PET and SPECT imaging for stereotactic localization

Current concepts in stereotactic radiosurgery - a neurosurgical and radiooncological point of view

Stereotactic radiosurgery is related to the history of "radiotherapy" and "stereotactic neurosurgery". The concepts for neurosurgeons and radiooncologists have been changed during the last decade and have also transformed neurosurgery. The gamma knife and the stereotactically modified linear accelerator (LINAC) are radiosurgical equipments to treat predetermined intracranial targets through the intact skull without damaging the surrounding normal brain tissue. These technical developments allow a more precise intracranial lesion control and offer even more conformal dose plans for irregularly shaped lesions. Histological determination by stereotactic biopsy remains the basis for any otherwise undefined intracranial lesion. As a minimal approach, it allows functional preservation, low risk and high sensitivity. Long-term results have been published for various indications. The impact of radiosurgery is presented for the management of gliomas, metastases, brain stem lesions, benign tumours and vascular malformations and selected functional disorders such as trigeminal neuralgia. In AVM's it can be performed as part of a multimodality strategy including resection or endovascular embolisation. Finally, the technological advances in radiation oncology as well as stereotactic neurosurgery have led to significant improvements in radiosurgical treatment opportunities. Novel indications are currently under investigation. The combination of both, the neurosurgical and the radiooncological expertise, will help to minimize the risk for the patient while achieving a greater treatment success.

Comparison of different techniques for stereotactic positron emission tomography imaging

BACKGROUND AND PURPOSE

The aim of this study was to evaluate three different techniques used for stereotactic positron emission tomography (PET) image definition: (1) PET imaging with external stereotactic radioactive markers, (2) PET imaging without external stereotactic markers and subsequent coregistration with stereotactically defined imaging modality such as computed tomography (CT) or magnetic resonance imaging (MRI), (3) PET/CT imaging with utilization of external nonradioactive markers.

MATERIALS AND METHODS

Special head phantom that could be fixed in the Leksell stereotactic frame was used. The phantom was filled with fluorodeoxyglucose ((18)F-FDG) in water solution at an activity concentration of 17.5 kBq/ml simulating counts from standard brain. A spherically shaped glass test vessel (inner diameter 46 mm and wall thickness 3 mm) positioned in the head phantom was filled with FDG water solution at an activity concentration of 52.5 kBq/ml corresponding to pathologic lesion during PET imaging. Leksell stereotactic MRI indicator box was filled with FDG water solution at an activity concentration of 3.1 MBq/ml. The phantom was then stereotactically investigated on PET, PET/CT, CT and MRI. Deviations between stereotactic X, Y, Z PET coordinates of the center of the spherical vessel (simulating pathological lesion) were determined in the treatment planning system according to reference image and represented inaccuracy in stereotactic PET image definition for each of three tested methods of stereotactic PET definition.

RESULTS

Total spatial inaccuracy for stereotactic PET image definition based on radioactive fiducials was 1.7 and 0.7 mm for 3.4- and 2.0-mm PET slices, respectively. Total spatial PET image definition inaccuracy based on PET/CT imaging and stereotactic definition using nonradioactive CT fiducials was 0.7 mm. Total spatial PET image definition inaccuracy based on coregistration was 0.5 and 0.9 mm for coregistration with MRI and CT, respectively.

CONCLUSION

All three evaluated stereotactic PET image definition techniques indicated very good accuracy in this phantom study entirely accepted by clinical requirements for functional imaging. The most convenient stereotactic PET image definition technique seemed to be PET image coregistration either on CT or MRI. In this situation, PET imaging can be done independently on frame application (for example few days before stereotactic frame application or even in a different centre) and then coregistered with stereotactically performed CT or MRI during the stereotactic procedure. However, detailed patient study has to be performed to test image coregistration inaccuracy on real clinical data.

Integration of three-dimensional corticospinal tractography into treatment planning for gamma knife surgery

OBJECT

In the radiosurgical treatment of critically located lesions, the effort to minimize the risk of complication is essential. In this study the integration of diffusion-tensor (DT) imaging-based tractography was clinically applied to treatment planning for gamma knife surgery (GKS).

METHODS

Seven patients with cerebral arteriovenous malformations located adjacent to the corticospinal tract (CST) underwent this technique. Data provided by DT imaging were acquired before the frame was affixed to the patient's head and the CST of the DT tractography was created using our original software. Stereotactic three-dimensional imaging studies were obtained after frame fixation and then coregistered with the data from DT tractography. After image fusion of the two studies, the combined images were transported to a GKS treatment-planning workstation. The spatial relationship between the dose distribution and the CST was clearly demonstrated within the 2 hours it took to complete the entire imaging process. The univariate logistic regression analysis of transient or permanent motor complications revealed a significant independent correlation with the volume of the CST that received 25 Gy or more and with a maximum dose to the CST (p < 0.05).

CONCLUSIONS

The integration of DT tractography into the GKS treatment planning was highly useful in confirming the dose to the CST during treatment planning.

Integration of the metabolic data of positron emission tomography in the dosimetry planning of radiosurgery with the Leksell gamma knife: early experience with brain tumors

✓ The purpose of this study was to assess the use of positron emission tomography (PET) as a stereotactic planning modality for gamma knife radiosurgery (GKS).

The authors developed and validated a technique for fiducial marker imaging, importation, and handling of PET data for integration into GammaPlan planning software. The clinical feasibility in applying this approach to a selected group of patients presenting with recurrent glial tumors or metastases was evaluated.

Positron emission tomography data can be integrated into GammaPlan, allowing a high spatial accuracy, as validated using a phantom. Positron emission tomography data were successfully combined with magnetic resonance (MR) images to define the target volume for the radiosurgical treatment of patients with recurrent glioma or metastasis. This approach may contribute to optimizing target selection for infiltrating or ill-defined brain lesions. Because PET is also useful for the pretreatment and follow-up evaluation, the use of stereotactic PET in these patients can enable an accurate comparison of PET-based metabolic data with MR-based anatomical data. This could give a better understanding of the metabolic changes following radiosurgery.

The ability to use PET data in GKS represents a crucial step toward further developments in radiosurgery, as this approach provides additional information that may open new perspectives for the optimization of the treatment of brain tumors.