Stereotactic radiosurgery versus stereotactic radiotherapy for patients with vestibular schwannoma: a Leksell Gamma Knife Society 2000 debate

By definition, the term "radiosurgery" refers to the delivery of a therapeutic radiation dose in a single fraction, not simply the use of stereotaxy. Multiple-fraction delivery is better termed "stereotactic radiotherapy." There are compelling radiobiological principles supporting the biological superiority of single-fraction radiation for achieving an optimal therapeutic response for the slowly proliferating, late-responding, tissue of a schwannoma. It is axiomatic that complication avoidance requires precise three-dimensional conformality between treatment and tumor volumes. This degree of conformality can only be achieved through complex multiisocenter planning. Alternative radiosurgery devices are generally limited to delivering one to four isocenters in a single treatment session. Although they can reproduce dose plans similar in conformality to early gamma knife dose plans by using a similar number of isocenters, they cannot reproduce the conformality of modern gamma knife plans based on magnetic resonance image-targeted localization and five to 30 isocenters. A disturbing trend is developing in which institutions without nongamma knife radiosurgery (GKS) centers are championing and/or shifting to hypofractionated stereotactic radiotherapy for vestibular schwannomas. This trend appears to be driven by a desire to reduce complication rates to compete with modern GKS results by using complex multiisocenter planning. Aggressive advertising and marketing from some of these centers even paradoxically suggests biological superiority of hypofractionation approaches over single-dose radiosurgery for vestibular schwannomas. At the same time these centers continue to use the term radiosurgery to describe their hypofractionated radiotherapy approach in an apparent effort to benefit from a GKS "halo effect." It must be reemphasized that as neurosurgeons our primary duty is to achieve permanent tumor control for our patients and not to eliminate complications at the expense of potential late recurrence. The answer to minimizing complications while maintaining maximum tumor control is improved conformality of radiosurgery dose planning and not resorting to homeopathic radiosurgery doses or hypofractionation radiotherapy schemes.     

Chained lightning: part III--Emerging technology, novel therapeutic strategies, and new energy modalities for radiosurgery

Radiosurgery is fundamentally the harnessing of energy and delivering it to a focal target for a therapeutic effect. The evolution of radiosurgical technology and practice has served toward refining methodologies for better conformal energy delivery. In the past, this has resulted in developing strategies for improved beam generation and delivery. Ultimately, however, our current instrumentation and treatment modalities may be approaching a practical limit with regard to further optimizing energy containment. In looking forward, several strategies are emerging to circumvent these limitations and improve conformal radiosurgery. Refinement of imaging techniques through functional imaging and nanoprobes for cancer detection may benefit lesion localization and targeting. Methods for enhancing the biological effect while reducing radiation-induced changes are being examined through dose fractionation schedules. Radiosensitizers and photosensitizers are being investigated as agents for modulating the biological response of tissues to radiation and alternative energy forms. Discovery of new energy modalities is being pursued through development of microplanar beams, free electron lasers, and high-intensity focused ultrasound. The exploration of these future possibilities will provide the tools for radiosurgical treatment of a broader spectrum of diseases for the next generation.     

An analysis of the accuracy of the CyberKnife: a robotic frameless stereotactic radiosurgical system

OBJECTIVE: The use of stereotactic radiosurgical systems to treat intracranial and extracranial tumors and other lesions requires a high degree of accuracy in target identification and localization. The purpose of this study was to evaluate the total system accuracy of the CyberKnife (Accuray, Inc., Sunnyvale, CA), a frameless, image-guided, stereotactic radiosurgery system. METHODS: Clinically relevant accuracy or application accuracy of the CyberKnife radiosurgery system is based on 1) the beam delivery accuracy, which combines the robot and the camera image tracking system, and 2) target localization accuracy, which combines computed tomographic (CT) imaging and treatment planning. Clinically relevant accuracy can be measured by delivering a radiation dose to phantoms, in which the target is defined on a set of CT images using all components of the CyberKnife system, including the treatment planning software, the robot, the camera tracking system, and the linear accelerator. Clinically relevant accuracy was measured in head phantoms loaded with packs of radiochromic film. The accuracy measured is the displacement of the dose contours from the treatment plan to that measured in the radiosurgically exposed phantom. RESULTS: Measurements of mean errors of the second-generation CyberKnife system at Stanford University Medical Center, installed in 2001, ranged from 0.7 mm for a CT slice thickness of 0.625 mm to 1.97 mm for a CT slice thickness of 3.75 mm. CONCLUSION: The frameless, image-guided, second-generation CyberKnife radiosurgery system has a clinically relevant accuracy of 1.1 +/- 0.3 mm when CT slice thicknesses of 1.25 mm are used. CyberKnife precision is comparable to published localization errors in current frame-based radiosurgical systems.     

