression. Cues from multiple areas of the tumor's biological context, such as the cellular makeup of the microenvironment and its biochemical properties, can cause the tumor to spread. Having a full view of the tumor's potential metastatic behavior means that anti-cancer agents can be studied and tested in similar circumstances to the in vivomalignancy.
This matters because the majority of cancer deaths are caused by metastasized cancers, rather than origin tumors. That's especially true of breast cancer: Metastatic breast cancer causes most of the 42,000 annual deaths from breast cancer.
Protecting the integrity of the microenvironment can help replicate the metastatic nature of the tumor as it moves from formation and angiogenesis to invasion, intravasation, extravasation, and, ultimately, circulation.