Stereotactic Radiosurgery

Michael R. Kuettel, M.D., Ph.D.

 

Abstract: Stereotactic radiation is a method of delivering focused irradiation to both benign and cancerous lesions. This form of treatment can be administered in single or multiple fractions (radiosurgery or stereotactic radiotrherapy, respectively). This technology has evolved rapidly due to advances in both hardware and software design. Clinical indications for the use of this treatment are gaining acceptance through prospective randomized trials.

Introduction: The term radiosurgery (known as single fraction radiosurgery) was defined one half century ago to characterize the method of destroying diseased or dysfunctional tissue with a single large dose of irradiation delivered through stereotactically directed narrow radiation beams. Currently, there are three primary methods to deliver stereotactic irradiation: heavy charged particle beams (protons, helium, etc.), gamma irradiation emitted from a fixed array of cobalt-60 sources (gamma knife), and high energy photon irradiation produced with a linear accelerator (modified or dedicated units). In each of these three systems, the patient's head is attached securely to a stereotactic apparatus, providing a reference frame with a coordinate system for target determination and means of precise patient positioning for treatment. All three of these approaches rely on techniques that sharply focus the irradiation to converge on a predetermined volume. Although this technique was initially described 50 years ago, the past few years have resulted in a rapid of evolution of this technology. With new developments in this technology and with new clinical applications, this treatment is now the subject of prospective national trials, which will establish indications and benefits of this clinical approach. This article serves as a review of the terminology, recent advances, and prospective trials using this treatment.

Charged-Particle Irradiation. Charged-particle irradiation refers to the use of beams that exhibit a unique radiation distribution profile (known as the Bragg peak), resulting in a sharp dose peak deep within tissue. With proper planning, this radiation peak can be manipulated to allow a minimum exit dose. Because charged-particle facilities are expensive and available in only a few centers in the United States, the subject will be omitted from the remainder of this review.

Gamma Knife. The gamma knife is a dedicated machine used solely for stereotactic radiosurgery. This device uses 201 cobalt-60 sources positioned so that each source is oriented along the radius of a sphere and aimed precisely at a central point. Circular beams of radiation are produced that can measure 4 to 18 mm in diameter. Selective blocking of sources can alter the shape of the dose delivered. The irradiation produced from cobalt-60 is similar to that achieved from a low level energy linear accelerator x-ray beam. The amount of irradiation is delivered by precisely timing the duration of irradiation. Currently, 38 gamma knife systems are in clinical operation in the United States and used only for the treatment of brain lesions. Computerized treatment planning allows the physician to precisely target the brain lesion. Current software can compile images from CT and MRI scan to construct 3-D views.

Linear Accelerators. Several recent advances have been made whereby a linear accelerator can be modified using a simple, cost effective system for beam shaping. Currently conformal arc therapy can be delivered optimizing dose to tumors while sparing normal tissue. Similar to the gamma knife, a customized head holder is used to position patients throughout treatment. Instead of multiple cobalt sources with the gamma knife, the linear accelerator generates x-rays with variations in dose distribution made via changes in machine movement as well as patient alignment. Although most linear accelerators currently in use can be modified to deliver stereotactic irradiation, a dedicated unit is preferred.

Stereotactic Radiotherapy. Fractionated treatment with stereotactic irradiation is becoming more available at many centers. This application of radiosurgery was developed by the recognition that tumors located outside the brain could benefit from stereotactic technology. The basic concept is that normal tissue is invariably irradiated even by the most elaborate treatment plans. Conceivably, delivery of smaller fractions of daily irradiation would be more forgiving to adjacent normal tissue resulting in fewer complications. Noninvasive stereotactic head and body frames have been developed for patients receiving fractionated treatments. To date, several institutions, including Georgetown, have reported preliminary experience with this treatment. Preliminary results are comparable with those achieved by stereotactic radiosurgery. Longer follow-up, however, is necessary.

Clinical Research. During the past few years, clinical scientists have explored new dimensions in the treatment of several diseases with stereotactic irradiation strategies. In addition, an effort has been mounted to resolve clinical dilemmas through the mechanism of scientifically designed prospective trials. These trials have been developed to eliminate subjectivity and bias that could diversely influence the interpretation of results and to define further the role of stereotactic radiosurgery. Both benign brain lesions (including arterial venous malformations), as well as malignant diseases (including primary brain lesions and metastases) are currently being evaluated.

Conclusions: The conversions of new technologies and new clinical frontiers have made stereotactic irradiation an important cancer treatment modality. Clinical trials, however, will be necessary to critically access the benefit of these treatment advances for a relatively rare set of diseases with unfavorable prognoses as currently managed. Georgetown University Medical Center is dedicated to offer both stereotactic radiosurgery and radiotherapy for patients needing such treatment in the greater Washington area. Currently, we have purchased a dedicated stereotactic radiosurgery and radiotherapy unit to be placed in our Northern Virginia facility in Fairfax City. This state-of-the-art stereotactic unit will be available in spring, 1999.

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