1 Ministry of Health Holdings, Singapore, 099253, Singapore.
2 Department of Radiation Oncology, Stanford University, California, 94305, USA.
3 Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, 10065, USA.
4 Department of Head and Neck and Thoracic Cancers, Division of Radiation Oncology and Division of Medical Sciences, National Cancer Centre Singapore, Singapore, 169610, Singapore.