1. Department of Mathematical Sciences “G. L. Lagrange”, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
2. Department of Radiotherapy and Radiation Oncology, Saarland University Medical Centre, Kirrberger Straße Geb. 6.5, 66421 Homburg/Saarland, Germany
3. Felix-Klein-Zentrum für Mathematik, Technische Universität Kaiserslautern, Paul-Ehrlich-Straße 31, 67663 Kaiserslautern, Germany