Editor’s Note: This column is an update to the former “Timeline” column readers saw in previous issues of AANS Neurosurgeon. It reflects the ideas and thoughts of the publication’s editor.

When did neurosurgery begin? Some say it started when Harvey Cushing defined “the special field of neurological surgery” in 1905. Of course, surgery of the central nervous system was being done by others at that time and long before that, as well. Stereotactic radiosurgery (SRS), on the other hand, has an undeniable point of origin. The concept and, indeed the very term, were coined by Dr. Lars Leksell in 1951. (For those who may not know, Leksell was the Swedish neurosurgeon who also devised your double-action rongeurs.) So it is clear that SRS is not at all a brand-new technology. In fact, 60 years after its birth, SRS is at least half as old as neurosurgery. It is equally clear that SRS was invented by a neurosurgeon. (For more about the history of radiosurgery, click here.)

Leksell conceived of SRS primarily as a tool for minimally invasive surgery for the relief of pain and movement disorders. (Indeed, it is ironic that after decades of SRS aimed at treating tumors and arteriovenous malformations (AVMs), attention again is being paid to its use in functional neurosurgery). Working with partners in physics, Leksell tried various methods of focusing radiation, including orthovoltage x-rays, linear accelerators and heavy particle beams. Not finding satisfaction with these energy sources, he devised a new one based on radioactive sources — the Gamma Knife (GK), which made its debut in 1968. Leksell and colleagues began to treat vestibular schwannomas (as they now are called) and intracranial AVMs, as these lesions could be visualized in that pre-CT era. The GK also was used to treat patients with trigeminal neuralgia. For more than a decade, SRS meant, in essence, the sole GK in the world at the Karolinska Institute in Stockholm. In the 1980s, other GKs were installed in England, Chile and the United States (at the University of Pittsburgh). There now are 125 GKs in the United States and hundreds more around the world.

Neurosurgeons have been at the forefront of parallel developments in SRS. Heavy particle irradiation was promoted in large part by the work of Dr. Raymond Kjellberg, a neurosurgeon at Massachusetts General Hospital. We now think of heavy particle therapy as a form of SRS, whose application has been limited by the expense and complexity of the cyclotrons needed to generate the beams. Linear accelerators (LINACs) were first adapted for SRS in the early 1980s by Drs. Federico Colombo and Osvaldo Betti, neurosurgeons working in Vicenza, Italy, and Buenos Aires, Argentina, respectively. Shortly thereafter, Dr. Ken Winston at the Brigham and Women’s Hospital led the adaptation of a commercially available stereotactic frame for linear-accelerator (LINAC) -based SRS, a project that led to invention of the Radionics X-Knife — an integrated, turnkey SRS system. Other such LINAC-based SRS tools were developed, including the University of Florida system by neurosurgeon Dr. William Friedman. (Remember that, like Leksell, all the other pioneering neurosurgeons mentioned here worked in close collaboration with physicists.)

Twenty years ago, SRS was an infrequent topic in our main neurosurgical journals. At our annual meetings, it typically was consigned to a poorly attended concurrent afternoon talk where the main issue was the attempt by LINAC users to prove their devices were as reliable and effective as the GK. These unedifying debates mostly came to an end by the late 1990s as more literature emerged demonstrating the utility of LINAC-based SRS. After all, both of these competing approaches used a stereotactic frame to immobilize the patient and to establish a stereotactic space for treatment planning and delivery.

A major paradigm shift in SRS resulted from the work of neurosurgeon Dr. John Adler at Stanford University. While a fellow at the Karolinska Institute under Dr. Leksell’s tutelage, he first conceived of delivering SRS without the need for a stereotactic frame. This would have the advantage of allowing any target in the head — indeed, a target anywhere in a patient’s body — to be treated with SRS. His work resulted in the Cyberknife, which uses a robot-mounted linear accelerator. The frameless method devised by Adler uses registration of repeated fluoroscopic images to preoperative CT in order to do patient position correction as needed. The Cyberknife has been a commercial product for about a decade.

The ingenious engineering behind the Cyberknife let the frameless SRS genie out of the bottle. Brainlab adapted its Novalis SRS system for frameless use. And every new updated LINAC began to come with the assurance that it, too, could perform frameless SRS, using the same concepts of image registration (although the accuracy and applicability of these devices for true SRS remains unproven). Now the role of neurosurgeons performing SRS began to blur. When a frame was no longer needed for the procedure, was the neurosurgeon still needed? After all, radiation oncologists were used to contouring structures and, as a rule, knew more than neurosurgeons about the underlying principles of therapeutic irradiation. And, for that matter, why did SRS need to be delivered in a single session? Without a frame, patients could easily be brought back for multiple treatment fractions and repositioned accurately. And, if so, at what transition point of multiple fractions did SRS become radiation therapy?

These concerns led to some uncomfortable times between neurosurgeons and radiation oncologists. However, five years ago, their respective organizations agreed on a definition of SRS. Up to five fractions would be considered SRS, while any more than that would be RT. Neurosurgeons and radiation oncologists were partners in intracranial SRS. The neurosurgeon’s role in frameless treatments was to contour targets and related structures; to ensure proper dose selection; to confirm the treatment plan to minimize complications; and to be present for the procedure, including at least for one fraction if more than one was to be delivered.

The issue of spinal SRS, however, has remained a gray area. Although this, too, was a method developed by neurosurgeons (beginning with the work of Dr. Allan Hamilton in the 1990s, and kicked into high gear by the Cyberknife in the work of Drs. John Adler and Peter Gerszten), spinal SRS has not been as thoroughly embraced by the neurosurgical community as have intracranial treatments. Reasons for this may include the need for additional training and complication avoidance of non-neurological organs at risk (such as the lungs and esophagus). Regardless, it is a fact that spinal SRS also was a concept developed and nurtured by neurosurgeons. You, the readers, are well aware that management of spinal tumors falls well within the purview of neurosurgical practice, and that neurosurgeons are experts in the questions of when and which surgery is appropriate for decompression of the spinal cord or stabilization of the spine — analogous to similar questions that pertain to patients with intracranial lesions.

Neurosurgeons still have an opportunity to confirm their role as practitioners of SRS. We must remember that more than “putting on a frame,” SRS is about the cognitive aspects of patient selection, treatment planning, follow up, outcome analysis and research. Technological advances by neurosurgeons have brought us to this point. We should embrace them and remain leaders in stereotactic radiosurgery.

Michael Schulder, MD, FAANS, is editor of AANS Neurosurgeon and serves as chair of the AANS Neurosurgeon Editorial Board Committee. Dr. Schulder also is a professor and vice chairman of the Department of Neurosurgery at the Hofstra North Shore LIJ School of Medicine in New York. The author reported no conflicts to disclose.