BACKGROUND & AIMS: Standardized phase angle (SPA) is a value derived from bioelectrical impedance phase angle analysis and has been recognized as marker of nutritional risk and clinical status of cancer patients. This study aimed to evaluate the changes in SPA during radiation therapy of cancer patients and identify possible nutritional and clinical determinants of these changes. METHODS: Nutritional assessment was performed before the initiation and at the end of the radiotherapy. It included anthropometrics and bioelectrical impedance analysis. A generalized linear mixed model was used to evaluate the effects of selected independent variables on the changes in standardized phase angle. RESULTS: A total of 104 patients entered the study and those with complete information were included in the analysis (n = 62). Most patients (61%) were at clinical stage I and II, the most prevalent irradiation site was the pelvis (39%) and 98% were well nourished or overweight. According to the regression analysis, changes in body weight and irradiation at head and neck/upper abdomen and thorax were associated with standardized phase angle modifications during radiotherapy. Clinical stage and radiation dose were not associated with changes in standardized phase angle. Compared to baseline values, for every 1 kg weight reduction during the radiation therapy, the standardized phase angle also decreased by 0.107 (p < 0.0001). CONCLUSIONS: In well nourished or overweight patients, weight loss during radiation therapy is closely associated with decrease in standard phase angle.