Pretreating tumors with manganese dioxide nanoparticles may increase the efficacy of radiation therapy, according to a new study in mice. Radiation therapy depends on oxygen molecules to create reactive species that damage cancer cell DNA. Unfortunately, tumors tend to have low O2 levels, which limits the treatment’s effectiveness. Ralph S. DaCosta of the Princess Margaret Cancer Center, in Toronto, and Xiao Yu (Shirley) Wu of the University of Toronto wanted to fix this limitation by injecting tumors with materials that generate O2. MnO2 particles do so by reacting with H2O2 and H+ ions, both of which are abundant in tumors. Compared with untreated mice, animals injected with the nanoparticles had 45% higher levels of oxygenated hemoglobin in the blood vessels around their tumors. Also, after five days of radiation therapy, tumors injected with the nanoparticles were 66% smaller than tumors injected with saline (ACS Nano 2014, DOI: 10.1021/nn405773r). One drawback is that the particles must be injected directly into the tumor. The team is now working on a version that can target tumors from the bloodstream.