Change is the law of life. And those who look only to the past or present are certain to miss the future.
John F. Kennedy
Since its introduction five decades ago, stereotactic radiosurgery (SRS) has evolved from an investigational concept into a mainstream neurosurgical procedure for the management of a wide variety of brain disorders. Contemporary neurosurgeons routinely use radiosurgery either as a definitive or adjuvant treatment modality in the fields of neuro-oncology and cerebrovascular and functional neurosurgery. Stereotactic radiosurgery offers the surgical neurooncologist a precise and established treatment that, in combination with fractionated radiotherapy, chemotherapy, and conventional surgery, offers additional management options for the treatment of patients with brain tumors.4,5,12 The role of SRS in the management of vascular malformations is also well established. Furthermore, this modality has had a significant impact on the treatment of patients with brain metastases;4,26,51 in cases in which SRS is possible, these patients more commonly succumb to their uncontrolled extracranial disease than to their intracranial disease.
Recently there has been a spate of reports attempting to clarify or to (re)define the terms “stereotactic radiosurgery” and “stereotactic radiotherapy” (SRT).1,48,66 It has become increasingly clear that the evolution of radiosurgery and radiotherapeutic techniques demands a reevaluation of the definition of radiosurgery by organized neurosurgery. These factors led the American Association of Neurological Surgeons (AANS) and the Congress of Neurological Surgeons (CNS) to form the Stereotactic Radiosurgery Task Force under the auspices of the AANS/CNS Washington Committee. Members of the Stereotactic Radiosurgery Task Force were directed to review, clarify, and recommend to their parent organizations a contemporary definition of SRS, which would take into account historical, current, and potential applications of SRS. The purpose of this paper is to express the position of the AANS as well as that of the CNS on the definition of SRS.
Historical Review
“Stereotactic radiosurgery” was defined by the Swedish neurosurgeon Lars Leksell in 1951.57 At that time, Leksell sought to mimic destructive lesions in the brain produced by mechanically invasive stereotactic surgical procedures for movement and pain disorders by delivering a high dose of photon or proton energy to the intended target in a single session, while steep fall-off dose gradients protected the adjacent brain. Early efforts involving stereotactically applied ultrasound, orthovoltage x-ray, and accelerated particles such as protons proved inadequate to create these lesions deep in the brain or were otherwise too cumbersome. To overcome these shortcomings, Leksell, Liden, Larsson, and colleagues developed the Gamma Knife in 1967. This device focuses multiple beams of high-energy gamma rays to a common point directed by frame-based stereotactic guidance.55,58 Contemporaries such as Kjellberg, Winston, Lutz, Loeffler, Fabrikant, and others also developed systems using x-rays or particles to achieve the same ends.22,26,48,73,79
For decades, stereotactic localization was limited to information derived from atlases, plain radiographs, pneumo-encephalograms, and angiograms.37,38,42,56,71 Throughout his life, Leksell remained active in advancing the state of the art of SRS and was one of several visionaries who developed methods of exploiting the spatial information provided by computed tomography and, later, magnetic resonance (MR) imaging, thereby creating the field of image-guided stereotaxy.60 Although the radiosurgical treatment of intracranial malignancies became feasible, Leksell believed that SRS was best used for functional neurosurgery or to treat benign tumors and lesions such as arteriovenous malformations and not to treat malignant tumors.
Early neurosurgeons who performed radiosurgery found that collateral damage to adjacent structures occasionally occurred when treating benign disease; several strategies were devised to reduce complications.47,50 Stereotactic MR imaging was used to provide better visualization and definition of targets and anatomical structures at risk.23 Radiation doses directed to the lesion’s margin were gradually reduced while maintaining therapeutic efficacy.23,25 Computer-assisted planning systems aided the design of treatment plans that better conformed to the shape of the radiosurgery target.23,25 Rigid skull fixation, the “gold-standard” for stereotactic accuracy, was supplemented by relocatable frames that allowed radiosurgery to be performed in multiple sessions.13,16,18,24,39,43,59,63–65,69,70,77,78
Stereotactic radiosurgery became established and accepted as an important neurosurgical technique in the 1980s and 1990s.58,61 Its value transcended the original indications posed by Leksell to include proven efficacy for the most common central nervous system malignancy––metastatic disease.4,26,51 Neurosurgeons wished to extend the reach of this technology beyond the limits of cranial disease. The use of extracranial radiosurgery with the aid of a frame was first reported by Hamilton in 1996.41,72 Concurrently, conventional surgical stereotaxy was revolutionized by the neurosurgical development of frameless stereotactic techniques.8,62,67,74 The notion that radiosurgery could also be delivered without a stereotactic frame was brought to fruition by Adler and others.2,15,30,64,75 New linear accelerator (LINAC)–based radiosurgical instruments rely on image-guided stereotactic targeting and advanced beam delivery methods. In one system, radiosurgical delivery is performed by a lightweight LINAC that is robotically positioned,15,30,75 and in another, by a LINAC whose output is modulated by computer-controlled multileaf collimators.20 Today, radiosurgery can and has been performed on virtually any part of the body, and the fewer fixation requirements facilitate the performance of the procedure in multiple sessions.9–11,13,19,27–29,31–36,40,47,52,68,69,76
Recently developed alternative forms of energy include high-intensity focused ultrasound.17,44,45 When delivered stereotactically to destroy or injure tissue, these other forms of energy could be interpreted by some as falling within the umbrella of SRS.