Stereotactic radiosurgery. VII. Radiosurgery versus conventionally-fractionated radiotherapy in the treatment of cavernous sinus meningiomas

Twenty-eight patients with meningiomas involving the cavernous sinus were referred to the radiosurgical service at St. Bartholomew's Hospital 1989-1998. The majority (8/13) of patients with small (< 3 cm diameter) tumours received radiosurgery whereas the majority of large tumours (> 3 cm diameter; 12/15) received conventionally-fractionated radiotherapy. Other treatment recommendations were fractionated radiosurgery (one case) and conventionally-fractionated radiotherapy to the whole meningeal base and a radiosurgery boost (three cases). Fractionated radiosurgery is optimal where the target volume is small, but abuts critically sensitive nervous system. There have been no progressions of disease at relatively early follow-up. It is argued that subtotal excision followed by appropriate dose radiation therapy is often a strong competitor to attempted radical excision given the attendant morbidity of this latter operation for meningiomas at this site. Patients referred for radiation therapy are best served by a department with both radiosurgery and conventional radiation therapy facilities.     

Proton radiation therapy for chordomas and chondrosarcomas of the skull base

Most patients with conventional radiotherapy after surgery die with local disease progression. The superior local tumor control and overall survival achieved with fractionated proton RT can be attributed to improved dose localization characteristics of protons, resulting in higher doses delivered. Patients with base of skull neoplasms are increasingly considered for stereotactic radiosurgery. Recently, Muthukumar et al reported for the University of Pittsburgh group on cobalt-60 Gamma Knife (Elekta Instruments, Atlanta, GA) therapy for 15 patients with chordomas or chondrosarcomas of the base of the skull. With tumor volumes ranging between 0.98 and 10.3 mL (mean, 4.6 mL), doses to the tumor margin varying from 12 to 20 Gy (median, 18 Gy) were delivered. Two patients were treated without histologic tumor confirmation. After a median follow-up time of 40 months, 2 patients had died of disease, 2 patients had succumbed to intercurrent disease, and 1 patient surviving at the time of analysis had developed tumor progression. Neither actuarial local control nor actuarial survival data were presented. In the LLUMC series, most tumors exceeded sizes reportedly suitable for radiosurgery or were of a highly irregular configuration. Nevertheless, in 11 patients, tumors less than 15 mL in size remained locally controlled as did tumors sized between 15 and 25 mL in 11 additional patients; these patients were thus potential candidates for stereotactic radiosurgery. At present, too few reports on radiosurgery contain sufficient patient numbers and statistical analyses to permit one to draw conclusions about the feasibility of radiosurgery for chordomas and chondrosarcomas of the base of the skull. A principal difference between proton RT and radiosurgery as currently practiced in most centers concerns target definition. In proton RT, the GTV is treated. In addition, a clinical volume is defined, which is distinctly different from the GTV in size and shape, to include the operative site and other areas of microscopic risk. In many instances, only the GTV is targeted in radiosurgery. Although it is certainly appropriate to explore the role that radiosurgical techniques may have in treating these tumors, results should be evaluated against the excellent outcome that can be achieved with fractionated proton RT, particularly in patients with tumors small enough and of favorable configuration and location to make them candidates for radiosurgery. The present problem of particle therapy is its limited availability. In the United States, only two proton centers can currently provide treatment for base of skull lesions. The HCL is soon to be replaced by a hospital-based facility at the MGH. Several other proton centers in the United States are currently under active consideration. Proton RT is an evolutionary process. Recent developments in proton RT include intensity modulated therapy and improvements in beam delivery systems, namely, the introduction of active beam scanning. These should further increase the degree of dose conformity. In addition, other heavy particles are also being investigated so as to combine the physical advantages of protons with the differential increased biologic effectiveness of particles in tumor as compared to normal tissues. A report from the Heavy Ion Research Facility in Darmstadt, Germany, has not revealed any increased acute toxicities in the first 13 patients with skull base chordomas or chondrosarcomas treated using carbon ions. Several important factors have emerged from recently published results: Patients with low-grade chondrosarcomas and male patients with chordomas have an excellent chance of durable tumor control and long-term survival after proton RT. Severe complications are within the acceptable range considering the high doses delivered and given the major morbidity associated with uncontrollable tumor growth in such patients. Female patients with chordomas experience increased early and late failures     

