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Radiation Oncology: An Overview of the Treatment 

  Gamma Knife: A Tool for Brain Surgery 

by Dr. Timothy D. Nichols

Gamma Knife, the name conjures images of nuclear rays carving out its target with exquisite precision. In reality, the machine has nothing in common with a scalpel or even a modern cutting laser. During a Gamma Knife stereotactic radiosurgery procedure, there is no cutting at all, and, in fact, during the actual treatment, there is nothing on the treatment device even moving.

Radiosurgery is a term coined in 1951 to describe a procedure which ablates,


Dr. Tim Nichols, Radiation Oncologist at
Presbyterian Hospital of Dallas, plans a Gamma Knife treatment.

 

or destroys, a small portion of brain tissue with a high dose of radiation given in a single fraction, that is, all within a few minutes to a few hours on the same day.

The concept was first developed and tested by Lars Leksell, a Swedish neurosurgeon, for the treatment of certain "functional" disorders, such as Parkinson’s Disease and a pain syndrome called tic douloureux or trigeminal neuralgia. These were diseases not caused by tumors or strokes, but abnormalities in the function of otherwise normal appearing brain tissue. The motivation for this was straightforward -- the mortality rate for an open brain operation in the 1950’s was over 40% and hardly worth the risk for patients not in danger of dying from their disease.

Radiosurgery combines stereotaxis, a method for locating any point within the brain on an X,Y, Z coordinate system with equipment to deliver the high dose of radiation to the designated target. The Leksell Gamma Knife unit is one such radiosurgery device.

Dr. Richard Weiner, Neurosurgeon, places the head frame in
position to begin Gamma Knife radiosurgery.

The main components of the Leksell Gamma Knife U unit are truly mid 20th century technology. The 18,000 Kg device houses 201 high activity Co-60 radioactive sources, each situated over a carefully machined tube. The gamma-rays from all 201 sources are directed at the same point. The only moving parts are a door, to block radiation from escaping the machine when not in use, and an hydraulic or electric couch, to move the patient in and out of the machine. The treatment actually starts when the couch locks into its final position at the intersection point of the 201 Cobalt pencil beams. A very accurate timing device releases the couch once the treatment is complete. This machine can repeatedly lock in to the same spot within a fraction of a millimeter, and in practice, a lesion within the brain of a patient can be treated with an accuracy of between one third to one millimeter.

The Gamma Knife unit at Presbyterian Hospital of Dallas.

Why, one might ask, is a machine whose technology was available in the late 1960’s only now being used on a wide scale for the benefit of patients? Doctor Leksell was about two decades ahead of his time. He developed a machine which could reproducibly treat areas of the brain on a millimeter scale when imaging technology could only show those areas down to a centimeter level. Also, while the 201 Cobalt beams produced a beautifully tight area of high dose radiation around the target, the laborious calculations to determine a patient’s treatment time took hours. The full potential of this 1960’s machine could not be realized until MRI scanners and powerful small computers where available.

With the ability to identify small targets accurately, and to calculate the dose for complex treatment plans quickly, brain surgery without a knife is being realized for many patients. Arteriovenous malformations, an abnormal collection of blood vessels with a very high risk of bleeding, have been effectively treated for 10 years with this device. Benign brain tumors, such as meningiomas, pituitary adenomas and acoustic neuromas can also be treated in lieu of conventional surgery. Malignant brain tumors are also often treated effectively with radiosurgery. Lastly, with the most recent advances in MRI techniques and treatment planning software, those "functional" problems for which Lars Leksell originally developed the Gamma Knife, are being effectively tackled. The often severe facial pain syndrome, trigeminal neuralgia, has recently been approved for treatment with the Gamma Knife by the FDA as recent long term data on its effectiveness has become available. There are now several hundred patients treated for Parkinson’s Disease being followed, and initial results look promising.

With 45 centers in the U.S. alone, more than 80,000 patients have been treated with the Gamma Knife, world wide. The benefits of this treatment are available to most Texans, with centers in El Paso, Houston, San Antonio and Dallas. Of course, there are many factors which must be considered before any treatment modality is selected, and for some, open surgery is the best choice. For many, however, the availability of the Gamma Knife may give them an effective, less painful, less risky alternative.

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