This study is evaluating the effectiveness of a dietitian delivered health behaviour intervention to reduce malnutrition in head and neck cancer patients undergoing radiotherapy. You may be eligible to join this study if you have a confirmed diagnosis of cancer involving the nasopharynx, oropharynx, oral cavity, larynx, or hypopharynx requiring definitive or postoperative radiotherapy with curative intent. The 5 participating trial centres will begin in the ‘control’ condition i.e dietitians will provide ‘treatment as usual’. Participants recruited during this phase will receive treatment as usual by trained dietitians according to standard hospital practice. At a randomly determined time point, researchers will ask each centre to provide training to the dietitians. Dietitians will then provide ‘Eating as Treatment (EAT)’ as part of their standard dietetic consultations. EAT is designed to improve health behaviours of head and neck cancer patients and maintain their nutrition over the course of their radiotherapy. Participants will be regularly assessed for up to 12 weeks post radiotherapy treatment in order to evaluate nutrition status. Information about mood, smoking and alcohol use, therapeutic alliance and radiotherapy side effects will also be collected from participants. Medical records will be reviewed to collect a range of information including total radiotherapy treatment time, unplanned hospital visits, length of stay, dietitian contact and referral to appropriate services for depression.