PurposePatients with head and neck cancers are susceptible to malnutrition during radiotherapy. This study aimed to determine the changes in the nutritional status and its determinants in patients with head and neck cancer during radiotherapy.MethodsThis prospective observational study was performed in an outpatient Radiation Oncology clinic with a sample of 54 patients. An interview form (including anthropometric and laboratory parameters), the Patient-Generated Subjective Global Assessment to assess nutritional status, quality of life scales, and toxicity criteria were used for data collection at the baseline, the end of radiotherapy and 1 and 3months after radiotherapy.ResultsWhile the majority of the patients (90%) were well nourished at baseline, most of the patients (74%) were malnourished at the end of radiotherapy (p<0.001). During radiotherapy, patients developed malnutrition, reflected in a decrease in food intake, approximately 5% loss of body weight, a reduction in mid-arm upper circumference and mid-arm muscle mass, and reduced serum protein and albumin levels. The nutritional status was worse in oropharyngeal cancers (p=0.021), advanced stage (p=0.004), use of concomitant chemotherapy (p=0.041), and worse toxicity (p<0.001). Furthermore, the nutritional status was strongly associated with the quality of life.ConclusionsThis study demonstrated negative impact of radiotherapy on the nutritional status of patients with head and neck cancer. The study also showed the association of the nutritional status and the quality of life. The nutritional status should be assessed during different periods in the trajectory of treatment due to its significant contribution to the quality of life.