Development of a relocatable frame technique for gamma knife radiosurgery - Technical note

Object. One of the limiting factors in gamma knife radiosurgery is the rest riction to one treatment imposed by the fixed stereotactic frame. The abili ty, in selected cases, to remove the frame and replace it on a subsequent o ccasion in the same location would facilitate fractionated treatments and p rovide flexibility in the timing of treatment delivery. It is the purpose o f 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 s crews 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 locali zed (using the angiographic localizer) before and after each of five frame removal/reapplications to test the reproducibility of frame position. The m ean error in the observed target coordinates was 0.3 mm and the maximum err or observed was 0.7 mm, indicating that the frame can be reapplied with som e confidence. Repetition of bubble skull measurements has been investigated as a means of verifying that the frame was repositioned correctly; however, reproducibil ity of patient measurements was found to be poor even when no frame movemen t had occurred. In contrast, the use of a radiotherapy simulator to obtain repeated lateral and anteroposterior projections of the head was shown to b e capable of detecting frame movements of as little as 1 mm. Conclusions. Using this technique of frame application facilitates the reap plication of the frame with an accuracy of plus or minus 0.7 mm. Bubble mea surements are inadequate far the detection of frame movement. Simple techni ques in which a radiotherapy simulator is used can verify correct frame pla cement and indicate frame movements of less than 1 mm.