Stereotaxic Instrument for Use with the Bragg Peak of a Proton Beam

A technique is described that permits precise localization of brain targets and their irradiation using the end of the range of a proton beam, the Bragg peak, where ionization effects may be several times greater than elsewhere along the beam path. A stereotactic instrument was devised to guide a pencil- shaped beam of protons from a synchrocyclotron so that the Bragg peak of the beam falls precisely on the predetermined target area. The instrument is designed also for use with conventional probes such as electrodes or a balloon cannula. Protons, hydrogen nuclei stripped of their orbital electrons, are accelerated in a circular path to a speed of 93000 mi/sec or 0.5 the speed of light. Bending magnets divert the protons from the circular path to pass out the portal of the cyclotron. The emerging beam is shaped by 3 brass collimators to a desired configuration, maximizing the sharpness at the edge of the beam. Experimentally a round beam of 7-mm dia. and clinically a beam about 45-mm in its greatest diameter were used. An ionization chamber between the middle and final collimators monitors the integrated dose of radiation. Use of an x-ray tube on the beam axis permits radiologic verificationmore » of the beam spot on the brain target. The head holder of the stereotactic instrument has a set of cross hairs on either side that is aligned on the brain target, and the Bragg peak is precisely centered on the intersection of the cross hairs by adjustment of the variable saline absorber. The instrument described is designed for rapid application. It has built-in features to minimize and simplify calculations and to avoid and crosscheck against human error. (TCO)« less