1 Department of Physics, University of Pisa and INFN, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy. Electronic address: alberto.del.guerra@unipi.it.
2 Technical University of Munich, Institute for Advanced Study, Lichtenbergstr. 2 a, D-85748 Garching, Germany; Department of Radiation Oncology, Klinikum rechts der Isar, Ismaninger Straße 22, D-81675 München, Germany.