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AANS Neurosurgeon | Volume 28, Number 2, 2019

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Stereotactic Radiosurgery: A Neurosurgical Technology

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Your patients and families are excited to hear of their option to be treated with stereotactic radiosurgery (SRS): a high tech alternative to having your skull and brain open! Or is it?  Starting out as minimally invasive and fully in the realm of neurosurgery, this technology has involved to even less invasive and the neurosurgeons have sometimes been marginalized.  What is best in this swing of the pendulum for our patients? 

Consider that in 1905-114 years ago- Harvey Cushing defined “the special field of neurological surgery”. This, arguably, marks the recognized origin of neurosurgery as a specialty. If you insist on going back to 1884, when Bennett and Godlee published their case report on craniotomy based on neurological localization, we are 135 years ago. Well, SRS was invented and named by Lars Leksell in 1951 – a full 68 years ago. Now, it is true that 17 years passed between Lekell’s first publication on SRS and the appearance of the Gamma Knife, (GK) which he developed along with his colleagues in medical physics in Stockholm. But that still leaves us with 51 years of SRS as a neurosurgical tool in regular use.

What is Stereotaxis?

Stereotaxis, named and invented by Victor Horsley in 1908, required the use of a rigid frame attached to the patient’s skull as it came into practical use in the late 1940s. That is how Leksell conceived of SRS. The first adaptations of linear accelerators (Linacs) for SRS were reported by neurosurgeons in the early 1980s – Federico Colombo in Verona, and Osvaldo Betti in Buenos Aires. All of these SRS technologies – GK and various Linacs – were based on “wheels in to wheels out” methods, with a rhythm of a minimally invasive surgical procedure.  Patients came in, had a frame placed, were imaged, the treatment was planned, the treatment was delivered, the frame removed, then home. Neurosurgeons were happy with this arrangement, since it seemed “surgical” and also mandated their involvement with SRS.

Dawn of Noninvasive Frames

Then, in 1991, John Adler, a Stanford neurosurgeon, let the genie out of the bottle. After training in neurosurgery at the Brigham and Women’s Hospital, Adler did a year-long fellowship with Leksell. He was inspired to bring SRS to bear on any part of the human body, but this required liberating the procedure from the need for a rigidly attached frame. Thus was born the Cyberknife, the first mask-based tool for SRS or for any stereotactic procedure, for that matter. By the turn of the millennium, the Cyberknife had become a viable and commercially available tool.

Now, there was no turning back. Other Linac manufacturers soon followed suit, giving their practitioners a potential advantage over users of the frame-based GK. Not only could the frame application be bypassed, but as some neurosurgeons soon began to ask: who said that SRS had to be in a single fraction? That was necessitated by the pain and discomfort of the frame, but in the new era of mask-based SRS, this limitation no longer existed.  Lurking in the background was the potential for radiation oncologists to perform SRS without the need for neurosurgical participation. After much give-and-take with organized radiation oncology, and amongst neurosurgeons themselves, a new consensus definition of SRS was agreed and published in 2007. Now, SRS can be a treatment given in up to 5 fractions. The only holdouts to this new world were members of the GK community, who vociferously defended the use of a rigid frame and insisted on its superiority, along with disparaging the idea of fractionated SRS.

Can SRS be a Purely Radiation Oncology Tool?

Well, the chickens have come home to roost. Starting three years ago, the GK could now be used with a mask for single session or fractionated SRS. What is the role of neurosurgeons in these procedures? The fact is that the treatments can occur without neurosurgical presence or even involvement. Patients have the mask fitted in advance of the treatment, which is planned over 1-2 days. SRS delivery is done with the patients fixed in a mask, along with various methods for tracking that ensure that immobilization is maintained. These techniques are all a part of routine radiation oncology practice.

So how do we prevent SRS, a concept and a tool invented and perfected by neurosurgeons, from slipping away completely as part of our practice and research activities? It is clear that this is in our collective hands. Treating mask-bases SRS as an annoyance that isn’t surgery, then neurosurgeons will increasingly be marginalized and our involvement minimalized, until our very presence in the field we invented will be historical only. We must be actively engaged in the contouring of targets, of modifying the treatment plans as needed, and be present during the treatment. In addition, we must continue to drive the indications, research and innovation in both the cranial and spinal realms of SRS.  How unfortunate it would be for neurosurgeons to not be part of this new world of stereotactic radiosurgery!

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