Custom oral appliance for noninvasive immobilization during stereotactic radiotherapy.

S tereotactic pertains to a precise positioning in space. Stereotactic radiotherapy is a successful noninvasive modality for treating intracranial malignancies. Stereotactic radiosurgery was developed as a noninvasive tool to allow access to areas of the head that were previously treated by traditional surgical methods with a high rate of morbidity. As radiotherapy was refined, different stereotactic systems allowed for increasingly precise delivery of therapeutic radiation while minimizing collateral damage to normal tissues. Fractionated irradiation techniques generally have been found to be better tolerated by normal tissue and to allow for higher cumulative target doses as a result of repair between fractions. Intracranial neoplasms, such as pituitary adenoma, craniopharyngioma, large meningiomas, large acoustic neuromas, gliomas, as well as retinoblastomas are the lesions targeted for stereotactic radiotherapy. Even in these relatively small lesions, it is imperative that the beam position allow safe delivery of radiation therapy without an excessive dose to critical normal structures, such as the eyes. Some stereotactic devices use pins for fixation, which can cause superficial skin infections and cannot be tolerated for more than 58 days. Other devices have been developed to permit longer treatment times with lower doses of radiation at each session. For example the Laitinien stereoadaptor has a head frame that is mounted in a noninvasive way to the patient's head by means of two ear plugs and a nasal support. A second example is the Gill-Thomas frame, which is stabilized on the patient's head with polyethylmethacrylate polymer maxillary and mandibular dental impressions, a vinyl polysiloxane occipital support and three quick release nylon straps. This frame was modified in Boston to be the Gill-Thomas-Cosman [GTC] frame (Radionics, Burlington, MA) and uses a velcro headstrap, a vinyl polysiloxane occipital support with modified support mounting, and a specifically designed intraoral appliance fabricated in the pediatric dental clinic for repositioning. Fractionated irradiation techniques generally have been found to be better tolerated by normal tissue and to allow for higher cumulative target doses as a result of repair between fractions. Stereotactic radiotherapy often is performed by having the patient wear a stereotactic frame secured by pins to the soft and hard tissues of the cranial vault. The Gill-Thomas frame, which was invented in England, used extraoral devices to secure it in position, together with an intraoral dental impression. The GTC frame uses an intraoral device to allow rapid, precise alignment of the frame to the patient's head (Fig 1). This paper will review stereotactic radiation therapy and the current immobilization system based on the GTC frame. Because this technique achieves a high success rate, pediatric dentists should be knowledgeable in fabricating this appliance.