Radiotherapy mask. This mask is worn by patients undergoing radiotherapy. It is a rigid mask that is custom-made to hold the patient's head in position while the treatment is carried out. Markings on the mask can show where the radiation is to be directed. Guide lasers can also be used to help plan the radiotherapy. Radiotherapy is the use of radiation to treat diseases such as cancer. The radiation is focused on the tumour site, and used to disrupt and kill the cancer cells.