Acknowledgments of research support for the study: The Kom Op Tegen Kanker supported the development of this project through the "Emmanuel van der Schueren Fellowship for Quality Assurance in Radiotherapy" EORTC fellow grants (Oscar Matzinger, Melissa Christiaens). UK Coordinating Committee for Cancer Research (UKCCCR), Radiation Therapy Oncology Group (RTOG), French Federation Nationale des Centres de Lutte Contre le Cancer (FNLCC), European Organization for the Research and Treatment of Cancer (EORTC) and the ECOG-ACRIN Cancer Research Group (ECOG-ACRIN). ECOG-ACRIN grants U10CA180820 and UG1CA233270.