Clinical Evaluation of a Novel Thermoplastic Mask System With Intrafraction Motion Monitoring Using IR Tracking and Cone Beam CT for Gamma Knife Radiosurgery

Abstract

A non-invasive immobilization system consisting of a thermoplastic mask and image-guidance with cone-beam CT (CBCT) and infrared (IR) tracking has been developed to monitor intra- and inter-fractional movement during Gamma Knife® radiosurgery. This study aims to clinically evaluate the accuracy and stability of this novel immobilization system with image-guidance in patients treated with standard fractionated intensity modulated radiation therapy (IMRT) on a linear accelerator prior to application for longer treatments on the Gamma Knife Perfexion. Following ethics board approval, this prospective study evaluated adult patients planned for fractionated brain IMRT. Patients were immobilized with a thermoplastic mask (with the nose cut-out) and customized head cushion. A reflective marker was placed on the patient’s nose tip and tracked during each treatment using an IR camera fixated to the treatment couch aligned with a rigid reference coordinate system near the patient’s head. For each fraction, CBCT1 was acquired pre-treatment with verification CBCT2 (after any translational correction for interfraction set-up variation) and CBCT3 post-treatment. Using CBCT data, the intrafraction movement (combining translation and rotation) of the target was measured. Intrafraction movements of the nose tip were compared between CBCT and optical tracking methods. Preliminary data from 28 datasets (CBCT and optical data) from 4 patients are summarized. The mean and maximum intrafraction motion of the target was 0.27 mm and 3.63 mm based on pre- and post-treatment CBCT, but ranged from 0 - 3.63 mm. Mean intrafraction target movement for each patient varied from 0.04 - 0.69 mm. The mean intrafraction motion of the nose tip based on CBCT data was 0.49 mm compared with 0.51 mm using optical tracking. The maximum intrafraction motion of the nose tip was as large as 3.71 mm using CBCT in one patient and 3.16 mm using optical tracking. This initial clinical evaluation of intra-fraction motion measurements using both optical tracking and CBCT demonstrate that the mean intrafraction motion is small with a thermoplastic mask immobilization system. However, isolated measures of larger intrafraction motion, as high as 3 mm, support the need for image-guidance to monitor and manage intrafraction motion. Optical tracking of the nose marker in this system appears to be a sensitive tool that reflects motion of the nose tip and target detected using CBCT.