Imaging protocol
TUMOR RECURRENCE
All patients had undergone PET/CT imaging within 3 weeks prior to radiotherapy and by the time of suspected tumor recurrence. Whole-body PET scans had been acquired using three different models of PET/CT scanners (Siemens Biograph 16, Philips Gemini GXL, Philips Gemini TF16) with full-dose contrast-enhanced or low-dose non-contrast-enhanced CT for attenuation correction and anatomical co-registration. Image acquisition followed the mandatory PET-Plan trial protocol [