Cyberknife radiosurgery for metastatic spine tumors

Metastatic spine tumors affect a large number of patients each year, resulting in significant pain,destruction of the spinal column causing mechanical instability, and neurologic deficits. Standard therapeutic options include surgery and fractionated external beam radiotherapy. The first option can be associated with significant morbidity and limited local tumor control. Conversely, radiotherapy may provide less than optimal pain relief and tumor control, because the total dose is limited by the tolerance of adjacent tissues, such as the spinal cord. The emerging technique of spinal radiosurgery represents a logical extension of the current state-of-the-art radiation therapy. It has the potential to significantly improve local control of cancer of the spine, which could translate into more effective palliation and potentially longer survival. Spinal radiosurgery might offer improved pain control and a longer duration of pain control by giving larger radiobiologic doses.This technique also allows for the treatment of lesions previously irradiated using external beam radiation. Another advantage to the patient is that irradiation can be completed in a single day rather than several weeks, which is not inconsequential for patients with a limited life expectancy.In addition, cancer patients may have difficulty with access to a radiation treatment facility for prolonged daily fractionated therapy. This technique allows for the treatment of lesions previously irradiated using external beam radiation.Finally, the procedure is minimally invasive compared with open surgical techniques and can be performed in an outpatient setting. Similar to intracranial radiosurgery, stereo-tactic radiosurgery now has a feasible and safe delivery system available for the treatment of spinal metastatic lesions. The major potential benefit of radiosurgical ablation of spinal lesions is a relatively short treatment time in an outpatient setting combined with potentially better local control of the tumor with minimal risk of side effects. CyberKnife spinal radiosurgery offers a new and important alternative therapeutic modality for the treatment of spinal metastases in medically inoperable patients, previously irradiated sites, and for lesions not amenable to open surgical techniques or as an adjunct to surgery.Spinal radiosurgery is likely to become an essential part of any neurosurgical spine center that treats a large number of patients with spinal metastases.     

Linear accelerator radiosurgery at the University of Florida.

The University of Florida radiosurgical project began in 1986 with the following design criteria: the most accurate radiosurgical device possible, state-of-the-art computer hardware and software for dose planning, and a number of collimators sufficient to treat any lesion homogeneously. In this article we have reviewed how these goals have been met. Physical aspects of this device (accuracy, dose gradient, and dose-planning speed) as well as clinical results compare favorably with any other radiosurgical experience. We believe that LINAC radiosurgical systems are advantageous in terms of cost, variety of collimator sizes available, and currently available sophistication of computerized dose planning. In the near future, the development of conformal treatment may significantly change the entire field of radiosurgery by offering heretofore unobtainable dose plans for irregularly shaped lesions. In addition, LINAC systems may be adapted for stereotactically focused fractionated radiation therapy and for radiosurgical treatment of lesions elsewhere in the body. Accuracy and computer sophistication notwithstanding, we cannot emphasize strongly enough our belief that the least important determinant of radiosurgical results is the machine used to deliver the radiation. It is absolutely vital that all groups undertaking radiosurgery include neurosurgeons, radiation physicists, and radiation therapists who have spent considerable time studying and learning the myriad details necessary to produce consistently good results. All radiosurgical patients must be followed up carefully and studied so that we can learn how to better apply this technique. Only patients who are not candidates for conventional surgery should be treated radiosurgically, at least until much more is known about long-term success and complication rates. A patient never should be treated radiosurgically simply because the referring or treating neurosurgeon is uncomfortable with proven conventional procedures. All groups performing radiosurgery should strive to adhere to the highest possible standards. We are all responsible for verifying the adequacy of our radiosurgical systems. We are all responsible for selecting our patients well, treating them with a team approach that applies the latest available knowledge of our field, following up closely, and reporting our results honestly and thoroughly so that all can benefit. We owe this, at least, to our patients and to neurosurgery.     

