ADAPTATION AND VERIFICATION OF THE RELOCATABLE GILL-THOMAS-COSMAN FRAME IN STEREOTAXIC RADIOTHERAPY

Purpose: Stereotactic radiotherapy (SRT) combines techniques of stereo tactic radiosurgery (SRS) with radiation therapy fractionation schemes . Fractionation in SRT necessitates a relocatable immobilization syste m to precisely reproduce the patient's position at each treatment. The Gill-Thomas-Cosman (GTC) head frame is such an immobilization device compatible with the Brown-Roberts-Wells (BRW) stereotactic system. We describe this device, our modifications to the original design, the re peat position accuracy, and the daily verification procedure. Methods and Materials: The original GTC frame was tested on volunteers. This t esting led to an improved strapping system, the decision to construct the oral fixation appliance at our dental clinic, and the construction of a depth confirmation helmet to rapidly confirm the position of the frame on a daily basis. The GTC frame, at our institution, is not acc eptable for children requiring anesthesia, and a new frame, the ''Bost on Childrens' Hospital'' frame, was designed. This device uses the bas e ring of the GTC frame. Airway access is maintained through fixation on the nasal-glabellar region and the ear canal rather than the hard p alate and upper gingiva. Results: The modifications of the GTC frame a nd the verification protocol result in repeat positioning of the frame with respect to the patient anatomy, with a standard deviation of 0.4 mm for both the modified GTC frame and the Boston Childrens' Hospital frame. The relocatibility of the frames has been established in over 2,000 patient setups in over 60 patients to date. Discussion: The GTC frame is a noninvasive and versatile fixation system that provides pat ient comfort, as well as accurate relocatibility for SRT. The frame is not appropriate for single fraction radiosurgery, as a large setup er ror (> 2 mm) for a single treatment cannot be excluded. The GTC frame is compatible with the BRW system, and treatment planning for SRT and SRS patients is identical. We currently treat 10-13 SRT patients per d ay with intracranial neoplasms on a dedicated stereotactic therapy uni t. In addition, the Boston Childrens' Hospital frame allows the use of stereotactic therapy in the treatment of children under 6 years of ag e. This population will benefit especially from precise and highly foc al cranial irradiation.