Role of the Neurosurgeon in SRS
These advances notwithstanding, SRS remains a “team” discipline in which the roles of the surgeon, radiation oncologist, and physicist are essential, regardless of the target organ or site of service. As in any surgical procedure involving the brain or spine, the neurological surgeon provides preoperative assessment of the patient and a review of pertinent imaging studies so that therapeutic alternatives can be presented to the patient and informed consent can be obtained. After the procedure, the neurosurgeon provides continued reevaluation and follow-up review at clinically appropriate intervals in order to assess outcomes on a long-term basis. During the radiosurgical procedure itself, the neurosurgeon serves as the primary responsible healthcare provider. Separate tasks of a radiosurgical procedure, including the treatment setup, planning, and delivery that are performed by or directly supervised by the neurosurgeon, comprise the following: delivery of agents for appropriate conscious sedation; application of the stereotactic coordinate frame (when pertinent) based on lesion location; selection and creation of the appropriate imaging data set (for example, computed tomography scans, MR images, angio-grams, or positron emission tomography images) necessary for radiosurgical planning; computer-assisted delineation of target volumes and adjacent critical anatomical structures; creation of the 3D volumetric radiosurgical effect assisted by computer planning; setup, confirmation, and delivery of radiation; provision of additional sedation as required; monitoring of the patient’s vital signs during radiation delivery; removal of the stereotactic frame followed by bandaging or other wound care as needed; and standard postradiosurgery 90-day follow-up care. As the primary responsible health-care provider, the neurosurgeon assumes responsibility for chart completion as required by the patient’s inpatient or ambulatory status after radiosurgery.
Recent Publications on the Role of Radiosurgery Versus SRT
Because new technology now enables radiosurgery to be delivered in more than one session and because “radiation therapy” is sometimes administered with the aid of stereotactic localization, there have been several attempts in the neurosurgical literature during the past few years to define, redefine, or clarify the term SRS.1,48,66 At present there are “purists” who prefer the original definition of SRS offered by Lars Leksell some 50 years ago, while others subscribe to the concept of a procedure that has evolved with the emergence of new technology.
The Traditional Perspective
The principal argument made by authors espousing the traditional perspective is that the term radiosurgery must be restricted to a high dose of ionizing radiation delivered to a defined target in a single session.48,66 Stereotactic radiosurgery derives its safety by its high degree of conformality and high selectivity (shown by the steep dose falloff in the adjacent normal tissue), such that dose homogeneity within the target area is irrelevant. On the other hand, these authors contend that the delivery of fractionated radiation delivered in multiple sessions by daily application of a non–skeleton-affixed guiding device (SRT) is usually less conformal and precise than conventional frame-based SRS. This presumably makes dose homogeneity desirable. This group also maintains that the rationale for SRT is primarily an attempt to reduce the risks of radiation damage to the surrounding normal tissue. Finally, they state that the term “(hypo-)fractionated stereotactic radiosurgery” is an oxymoron.
Alternative Perspectives
All will agree that a high dose of ionizing radiation delivered to a stereotactically defined target in a single session is (a form of) SRS. Contemporary controversies focus on two areas: can “radiosurgery” be delivered in more than one session, and, if so, where does SRS delivered in multiple sessions end and SRT begin?
The historical review presented earlier demonstrates the evolutionary process of thought and practice in SRS throughout the past five decades. We believe that a reasonable person will recognize that this evolution includes radiosurgery delivered in more than one session. In his original description of SRS in 1951, Lars Leksell did not specifically state that the procedure needed be performed in a single session. In 1983, Leksell described SRS as “a technique for the non-invasive destruction of intracranial tissues or lesions. . . [in which] the open stereotactic method provides the basis. . . .”58—again without explicitly restricting its use to a single session. Statements limiting SRS to a single session arose years later, in describing the state of practice at that time.6,7,20,53 Today, the American Medical Association recognizes that SRS may be undertaken in one or more sessions according to Current Procedural Terminology,3 as does the Centers for Medicare and Medicaid Services.14
Ionizing radiation has been used for longer than a century in medical therapy. Much has been made of the differential radiobiology of SRS and fractionated radiotherapy––the “Four Rs” of reoxygenation, reassortment, repopulation, and repair1,20––to distinguish SRS from SRT. In truth, little is known about the true radiobiology of radiosurgery and these arguments are theoretical at best.49,54
What is known is the intent of the treatment. Radiosurgery aims to injure or destroy tissue at the target and preserve adjacent critical tissue, primarily due to steep dose gradients. Homogeneity within the lesion is generally not considered important and can be a disadvantage for achieving tumor shrinkage when treating lesions that do not contain normal tissue or for treating internal tumor areas of necrosis or hypoxemia. Tumors that may be resistant to fractionated radiotherapy may respond well to radiosurgery. Multiple sessions may be used to further reduce injury to adjacent normal tissue while maintaining the efficacy of radiosurgery. In fractionated radiotherapy abnormal tissue is differentiated from normal tissue within the target site by the differential sensitivity of these tissues to fractionated ionizing radiation.21 Dose homogeneity is desirable when the treatment volume contains sensitive normal tissue (either in the tumor or closely adjacent). Deleterious effects outside the treatment area may be further reduced by enhancing treatment conformality and by increasing the dose gradient. Either technique may be directed stereotactically (SRS and SRT).
