PURPOSE: Bodyweight loss is commonly found in Nasopharyngeal Carcinoma patients during Concurrent Chemo-radiotherapy (CCRT) and has implications for treatment decisions. However, the prognostic value of this weight loss remains uncertain. We addressed it by proposing a novel index Weight Censorial Score (WCS) that characterizes the patient-specific CCRT response on actual to estimated weight loss. METHODS: A retrospective study included 315 patients from two independent hospitals. An Estimated WCS (eWCS) was obtained through linear regression of image and dosimetry features. The eWCS was converted to an estimated net weight loss (nWL), with its accuracy evaluated. The Determined WCS (dWCS) was calculated by centering and scaling the post-RT actual nWL with patient's pre-RT body information. The ratio of dWCS to eWCS (WCS ratio) reflected the actual to estimated weight loss of a patient. The prognostic ability of WCS ratio dichotomized at 1 was evaluated. RESULTS: The mean absolute error of estimated to actual nWL was 1.84 kg. Patients who had their actual WL larger than estimated WL were found to have significantly worse OS (p = 0.005, HR = 3.35[1.45-7.73]), PFS (p = 0.038, HR = 1.86[1.03-3.35]), and DMFS (p = 0.050, HR = 2.20[1.00-4.85]), respectively, in multivariable cox analysis. They were also found not to benefit from adjuvant chemotherapy (p = 0.572), whereas the adjuvant chemotherapy provided significant PFS benefit in patients with actual WL smaller than estimated WL (p = 0.036, HR = 0.53[0.29-0.96]). CONCLUSION: The nWL of patient during CCRT can be reasonably estimated by dosimetry factors at pre-RT stage. The prognostic value of the actual to expected weight loss holds promise for highlighting vulnerable patients after CCRT.