Radiotherapy is an essential treatment approach in head and neck cancer. Many patients are offered radiotherapy or chemoradiotherapy following successful surgical tumour resection, but there are many complications associated with this treatment, and there is little evidence to determine if adding radiotherapy in this situation will give any survival benefit. This study will compare the genetic profile of patients with and without severe swallowing difficulties following radiotherapy and try to identify genetic factors associated with a higher risk of developing severe complications.