"Think of it like damming a river. Bevacizumab can block the main flow, but then once a tumor's need builds up behind this dam, water starts to flow around the blockage in the form of streams and tributaries. That's like these other growth factors -- eventually a tumor becomes able to use these tributaries of VEGF-C, VEGF-D and placental growth factor to supply itself with the 'water' it needs "