Few would disagree that the precise stereotactic delivery of a high dose of radiation for the purpose of tissue inactivation or destruction in a single session is within the scope of SRS, and that the precise stereotactic delivery of radiation in 30 sessions is not SRS but is better described as SRT. Conversely, such a single-session delivery should fall outside the scope of SRT. Between these extremes, however, are cases of potential overlap between the techniques. We believe that these are best differentiated by the intended mechanism of action and that data in the literature, federal policy, and contemporary practice indicate that the upper limit of sessions in which SRS may be delivered is five.14
After considerable debate and discussions, on June 29, 2005, the members of the AANS/CNS Stereotactic Radiosurgery Task Force (Appendix A) met in Chicago and arrived at a contemporary definition of SRS, which has subsequently been approved by both parent organizations. Thereafter, on March 20, 2006, representatives of the AANS/CNS met with the corresponding body of the American Society for Therapeutic Radiology and Oncology (ASTRO; Appendix B) and refined this definition of radiosurgery, subsequently sanctioned by the AANS, CNS, and ASTRO:
Stereotactic Radiosurgery is a distinct discipline that utilizes externally generated ionizing radiation in certain cases to inactivate or eradicate (a) defined target(s) in the head or spine without the need to make an incision. The target is defined by high-resolution stereotactic imaging. To assure quality of patient care the procedure involves a multidisciplinary team consisting of a neurosurgeon, radiation oncologist, and medical physicist.
Stereotactic Radiosurgery (SRS) typically is performed in a single session, using a rigidly attached stereotactic guiding device, other immobilization technology and/or a stereotactic image-guidance system, but can be performed in a limited number of sessions, up to a maximum of five.
Technologies that are used to perform SRS include linear accelerators, particle beam accelerators and multisource Cobalt 60 units. In order to enhance precision, various devices may incorporate robotics and real time imaging.
Appendix A
Members of the AANS/CNS Washington Committee Stereotactic Radiosurgery Task Force
Gene H. Barnett, M.D., Chair
Mark E. Linskey, M.D., Vice-Chair
John R. Adler, M.D.
Jeffrey W. Cozzens, M.D.
William A. Friedman, M.D.
M. Peter Heilbrun, M.D.
L. Dade Lunsford, M.D.
Michael Schulder, M.D.
Andrew E. Sloan, M.D.
Appendix B
Representatives at the March 20, 2006 Meeting of the AANS/CNS and the ASTRO
AANS/CNS
Gene Barnett, M.D., Chair, AANS/CNS Stereotactic Radiosurgery Task Force; Chair, AANS Representative Board of Directors
Mark Linskey, M.D., Vice-Chair, AANS/CNS Stereotactic Radiosurgery Task Force; Co-Chair, CNS Representative Executive Committee
Greg Przybylski, M.D., Chair AANS/CNS Coding and Reimbursement Committee; Member, AANS Relative Value Update Committee
Jeff Cozzens, M.D., Member, AANS/CNS Coding and Reimbursement Committee; Advisor, AANS Current Procedural Terminology
Troy Tippett, M.D., Chair, AANS/CNS Washington Committee; Member, AANS Board of Directors
Cathy Hill, Senior Manager for Regulatory Affairs, AANS/CNS
Katie Orrico, Director, AANS/CNS Washington Office
ASTRO
K. Kian Ang, M.D., Ph.D., President, ASTRO
Michael Steinberg, M.D., Member, ASTRO Board of Directors; Chair, Health Policy Council; Advisor, Current Procedural Terminology
Louis Potters, M.D., Member, ASTRO Board of Directors; Vice-Chair, Health Policy Council; Member, Ambulatory Payment Classification Panel
Timothy Williams, M.D., Co-Chair, Health Policy Committee
David Beyer, M.D., Co-Chair, Health Policy Committee; Advisor, Current Procedural Terminology
Najeeb Mohideen, M.D., Chair, Code Utilization, Application, Development and Valuation Committee; Representative, Relative Value Update Committee
Joel Cherlow, M.D., Chair, Regulatory Committee
Trisha Crishock, Director of Health Policy, ASTRO
Debra Lansey, Assistant Director of Health Policy, ASTRO
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