1 Research Department, Leo Cancer Care Ltd, Crawley, United Kingdom; Department of Medical Physics and Biomedical Engineering, University College London, London, United Kingdom.
2 Division of Radiation Biophysics, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
3 Department of Radiation Oncology, Centre Léon Bérard, Lyon, France.
4 Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland; Department of Physics, ETH Zürich, Zürich, Switzerland. Electronic address: tony.lomax@psi.ch.