AbstractID: 9062 Title: Accurate evaluation of the combined treatment plan for a staged
Gamma Knife procedure
Small to moderately sized arteriovenous malformations (AVMs) have traditionally been treated by surgery, embolization, or
stereotactic radiosurgery. For large AVMs, however, radiosurgery has proven less effective since the large single fraction doses
necessary to result in obliteration increase the risk of functionally apparent brain necrosis to unacceptable levels. An alternative is to
stage the radiosurgery procedure over multiple fractions (usually two), effectively irradiating a smaller volume of the AVM nidus
during each fraction. In this technique, a therapeutic dose can be delivered to a part of the nidus each time. The treatments are
separated by 6 months to allow for normal tissue recovery between fractions. The purpose of this work is to describe a methodology
of calculating a combined treatment plan for multiple Gamma Knife treatments. We assume that the difference between the
stereotactic coordinate systems defined by subsequent frame placements can be represented by a translation and a rotation. It is
possible to define a linear multi-dimensional set of equations that describes this transformation provided there are at least 4 fiducial
markers invasively attached to the patient’s skull that will not move between fractions. A unique transformation can be determined
based on the coordinates of the fiducial markers in both stereotactic coordinate systems. Using the transformation matrix, the shots
from the first fraction can then be displayed in the coordinate system of subsequent fractions allowing the computation of a combined
dose distribution covering the entire AVM. Both phantom studies and clinical experience will be presented.