1. Department of Nuclear Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, China
2. Key Laboratory of Nuclear Technology Application and Radiation Protection in Astronautics, Ministry of Industry and Information Technology, Nanjing, China
3. Joint International Research Laboratory on Advanced Particle Therapy, Nanjing University of Aeronautics and Astronautics, Nanjing, China
4. Department of Physics, University of Pavia, Pavia, Italy
5. Department of Radiation Physics, Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, USA