CyberKnife frameless stereotactic radiosurgery for spinal lesions: clinical experience in 125 cases

OBJECTIVE: The role of stereotactic radiosurgery for the treatment of intracranial lesions is well established. Its use for the treatment of spinal lesions has been limited by the availability of effective target-immobilizing devices. Conventional external beam radiotherapy lacks the precision to allow delivery of large doses of radiation near radiosensitive structures such as the spinal cord. The CyberKnife (Accuray, Inc., Sunnyvale, CA) is an image-guided frameless stereotactic radiosurgery system that allows for the radiosurgical treatment of spinal lesions. This study evaluated the feasibility and effectiveness of the treatment of spinal lesions with a single-fraction radiosurgical technique using the CyberKnife. METHODS: The CyberKnife system uses the coupling of an orthogonal pair of x-ray cameras to a dynamically manipulated robot-mounted linear accelerator with six degrees of freedom that guides the therapy beam to the intended target without the use of frame-based fixation. Real-time imaging allows the tracking of patient movement. Cervical spine lesions were located and tracked relative to cranial bony landmarks; lower spinal lesions were tracked relative to fiducial bone markers. In this prospective cohort evaluation of a spine radiosurgery technique, 125 spinal lesions in 115 consecutive patients were treated with a single-fraction radiosurgery technique (45 cervical, 30 thoracic, 36 lumbar, and 14 sacral). There were 17 benign tumors and 108 metastatic lesions. All dose plans were calculated on the basis of computed tomographic images acquired from 1.25-mm slices with an inverse treatment planning technique. Radiosurgical circular cones ranging in diameter from 5 to 40 mm were used. RESULTS: Tumor volume ranged from 0.3 to 232 cm(3) (mean, 27.8 cm(3)). Seventy-eight lesions had received external beam irradiation previously. Tumor dose was maintained at 12 to 20 Gy to the 80% isodose line (mean, 14 Gy); canal volume receiving more than 8 Gy ranged from 0.0 to 1.7 cm(3) (mean, 0.2 cm(3)). No acute radiation toxicity or new neurological deficits occurred during the follow-up period (range, 9-30 mo; median, 18 mo). Axial and radicular pain improved in 74 of 79 patients who were symptomatic before treatment. CONCLUSION: This is the first large prospective evaluation of this frameless image-guided spinal radiosurgery system. The CyberKnife system was found to be feasible, safe, and effective. The major potential benefits of radiosurgical ablation of spinal lesions are short treatment time in an outpatient setting with rapid recovery and symptomatic response. This technique offers a successful therapeutic modality for the treatment of a variety of spinal lesions as a primary treatment or for lesions not amenable to open surgical techniques, in medically inoperable patients, in lesions located in previously irradiated sites, or as an adjunct to surgery.     

Chained lightning, part II: neurosurgical principles, radiosurgical technology, and the manipulation of energy beam delivery

The fundamental principle in the radiosurgical treatment of neurological conditions is the delivery of energy to a lesion with minimal injury to surrounding structures. The development of radiosurgical techniques from Leksell's original design has focused on the refinement of various methodologies to achieve energy containment within a target. This article is the second in a series reviewing the evolution of radiosurgical instruments with respect to issues of energy beam generation and delivery for improved conformal therapy. Continuing with concepts introduced in an earlier article, this article examines specific aspects of beam delivery and the emergence of stereotactic radiosurgery as a measure for focusing energy beams within a target volume. The application of stereotactic principles and devices to gamma ray and linear accelerator-based energy sources provides the methodology by which energy beams are generated and targeted precisely in a focal lesion. Advanced technological systems are reviewed, including fixed beams, dynamic radiosurgery, multileaf collimation, beam shaping, and robotics as various approaches for manipulating beam delivery. Radiosurgical instruments are also compared with regard to mechanics, geometry, and dosimetry. Finally, new radiosurgical designs currently on the horizon are introduced. In exploring the complex history of radiosurgery, it is evident that the discovery and rediscovery of ideas invariably leads to the development of innovative technology for the next generation.     

