Figuring out how to use the treatment on other types of cancer, including ones outside the blood that create solid tumors, is the next big hurdle for scientists to clear. The problem, according to June, is that in cancers that create tumors, the bad cells are really good at keeping invaders out of the growing mass. In pancreatic cancer, for instance, the cancer cells basically build a wall around the tumor, which poses a problem for T-cells trying to get to it. That’s also true for brain cancers. To make CAR T-cell therapy a viable option for those patients, scientists will have to figure out how to safely break down those walls.