Background/Objectives:Thoracic radiotherapy (RT) is associated with acute toxicities, including oesophagitis, which can have an impact on nutritional intake and subsequently lead to malnutrition. This study aimed to identify RT dosimetric factors associated with 5 weight loss in patients receiving treatment for non-small-cell lung cancer (NSCLC).Methods:Radiation dose data to the oesophagus (including mean, maximum dose and oesophageal length) were retrospectively analysed for a cohort of 54 NSCLC patients treated with concurrent chemoradiotherapy between 2004 and 2006. Weight change was calculated using the lowest weight during the 90 days from RT commencement compared with the start of RT.Results:Four patients for whom weight was not available at the start or end of treatment were excluded, leaving 50 patients for analysis. The prevalence of significant weight loss during the 90 days from RT commencement was 22 (median weight loss=9.1 , range=5.9-22.1). Dosimetric factors significantly associated with 5 weight loss were maximum dose to the oesophagus (P=0.046), absolute oesophageal length receiving 40 Gy (odds ratio (OR)=1.18, P=0.04), 50 Gy (OR=1.20, P=0.02) and 60 Gy (OR=1.32, P=0.005) to the partial circumference, relative oesophageal length receiving 50 Gy (OR=1.03, P=0.03) and 60 Gy (OR=1.07, P=0.005) to the partial circumference.Conclusions:Multiple dosimetric factors were associated with significant weight loss. Of these factors, absolute and relative length of the oesophagus receiving 60 Gy to the partial circumference were more strongly related. Understanding the dosimetric factors associated with weight loss may aid early identification and intervention in patients at nutritional risk.European Journal of Clinical Nutrition advance online publication, 13 August 2014; doi:10.1038/ejcn.2014.166.