Fractionated radiotherapy techniques

A convergence of advances in patient immobilization and localization, patient imaging, beam shaping and delivery, and treatment planning has led to considerable improvement in the ability to deliver highly conformal radiation treatments by radiosurgical or fractionated radiotherapy techniques. The selection of the "best" treatment technique for any given patient needs to consider the morphology of the target and regional organs at risk as well as available technology and institutional expertise.     

Neurosurgical applications of radiotherapy

The term ‘radiosurgery’ (RS) indicates a high precision localized technique of irradiation used as an alternative to surgical excision in patients with malignant or benign conditions, both in the brain and in the body. Brain RS has been historically identified with ‘stereotactic radiotherapy’. The term refers to the long-established neurosurgical technique of localizing the position of a lesion in the brain by using a system of external 3D co-ordinates coupled with rigid head immobilization device (often fixed to the skull). A high dose of radiation is delivered to the target stereotactically identified and a safe and accurate treatment is achieved, minimizing the dose of radiation to the surrounding brain. While for some techniques the traditional stereotactic localization has been replaced by the integration of modern imaging with non-invasive accurate immobilization, the term ‘stereotactic’ is still maintained in the clinical practice. Over the past 30 years, the implementation of powerful diagnostic imaging devices and of new radiotherapy equipment has contributed to the large diffusion of brain RS. RS plays an important role in the management of brain tumours, vascular and functional brain lesions and the expertise of the multidisciplinary treating team (clinical oncologists, neurosurgeons, neuro-radiologists and medical physics) contributes to the treatment success rate.     

Neurosurgical applications of radiotherapy

The term ‘radiosurgery’ (RS) indicates a high precision localized technique of irradiation used as an alternative to surgical excision in patients with malignant or benign conditions, both in the brain and in the body. Brain RS has been historically identified with ‘stereotactic radiotherapy’. The term refers to the long-established neurosurgical technique of localizing the position of a lesion in the brain by using a system of external 3D co-ordinates coupled with rigid head immobilization device (often fixed to the skull). A high dose of radiation is delivered to the target stereotactically identified and a safe and accurate treatment is achieved, minimizing the dose of radiation to the surrounding brain. While for some techniques the traditional stereotactic localization has been replaced by the integration of modern imaging with non-invasive accurate immobilization, the term ‘stereotactic’ is still maintained in the clinical practice. Over the past 30 years, the implementation of powerful diagnostic imaging devices and of new radiotherapy equipment has contributed to the large diffusion of brain RS. RS plays an important role in the management of brain tumours, vascular and functional brain lesions and the expertise of the multidisciplinary treating team (clinical oncologists, neurosurgeons, neuro-radiologists and medical physics) contributes to the treatment success rate.     

Stereotactic radiosurgery. VI. Posterior displacement of the brainstem facilitates safer high dose radiosurgery for clival chordoma.

The current 'best treatment method' for clival chordoma is regarded as radical surgical resection followed by radiation therapy; radiosurgery usually plays a major part in the radiation therapy programme. From primate radiation biology studies and from clinical observations, the brainstem is known to be the most radiosensitive part of the central nervous system. The tolerance of the brainstem to high single radiosurgical doses of radiation is limited (all the more so in programmes such as our own where conventionally fractionated radiotherapy precedes radiosurgery or the patient has relapsed after conventional radiotherapy--as in the patient reported here). In this report we describe the operative displacement of the brainstem posteriorly at time of resection such that the proportion of the prescribed postoperative radiosurgical dose received by the brainstem is greatly reduced (by the order of 50%). The gains perceived to accrue from this technique are quantified from isodosimetric considerations not only in dose sparing to the brainstem, but importantly in that the dose to the clival chordoma may be highly significantly increased without exceeding current accepted tolerance brainstem dose limits. Two patients have received this joint surgical/radiosurgical approach to date; the second case is presented here in detail.     

