Development of a relocatable frame technique for gamma knife radiosurgery

Abstract

Object. One of the limiting factors in gamma knife radiosurgery is the restriction to one treatment imposed by the fixed stereotactic frame. The ability, in selected cases, to remove the frame and replace it on a subsequent occasion in the same location would facilitate fractionated treatments and provide flexibility in the timing of treatment delivery. It is the purpose of this work to investigate techniques for frame fixation and for essential verification of frame position once it has been reapplied. Methods. A technique is proposed that requires four surgical self-tapping screws to be inserted into the skull. Aluminum pins are inserted through the frame pillars and are tightened against the head of the screws, providing a firm fixation of the frame. Pin lengths are recorded on gauges to ensure reproducibility of the position. In phantom tests, test objects were localized (using the angiographic localizer) before and after each of five frame removal/reapplications to test the reproducibility of frame position. The mean error in the observed target coordinates was 0.3 mm and the maximum error observed was 0.7 mm, indicating that the frame can be reapplied with some confidence. Repetition of bubble skull measurements has been investigated as a means of verifying that the frame was repositioned correctly; however, reproducibility of patient measurements was found to be poor even when no frame movement had occurred. In contrast, the use of a radiotherapy simulator to obtain repeated lateral and anteroposterior projections of the head was shown to be capable of detecting frame movements of as little as 1 mm. Conclusions. Using this technique of frame application facilitates the reapplication of the frame with an accuracy of plus or minus 0.7 mm. Bubble measurements are inadequate for the detection of frame movement. Simple techniques in which a radiotherapy simulator is used can verify correct frame placement and indicate frame movements of less than 1 mm.