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Gamma Knife Radiosurgery
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INTRODUCTION

Technical Overview

The Gamma Knife is approximately the size of an MRI or CT scanner. It consists of a heavily shielded vault containing 201 separate cobalt-60 sources, (measuring 1mm in diameter and 20mm in length, with a specific activity of 150 Curie per gram) arranged around a hemisphere. Each source emits a beam of gamma rays, directed to a set point at the centre of the unit. The beams are further focused by collimator helmets, with apertures of 4 to 18 mm and result in "shots" of corresponding size. Since each of the cobalt radiation beams is relatively weak, the tissues are exposed to minimal amounts of radiation, except where these 201 beams converge, or meet, to provide the necessary radiation dose.

The patient is fitted with a stereotactic frame that serves as a reference for localizing the target and preventing movement of the head during treatment. An MRI scan is then performed to identify the target or targets. In some cases, CT scan or angiogram may also be used. Information about the target location within the frame is then transferred to the Leksell Gamma Plan computer planning station.

A GKS plan is designed to precisely cover the defined target, be it a tumor, AVM, or functional brain area with a prescribed dose of high energy gamma radiation (i.e. conformal) while minimizing the radiation exposure to immediately surrounding brain and associated structures. To create a treatment plan, one or more exposures or "shots" of gamma radiation are prescribed. The cumulative dose of these shots is displayed on the planning station computer, and adjustments made until an ideal plan is created. Once the plan is completed, all doses prescribed are given in one treatment.

The treatment plan is completed by the neurosurgeon, radiation oncologist and medical physicist, each ensuring optimal safety and effectiveness. The approved GKS plan is then transferred to the operator console that controls the procedure.

GKS is performed with the patient lying on the gamma knife couch and their head frame docked with the selected collimator helmet. During a treatment, the shielding doors open and the patient is automatically positioned within the unit. The collimators and helmet focus the beams to converge on the treatment target. Movement of the patient couch in and out of the radiation unit and the opening and closing of the shielding doors are the only sources of movement during the procedure. During the entire procedure, audio and visual communication between the patient and the GKS team is maintained via TV monitor and intercom. You will move in and out at the beginning and end of the procedure and part way out for the position changes.

One recent advance in technology is the Automated Positioning System (APS). This computer guided robotic modification maintains the head position during each GKS "shot", and then makes micro-millimetric adjustments as prescribed by the treatment plan. The APS has increased the speed and efficiency of performing GKS, and that results in a more comfortable and potentially more effective treatment for the patient because it makes use of multiple focus points (isocenters) easier which allows one to produce a more conformal plan.


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