1 Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels B-1090, Belgium; Department of Radiotherapy, Institut Andrée Dutreix, Dunkerque, France.
2 Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels B-1090, Belgium. Electronic address: mark.deridder@uzbrussel.be.