1 Department of Radiotherapy, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands. Electronic address: n.russell@nki.nl.
2 Radiotherapiegroep, Arnhem, The Netherlands.
3 Department of Radiation Therapy, Catharina Hospital, Eindhoven, The Netherlands.
4 Dutch Breast Cancer Research Group, BOOG Study Center, Amsterdam, The Netherlands.