1. Department of Radiation Oncology, Stanford University, Stanford, California 94305
2. Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, California 90095
3. Institute for Mathematics and Its Applications, University of Minnesota, Mineapolis, Minnesota 55455
4. Biomedical Physics Interdepartmental Program, David Geffen School of Medicine at UCLA, Los Angeles, California 90095‐1770
5. Department of Mathematics, University of California, Los Angeles, California 90095