Radiotherapy is the treatment of a malignant disease (cancer) by applying radiation. Radiotherapy is a common treatment for cancer. Approximately fifty per cent of patients are either treated by radiotherapy only or in combination with other treatments during the course of their disease. External radiotherapy is the most commonly used and involves administering radiation through the skin and tissues in order to irradiate the entire region of the tumour. It can be distinguished from internal radiotherapy or curietherapy. For example, in the case of thyroid cancer metabolic radiotherapy involves the administration of radioactive iodine that is then absorbed by the gland where it releases radiation directly into the cancer cells.