Gamma Knife Radiosurgery for Arteriovenous Malformations

Operative Planning

Special Equipment

Anesthetic Issues

Local anesthesia (lidocaine 1% or 2% with epinephrine, diluted 1:1 with sodium bicarbonate)

Frame Fixation

Stereotactic Magnetic Resonance Imaging Scan

MR indicator box is placed.
Volumetric MR (1 mm, skip 0) (1-mm thick slices with no interslice interval) is performed along with MR angiography and sent via computer transfer to the gamma knife suite.

Stereotactic Angiogram

Angiogram indicator box consists of five panels; one top panel and two pairs of panels (right-left and anteroposterior panels) that mirror each other in appearance but have different fiducial markers
The markers are projected on films.
It used to be important that the films be taken perpendicularly; this is no longer the case: most computer systems now correct for any angulation that may occur.
Therefore, it is now possible to tilt the x-ray tube to obtain a semioblique projection as long as all fiducial markers can be defined.
The anteroposterior and lateral films of the outlined AVM nidus are scanned and transferred into the gamma plan.

Gamma Planning—Image Definition

Dose-planning procedures using the Leksell Gamma Plan (Elekta) are performed in three dimensions, using angiograms and MR images.
Except in some cases in which an MR imaging study is contraindicated for various reasons or if a patient is allergic to contrast material, both angiogram and MR imaging can be performed.
Although anatomic and pathologic resolution is much better with MR than with angiography, there is a degree of anatomic distortion in the MR images; therefore, if the angiogram and MR images are not congruent, which is not uncommon, the angiogram images are more likely to be accurate.
Because both the coronal and sagittal views are more sensitive to distortion than the axial MR, dose planning, in general, is based primarily on the axial view, with reference to both the coronal and sagittal views that are reconstructed from the axial views.
Because angiography provides two-dimensional information only, the dose planning for an AVM is performed primarily based on MR images, with angiography being used only to outline the maximal dimension of the nidus.

Gamma Planning—Treatment Planning

Gamma Planning Dosimetry

Achieve good conformity between the ROI and dose distribution.
Where necessary, select more target points, using smaller collimators for treating lesions located near critical brain structures (i.e., conformal multiple-isocenter radiosurgery); although the total procedure time is considerably prolonged, better conformity can be achieved and the integrated dose to the surrounding brain can be markedly decreased, despite the same marginal dose; the volume of normal brain receiving 10 Gy or more is decreased.

Closure