Concepts of Conformality and Selectivity in Acoustic Tumor Radiosurgery

Abstract

The goals of acoustic neuroma radiosurgery include cessation of tumor growth, tumor volume regression, elimination of open surgical risks, maintenance of existing cranial nerve function and improvement of patient’s symptoms. In order to evaluate current technology usage at the University of Pittsburgh, we reviewed our current stereotactic imaging, and dose planning techniques for acoustic neuroma radiosurgery. We then assessed the impact of highly conformal and selective dose plans on the outcome of acoustic neuroma. Conformality denotes that selected isodose conforms to the three-dimensional target volume and the target dose is high. Selectivity refers to the fact that the integral dose to surrounding tissues outside of the target volume is low. Various conformality indices have been proposed to measure the perfection of a radiosurgical plan where selected isodose volume perfectly matches the 3-D geometry of the image defined target volume. A variety of technologies have been developed to enhance conformality and selectivity over the years. High resolution MRI is necessary to define the 3-D tumor volume and to delineate adjacent critical structures including the cochlea, semi-circular canals, and cranial nerves. Multiple isocenter plans using the Gamma Knife achieves a very high conformality and permits very high selectivity using small beams. The incorporation of robotic dose delivery facilitates highly accurate delivery of radiation, maximizing both conformality and selectivity.