Stereotactic radiosurgery for pituitary adenomas: an intermediate review of its safety, efficacy, and role in the neurosurgical treatment armamentarium

OBJECT: Pituitary adenomas are very common neoplasms, constituting between 10 and 20% of all primary brain tumors. Historically, the treatment armamentarium for pituitary adenomas has included medical management, microsurgery, and fractionated radiotherapy. More recently, radiosurgery has emerged as a viable treatment option. The goal of this research was to define more fully the efficacy, safety, and role of radiosurgery in the treatment of pituitary adenomas. METHODS: Medical literature databases were searched for articles pertaining to pituitary adenomas and stereotactic radiosurgery. Each study was examined to determine the number of patients, radiosurgical parameters (for example, maximal dose and tumor margin dose), duration of follow-up review, tumor growth control rate, complications, and rate of hormone normalization in the case of functioning adenomas. A total of 35 peer-reviewed studies involving 1621 patients were examined. Radiosurgery resulted in the control of tumor size in approximately 90% of treated patients. The reported rates of hormone normalization for functioning adenomas varied substantially. This was due in part to widespread differences in endocrinological criteria used for the postradiosurgical assessment. The risks of hypopituitarism, radiation-induced neoplasia, and cerebral vasculopathy associated with radiosurgery appeared lower than those for fractionated radiation therapy. Nevertheless, further observation will be required to understand the true probabilities. The incidence of other serious complications following radiosurgery was quite low. CONCLUSIONS: Although microsurgery remains the primary treatment modality in most cases, stereotactic radiosurgery offers both safe and effective treatment for recurrent or residual pituitary adenomas. In rare instances, radiosurgery may be the best initial treatment for patients with pituitary adenomas. Further refinements in the radiosurgical technique will likely lead to improved outcomes.     

Spinal radiosurgery: technology and clinical outcomes

The development of computer-based image guidance has allowed stereotactic radiosurgery and radiotherapy to be freed from the constraints imposed by the stereotactic frames once required for intracranial radiosurgery. This freedom has led to the application of radiosurgery to targets outside the brain. In this paper, we briefly review the technologies, treatment parameters, and clinical outcomes of radiosurgical treatment for spinal pathology, including metastatic tumors and rare but challenging lesions such as arteriovenous malformations and benign tumors. A special emphasis is put on the newest development, fiducial-less robotic radiosurgery. Spinal radiosurgery is associated with excellent rates of tumor control and pain relief with a good dose sparing of the highly sensitive spinal cord. Further research is required to optimize treatment strategies and to assess clinical benefits and toxicity in the long term.     

Extracranial radiosurgery—applications in the management of benign intradural spinal neoplasms

Stereotactic radiosurgery has enabled the delivery of higher doses of radiation and decreased fractionation due to improved accuracy. Spinal radiosurgery has been increasingly utilized for the management of metastatic extradural spinal disease. However, surgical resection remains the primary treatment strategy for intradural spinal tumors. Preliminary evidence suggests that radiosurgical ablation with stereotactic radiation for intradural spinal lesions may be efficacious in certain clinical scenarios. Local tumor control, pain relief, and improvement in neurologic function with minimal morbidity have been reported in short-term follow-up. However, long-term efficacy of radiosurgery in the management of intradural spinal neoplasms necessitates further validation. As extracranial radiosurgery is a newly evolving modality, a continuative review of the current literature is appropriate. Until a standardized therapeutic window of safety and efficacy can be determined, the recommendation of radiosurgical applications for benign spinal tumors should be reserved for carefully selected cases.     

Toward an expanded view of radiosurgery

Radiosurgery and radiotherapy were originally distinguished on the basis of the manner in which they protected normal tissues from radiation injury. Radiosurgery does so by precise targeting of cross-fired radiation beams to abnormal tissue, with abrupt falloff of radiation doses to surrounding normal tissue. Radiotherapy was historically less concerned with targeting accuracy and anatomic precision; normal tissues were protected by dividing doses into multiple fractions separated by time to allow recovery of normal tissues. By this means, radiotherapy applied radiobiological principles to disrupt dividing cells selectively. Despite the development of computer-based, image-guided frameless technology that eliminates the necessity to perform radiosurgery in a single session, there are some who still insist that radiosurgery be distinguished from radiotherapy on the basis of whether treatment is delivered in a single session. Here, we propose that this definition of radiosurgery is needlessly restrictive and that staging or hypofractionation of radiosurgical treatment permits the limited application of radiobiological principles of radiotherapy to improve radiosurgical treatment. We therefore define radiosurgery as a procedure that involves the active participation of a surgeon and in which spatially accurate and highly conformal doses of radiation are targeted at well-defined structures with an ablative intent.     

