Introduction to Radiosurgery

Photon Radiosurgery: A Clinical Review

The term radiosurgery has been used to describe a variety of radiotherapy techniques which deliver high doses of radiation to small, stereotactically defined intracranial targets in such a way that the dose fall-off outside the targeted volume is very sharp. Proton, charged particle, gamma unit, and linear accelerator-based techniques appear to be equivalent from the standpoint of accuracy, dose distributions, and clinical results. However, capital and operating costs associated with the use of linear accelerators in general clinical use are much lower. Radiosurgery has an established role in the treatment of arteriovenous malformations and acoustic neurinomas. Interest in these techniques is increasing in neurosurgical and radiation oncological communities, as radiosurgery is rapidly assuming a place in the management of several other conditions, including craniopharyngiomas, meningiomas, and selected malignant lesions.

Dosimetric evaluation of lithium carbonate (Li2CO3) as a dosemeter for gamma-radiation dose measurements

This work reports the possibility of using lithium carbonate as a dosimetric material for gamma-radiation measurements. Carboxi-radical ions, CO(2)(-) and CO(3)(-), arise from the gamma irradiation of Li(2)CO(3), and these radical ions can be quantified by electron paramagnetic resonance (EPR) spectrometry. The EPR-signal response of gamma-irradiated lithium carbonate has been investigated to determine some dosimetric characteristics such as: peak-to-peak signal intensity versus gamma dose received, zero-dose response, signal fading, signal repeatability, batch homogeneity, dose rate effect and stability at different environmental conditions. Using the conventional peak-to-peak method of stable ion radicals, it is concluded that lithium carbonate could be used as a gamma dosemeter in the range of 3-100 Gy.

CHAINED LIGHTNING, PART II

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.

Characteristics of a dedicated linear accelerator-based stereotactic radiosurgery-radiotherapy unit

A stereotactic radiosurgery and radiotherapy (SRS/SRT) system on a dedicated Varian Clinac-600SR linear accelerator with Brown-Roberts-Wells and Gill-Thomas-Cosman relocatable frames along with the Radionics (RSA) planning system is evaluated. The Clinac-600SR has a single 6-MV beam with the same beam characteristics as that of the mother unit, the Clinac-600C. The primary collimator is a fixed cone projecting to a 10-cm diameter at isocenter. The secondary collimator is a heavily shielded cylindrical collimator attached to the face plate of the primary collimator. The tertiary collimation consists of the actual treatment cones. The cone sizes vary from 12.5 to 40.0 mm diameter. The mechanical stability of the entire system was verified. The variations in isocenter position with table, gantry, and collimator rotation were found to be < 0.5 mm with a compounded accuracy of < or = 1.0 mm. The radiation leakage under the cones was < 1% measured at a depth of 5 cm in a phantom. The beam profiles of all cones in the x and y directions were within +/- 0.5 mm and match with the physical size of the cone. The dosimetric data such as tissue maximum ratio, off-axis ratio, and cone factor were taken using film, diamond detector, and ion chambers. The mechanical and dosimetric characteristics including dose linearity of this unit are presented and found to be suitable for SRS/SRT. The difficulty in absolute dose measurement for small cone is discussed.

Modifications based on computed tomographic imaging in planning the radiosurgical treatment of arteriovenous malformations

Early in the course of treating arteriovenous malformations with radiosurgery, it was noted that the nidus isocenter and diameter, as identified by stereotactic angiography, often differed from that identified by stereotactic enhanced computed tomography (CT). To assess the sources of discrepancy between the arteriographic and CT representations of the nidus, dosimetry (from treatment records stored on an optical disk) was reviewed in 81 consecutive cases. In 44 cases, the isocenters differed by an average of 3.6 mm and the collimator size differed. Fourteen nidi were larger on CT (average, 2.6 mm), and 30 were smaller on CT (average, 4.0 mm). Overall, the angiographic and the CT nidus differed in 75% of the cases reviewed. Sources of error in the angiographic nidus determination included overlapping vessels, bony structures, fine filamentous arterioles, and irregular shapes.