Early experience with the Gill Thomas Locator for computed tomography-directed stereotactic biopsy of intracranial lesions

Intracranial stereotactic positioning systems: Report of the American Association of Physicists in Medicine Radiation Therapy Committee Task Group No. 68

Intracranial stereotactic positioning systems (ISPSs) are used to position patients prior to precise radiation treatment of localized lesions of the brain. Often, the lesion is located in close proximity to critical anatomic features whose functions should be maintained. Many types of ISPSs have been described in the literature and are commercially available. These are briefly reviewed. ISPS systems provide two critical functions. The first is to establish a coordinate system upon which a guided therapy can be applied. The second is to provide a method to reapply the coordinate system to the patient such that the coordinates assigned to the patient's anatomy are identical from application to application. Without limiting this study to any particular approach to ISPSs, this report introduces nomenclature and suggests performance tests to quantify both the stability of the ISPS to map diagnostic data to a coordinate system, as well as the ISPS's ability to be realigned to the patient's anatomy. For users who desire to develop a new ISPS system, it may be necessary for the clinical team to establish the accuracy and precision of each of these functions. For commercially available systems that have demonstrated an acceptable level of accuracy and precision, the clinical team may need to demonstrate local ability to apply the system in a manner consistent with that employed during the published testing. The level of accuracy and precision required of an individual ISPS system is dependent upon the clinical protocol (e.g., fractionation, margin, pathology, etc.). Each clinical team should provide routine quality assurance procedures that are sufficient to support the assumptions of accuracy and precision used during the planning process. The testing of ISPS systems can be grouped into two broad categories, type testing, which occurs prior to general commercialization, and site testing, performed when a commercial system is installed at a clinic. Guidelines to help select the appropriate tests as well as recommendations to help establish the required frequency of testing are provided. Because of the broad scope of different systems, it is important that both the manufacturer and user rigorously critique the system and set QA tests appropriate to the particular device and its possible weaknesses. Major recommendations of the Task Group include: introduction of a new nomenclature for reporting repositioning accuracy; comprehensive analysis of patient characteristics that might adversely affect positioning accuracy; performance of testing immediately before each treatment to establish that there are no gross positioning errors; a general request to the Medical Physics community for improved QA tools; implementation of weekly portal imaging (perhaps cone beam CT in the future) as a method of tracking fractionated patients (as per TG 40); and periodic routine reviews of positioning accuracy.

A method for improving the accuracy of stereotaxic procedures in monkeys using implanted fiducial markers in CT scans that also serve as anchor points in a stereotaxic frame

We developed a relatively inexpensive method for stereotaxic placement of electrodes or needles in the brains of monkeys. Steel balls were affixed to the skulls of monkeys. These balls served as fiducial markers and were also used as points at which the monkey's skull was held in a modified stereotaxic apparatus. Computed tomography (CT) was used to establish the location of an injection target with respect to the fiducial markers. A computer program related the CT coordinates to stereotaxic coordinates. These were used to direct an electrode marker toward a target in the hypothalamus. With the marker left in place, the monkey was removed from the stereotaxic frame and a second CT scan was performed. Corrections for errors in marker placement were made and retrograde tracers were injected. This procedure was found to be more accurate and reliable than conventional stereotaxic procedures. The accuracy and repeatability of the technique were also established using a phantom model of a monkey's skull. Two important advantages of this method are that animals can be repeatedly placed into the stereotaxic frame in precisely the same position and that there are many opportunities during the procedure to check for and correct errors.

[Stereotactic surgery for intracranial mass lesions: diagnosis and treatment]

The authors present a study about 50 stereotactic procedures for intracranial lesions. In 12 cases stereotaxis was used as a treatment: stereotactic surgical resection (5 patients) and aspiration of lesions (7 patients). This study confirms the great accuracy and low morbidity of the stereotactic techniques. Computed tomography and magnetic resonance imaging determinate the precise stereotactic coordinates, but in some cases it is necessary to perform stereo-angiography. The stereotactic surgical resection of brain tumors allows the treatment of deep lesions or located at functional areas and considered to be inoperable before. The related literature is discussed.