An oral device for use in radiotherapy of the head and neck includes a base member and a bite member or “mouthpiece,” which is selectively positionable relative to the base member. The device provides immobilization of the tongue for accurate reproducible administration of radiation therapy. The device fits within the oral cavity of a patient and is maintained in position by a bite member that fits over the maxillary teeth of the patient. The position of the bite member relative to the base member may be adjustable to accommodate differences in mouth cavities and bite profiles. For example, the base member may include a platform or series of members configured to be removably coupled to the bite member in multiple different positions.
