Dynamic stereotactic radiosurgery.

Two radiosurgical procedures using a stereotactic frame and a linear accelerator X ray beam with a circular field diameter between 0.5 and 3 cm are presented. One technique is based on a single plane rotation (single plane radiosurgery) whereas the other uses simultaneous and continuous motions of both the gantry (approximately 360 degrees) and couch (approximately 180 degrees) during the radiosurgical procedure (dynamic radiosurgery). The dose, typically a few thousand cGy, is prescribed to the 90% isodose line which just covers the target volume. The dose fall-off outside the spherical target volume is considerably sharper for the dynamic rotation than for the single plane rotation, and is comparable to the dose fall-off obtained with the two presently known dedicated radiosurgical techniques: one based on focused cobalt beams and the other on proton beams. The dose fall-off in the dynamic radiosurgery discussed here is also comparable to that of previously described linear accelerator based multiple converging are techniques, making the dynamic radiosurgery an attractive alternative to presently known radiosurgical procedures. The radiation beam parameters are discussed and the stereotactic frame described. The dose distributions for both radiosurgical techniques are calculated in a single plane and then corrected for the attenuation effects in the stereotactic frame (approximately 2%) and for the effects of the dynamic rotation (approximately 2%). The skin doses are 0.7% and 2%, and the lens doses, if the beam passes through the eyes, are 2.5% and 3.5% for the dynamic rotation and single plane rotation, respectively. The scatter and leakage dose for the radiosurgical procedures is typically 0.2% to the patient's thyroid, 0.06% to the breast, and 0.02% to the gonads.