1Medical University of Vienna, Comprehensive Cancer Center Vienna, Department of Radiation Oncology, Vienna, Austria; 2Medical University of Vienna, Department of Statistics, Vienna, Austria; 3Department of Oncology, Aarhus University Hospital, Denmark, Department of Experimental Clinical Oncology, Aarhus, Denmark; 4Gloucestershire Oncology Centre, Cheltenham General Hospital, Department of Oncology, Gloucester, United Kingdom; 5Mount Vernon Cancer Centre, Northwood, Hertfordshire, Department of Oncology, London, United Kingdom; 6Neolife Enayati Medical City, Department of Radiation Oncology, Bucharest, Romania; 7Centre Oscar Lambret, Department of Radiation Oncology, Lille, France; 8University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany; 9Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 10Amsterdam University Medical Centers, AMC, Department of Radiation Oncology, Amsterdam, Netherlands Antilles; 11Interventional Oncology Centre (IOC), Fondazione Policlinico Universitario A. Gemelli IRCCS, Dipartimento di Diagnostica per immagini, Radioterapia Oncologica ed Ematologia - Gemelli ART (Advanced Radiation Therapy), Rome, Italy; 12Hospital Duran iReynals, Institut Catalàd'Oncologia (ICO), Radiobiology and Cancer Group, ONCOBELL Program, Institut d'Investigació Biomèdica de Bellvitge (IDIBELL), L'Hospitalet de Llobregat, Barcelona, Department of Radiation Oncology, Barcelona, Spain; 13Wroclaw Comprehensive Cancer Center, Wroclaw Medical University, Department of Oncology, Wroclaw, Poland; 14Institute for Oncology and Radiology of Serbia, Department of Radiotherapy, Belgrad, Serbia; 15Cambridge University Hospitals NHS Trust, Department of Oncology, Cambridge, United Kingdom; 16Erasmus MC, Department of Radiation Oncology, Rotterdam, Netherlands Antilles; 17Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 18University Medical Centre Utrecht, Department of Radiation Oncology, Utrecht, Netherlands Antilles; 19Cedars Sinai Medical Center, Los Angeles, CA, Department of Radiation Oncology, Los Angeles, USA