This is Episode 11 Part B, Tumor Microenvironment - Angiogenesis & HIF-1. In our last episode, we explored the critical issue of tumor hypoxia and how reoxygenation plays a role during fractionated radiotherapy. Today, we're going to build on that by looking at how tumors try to get more oxygen by forming new blood vessels – a process called angiogenesis – and the master switch that controls this, a protein called HIF-1α.