[1] Paul Scherrer Inst, Ctr Proton Therapy, Villigen, Switzerland
[2] Swiss Fed Inst Technol, Dept Phys, Zurich, Switzerland
[3] Maastricht Univ Med Ctr, GROW Sch Oncol, Dept Radiat Oncol Maastro, Maastricht, Netherlands
[4] Univ Hosp Zurich, Dept Radiat Oncol, Zurich, Switzerland
[5] Univ Bern, Univ Hosp Bern, Dept Radiat Oncol, Inselspital, Bern, Switzerland