Fractionated stereotactic radiotherapy for intracranial neoplasms.

Fractionated stereotactic radiotherapy is a method which attempts to combine the radiobiological advantages offered by dose fractionation with a technique for focal delivery of radiation. At McGill University, fractionated stereotactic radiotherapy is given with a linear accelerator-based dynamic stereotactic radiosurgery unit. The first treatment is given using the stereotactic frame for target localization and head immobilization. Subsequent treatments are given using skin tattoos and laser alignment for target placement within the isocenter of the linear accelerator, and a modified portable halo-ring device is used for skull immobilization. Typically, a marginal dose of 42 Gy was prescribed at the margins of the lesion, divided in 6 fractions and given over a 2-week period. We report the pathological profile and treatment results in a series of 21 patients with a variety of intracranial tumors, treated in this manner between May 1987 and April 1990. Fractionated stereotactic radiotherapy appears to be a worthwhile procedure for the treatment of well-selected patients with intracranial neoplasms.     

Functional neurosurgery--a future for the gamma knife?

The Gamma Knife is currently the only radiosurgical device which has been used in functional neurosurgery. This mode of utilization is possible because the instrument can make lesions in normal brains with a volume as small as 50 mm3. The experience of functional radiosurgery accumulated at the Karolinska Institute over 21 years is reviewed, and the possible implications of the new developments in imaging techniques for the future of functional radiosurgery are considered. The review covers gamma thalamotomy for pain and tremor, radiosurgery for trigeminal neuralgia, gamma capsulotomy for severe anxiety and obsessive-compulsive neurosis, and Gamma Knife surgery for focal epilepsy. The important role of stereotactic MRI localization in functional radiosurgery is pointed out, and a preliminary report of the recent experience with stereotactic magnetoencephalography combined with stereotactic MRI for physiological and anatomic target localization is given. It is concluded that functional radiosurgery should only be performed with radiation of very small volumes of brain, as the very high doses required would be devastating if delivered to even small volumes.     

Dosimetric comparison of CyberKnife with other radiosurgical modalities for an ellipsoidal target

OBJECTIVE: To compare treatment plans obtained with the CyberKnife (CK) (Accuray, Inc., Sunnyvale, CA) with those of other commonly used radiosurgical modalities, such as the gamma knife (GK), linear accelerator multiple arcs, conformally shaped static fields, and intensity-modulated radiotherapy (IMRT). METHODS: An ellipsoidal simulated target was chosen centrally located in a three-dimensional model of a patient's head acquired with magnetic resonance or computed tomographic imaging. It was 25 mm in diameter and 35 mm long. The aims of treatment plans were 100% target volume coverage with an appropriate isodose line, minimum radiation dose to normal tissue, and clinically acceptable delivery. These plans were evaluated by use of a dose-volume histogram and other commonly used radiosurgical parameters such as target coverage, homogeneity index, and conformity index. RESULTS: All selected treatment modalities were equivalent in providing full target coverage. For dose homogeneity, all modalities except for multiple isocenter plans for GK (homogeneity index, 2.0) were similar (homogeneity index, congruent with 1.25). Dose conformity was essentially equivalent for all treatment plans except for IMRT, which had a slightly higher value (conformity index, congruent with 1.27). There was a substantial variation in the radiation dose to normal tissue between the studied modalities, particularly at the lower dose levels. CONCLUSION: CK plans seemed to be more flexible for a given target size and shape. For a target of limited volume and essentially of any shape, one could obtain similarly good conformal dosimetry with CK and GK. For a regular-shaped but other than spherical target, homogeneous dose distribution could be obtained with all selected modalities except for multiple isocenters, linear accelerator multiple arcs, or GK. Both IMRT and conformally shaped static fields offered good alternative treatment modalities to CK, GK, or linear accelerator multiple arc radiosurgery, with slightly inferior dosimetry in conformity (IMRT).