Light is used to treat cancer, but phototherapy is only effective where light can easily reach, limiting its use to cancers of the skin and areas accessible with an endoscope. Using a mouse model of cancer, researchers at Washington University School of Medicine have devised a way to apply light-based therapy to deep tissues never before accessible. Instead of shining an outside light, they can deliver light directly to tumor cells, along with a photosensitive source of free radicals that can be activated by the light to destroy cancer.