History, Philosophy & Terminology

1.The words, defined

Because these terms are used loosely in conversation, the hub holds to the consensus definitions used by the professional societies and physics task groups:

• SRS (stereotactic radiosurgery): strictly, stereotactically guided, high-precision delivery of a high dose to an intracranial (or skull-base/upper-cervical) target in a single fraction. This hub reserves SRS for the single fraction and names multi-fraction cranial treatment hSRT or FSRT. Note that this is a deliberately strict convention rather than the society definition: ASTRO's SRS model policy defines cranial SRS as one fraction or two to five fractions, so 1 to 5 fraction cranial treatment is correctly called SRS in society and payer documents. The hub separates them because the radiobiology, immobilization, and constraints differ, not because the broader definition is wrong.
• SRT / FSRT (stereotactic radiotherapy / fractionated SRT): the same stereotactic precision delivered in more than five fractions, used when a target is large or abuts a dose-limiting structure (e.g., optic apparatus). Treatments of two to five fractions are named hSRT, which is the more specific term and the one this hub uses.
• hSRT (hypofractionated SRT): the common middle ground of 2–5 fractions for cranial targets too large or too eloquent for a single session.
• SBRT (stereotactic body radiotherapy) and its synonym SABR (stereotactic ablative radiotherapy): the extracranial counterpart: stereotactic, ablative-intent treatment of body sites (spine, lung, liver, and others) in roughly 1–5 fractions, with the added challenge of organ motion.

The distinctions are not pedantic. Single-fraction cranial SRS, five-fraction cavity hSRT, and three-fraction spine SBRT differ in radiobiology, immobilization, image guidance, and the constraints that govern safety, which is why the hub separates them.

2.Leksell and the radiosurgical concept

The Swedish neurosurgeon Lars Leksell introduced the term and the concept of stereotactic radiosurgery in 1951, proposing that a stereotactic frame could be used to aim many cross-firing beams of radiation at a deep intracranial target, destroying it without a craniotomy. His earliest implementations used an orthovoltage X-ray tube mounted on his stereotactic arc, and collaborators in Uppsala explored crossed proton beams. Dissatisfied with both, Leksell and the radiobiologist Borje Larsson designed a purpose-built device using many fixed cobalt-60 sources arrayed in a hemisphere so their beams converged on a single point: the Gamma Knife. The first clinical units were installed in Stockholm in the late 1960s, initially used for functional targets (including pain and movement/psychiatric disorders) before the modern tumor and vascular indications matured. Leksell's 1983 summary cemented the principles that still define the field: rigid stereotactic fixation, a sharp dose gradient, and single-session ablation.

3.Particle beams and the parallel track

In parallel, charged-particle radiosurgery developed at physics laboratories: John Lawrence began human pituitary irradiation with cyclotron-produced protons, deuterons, and helium ions at Berkeley in 1954, and William Sweet and Raymond Kjellberg began single-fraction Bragg-peak proton treatment of pituitary tumors and arteriovenous malformations at the Harvard Cyclotron in 1961, exploiting the Bragg peak to deposit dose at depth. Particle radiosurgery remained confined to a few centers because of cost and complexity, but it established that the radiosurgical concept was not tied to cobalt photons.

4.The Gamma Knife reaches North America

The dedicated clinical Gamma Knife program crossed to North America through Pittsburgh. L. Dade Lunsford had trained at the Karolinska Institute directly under Leksell and Erik-Olof Backlund, and carried the method home. After a multi-year effort through the early and mid 1980s to satisfy regulators, hospital administrators, and skeptical colleagues, a Leksell Gamma Knife, the fifth unit built, was installed at the University of Pittsburgh, where the first patient was treated on August 14, 1987. Earlier machines and methods had reached the continent before this: Leksell's original gamma unit was moved to UCLA in 1974 and used experimentally under Robert Rand, and linac radiosurgery had already begun in Boston in January 1986 and Montreal in December 1986. What Pittsburgh established was the first North American program built around a dedicated cobalt unit for routine clinical cranial radiosurgery.

What made Pittsburgh consequential, however, was less the machine than the discipline built around it: a prospective clinical registry maintained across decades, a high-volume practice spanning the full range of cranial indications, and a training pipeline that seeded programs across the country and abroad. That data-driven culture was eventually institutionalized in multi-institutional consortia. The International Gamma Knife Research Foundation: later broadened to the International Radiosurgery Research Foundation (IRRF): was founded in Pittsburgh, with UPMC as its data-coordinating center, and now links more than forty centers worldwide in pooled retrospective and prospective studies. Much of the disease-specific Gamma Knife evidence cited across this hub originates in exactly this kind of multicenter collaboration, which converts single-center experience into generalizable practice. (The International Stereotactic Radiosurgery Society was likewise founded in Pittsburgh.)

5.The LINAC adaptation

Through the 1980s, several groups adapted the widely available medical linear accelerator (LINAC) to radiosurgery by adding stereotactic localization and arc-based delivery (Betti and Derechinsky; Colombo; and, in North America, the Harvard group), with Winston and Lutz formalizing the quality-assurance and accuracy testing that made LINAC radiosurgery trustworthy. The LINAC adaptation democratized radiosurgery (any department with a linac could, in principle, participate) and set up the enduring coexistence of dedicated cobalt systems and multipurpose photon linacs.

6.Frameless, robotic, and body radiosurgery

Two further developments opened the modern era. John Adler, who had trained with Leksell, conceived the CyberKnife (a compact linac on a robotic arm with image-guided, frameless targeting) extending radiosurgery to frameless cranial and, crucially, spinal and body treatment. Around the same time, Lax and Blomgren at the Karolinska built a stereotactic body frame and reported high-dose, few-fraction treatment of extracranial tumors, founding SBRT/SABR. Frameless image guidance (cone-beam CT, planar kV imaging, surface tracking) then spread to conventional linacs, so that today the radiosurgical concept is delivered by dedicated cobalt units, gantry linacs, robotic linacs, and particle systems alike: the subject of the platforms page.

7.A shared discipline with two parent specialties

Radiosurgery is unusual in being co-owned by neurosurgery and radiation oncology, with medical physics indispensable to both. That shared ownership is a strength, it pairs anatomical and surgical judgment with radiobiologic and dosimetric expertise, but it also means the literature, terminology, and even the names of the same target can differ by specialty. The hub deliberately writes for both audiences. It also stays platform-neutral: the evidence rarely supports one delivery technology as categorically superior for a given indication, and the choice is usually driven by target size and location, fractionation needs, institutional resources, and operator experience rather than by brand. Where a personal practice preference is offered, it is labeled as such, not presented as established fact.