Technical Note: Evaluation of the systematic accuracy of a frameless, multiple image modality guided, linear accelerator based stereotactic radiosurgery system.

PURPOSE: To evaluate the total systematic accuracy of a frameless, image guided stereotactic radiosurgery system. METHODS: The localization accuracy and intermodality difference was determined by delivering radiation to an end-to-end prototype phantom, in which the targets were localized using optical surface monitoring system (OSMS), electromagnetic beacon-based tracking (Calypso®), cone-beam CT, "snap-shot" planar x-ray imaging, and a robotic couch. Six IMRT plans with jaw tracking and a flattening filter free beam were used to study the dosimetric accuracy for intracranial and spinal stereotactic radiosurgery treatment. RESULTS: End-to-end localization accuracy of the system evaluated with the end-to-end phantom was 0.5 ± 0.2 mm with a maximum deviation of 0.9 mm over 90 measurements (including jaw, MLC, and cone measurements for both auto and manual fusion) for single isocenter, single target treatment, 0.6 ± 0.4 mm for multitarget treatment with shared isocenter. Residual setup errors were within 0.1 mm for OSMS, and 0.3 mm for Calypso. Dosimetric evaluation based on absolute film dosimetry showed greater than 90% pass rate for all cases using a gamma criteria of 3%/1 mm. CONCLUSIONS: The authors' experience demonstrates that the localization accuracy of the frameless image-guided system is comparable to robotic or invasive frame based radiosurgery systems.

Development of a 3-D convolution/superposition algorithm for precise dose calculation in the skull.

In this paper an algorithm for calculating 3-D dose distributions within the brain is introduced and adapted to the demands of modem radiosurgery. The dose calculation with this model is based on a 3-D distribution of the primary photon intensity which is calculated with a ray casting algorithm. A prelocated matrix takes into account field sizes as well as modifying elements as collimator positions (MLC), blocks, wedges and compensators. Monte Carlo precalculated monoenergetic kernels from 0.1 MeV to 50 MeV were at our disposal. The components of the spectrum were either determined by deconvoluting depth dose curves measured in water or analyzed with a Ge-Li detector system in the case of 60Co. The calculated fluence distribution has to be superposed to the complete kernel containing the spatial energy deposition. Inhomogeneities and tissue interface phenomena (rhoe, Z) have been investigated. The divergence of the rays and the curved surface of the patient are taken into account. Assuming homogenous media, it is possible to shorten the computation time by using the Fast Fourier Transformation (FFT) delivering a first overview within seconds. The algorithm was evaluated and verified under specific conditions of small fields as used in radiosurgery and compared to dose measurements and Monte Carlo calculations. In using both the fast algorithm (FFT) for mainly homogenous conditions on one hand and the very precise superposition for inhomogeneous cases on the other, this algorithm can be a very helpful instrument especially for critical locations in the skull.

Development of an interactive graphical user interface for therapy simulation.

Because of the ongoing development to more complex non-coplanar techniques in radiotherapy, the use of modem computer graphics while designing a dose plan becomes increasingly essential and more important. In this paper, we describe a concept to simulate 3D conformal treatment techniques on the computer. All important components of the treatment device and relevant patient structures are mapped to an internal model, which allows simulation of motion sequences as well as the interactive adjustment of treatment parameters. The intention of this user interface is to save time by using mainly graphical modules in the optimization process instead of running through the dose calculation every time.

High precision radiosurgery and technical standards.

BACKGROUND: A high degree of precision and accuracy in radiosurgery is a fundamental requirement for therapeutic success. Small radiation fields and steep dose gradients are clinically applied thus necessitating a dedicated quality assurance program in order to guarantee dosimetric and geometric accuracy. MATERIAL AND METHODS: A detailed analysis of the course of treatment independent of the irradiation technique used results in the so-called chain of uncertainties in radiosurgery (immobilisation, imaging, treatment planning system, definition of regions of interest, mechanical accuracy, dose planning, dose verification). Each link in this chain is analysed for accuracy and the established quality assurance procedures are discussed. A "System Test" was used to check the whole chain of uncertainties simultaneously. RESULTS: The tests described are compatible with published reports on quality assurance in radiosurgery. In terms of accuracy the weakest link in the chain of uncertainties is stereotactic MR imaging. Geometric overall accuracy measured in the "System Test" is less than 0.7 mm. CONCLUSION: The established quality assurance routines have clinically been validated. MR imaging dominates geometric overall accuracy in radiosurgery, which can be limited to less than 1 mm by an adequate quality assurance protocol.

VOLUMESERIES: a software tool for target volume follow-up studies with computerized tomography and magnetic resonance imaging. Technical note.

In clinical follow-up studies after radiosurgery, imaging modalities such as computerized tomography (CT) and magnetic resonance (MR) imaging are used. Accurate determination of the residual lesion volume is necessary for realistic assessment of the effects of treatment. Usually, the diameters rather than the volume of the lesion are measured. To determine the lesion volume without using stereotactically defined images, the software program VOLUMESERIES has been developed. VOLUMESERIES is a personal computer-based image analysis tool. Acquired DICOM CT scans and MR image series can be visualized. The region of interest is contoured with the help of the mouse, and then the system calculates the volume of the contoured region and the total volume is given in cubic centimeters. The defined volume is also displayed in reconstructed sagittal and coronal slices. In addition, distance measurements can be performed to measure tumor extent. The accuracy of VOLUMESERIES was checked against stereotactically defined images in the Leksell GammaPlan treatment planning program. A discrepancy in target volumes of approximately 8% was observed between the two methods. This discrepancy is of lesser interest because the method is used to determine the course of the target volume over time, rather than the absolute volume. Moreover, it could be shown that the method was more sensitive than the tumor diameter measurements currently in use. VOLUMESERIES appears to be a valuable tool for assessing residual lesion volume on follow-up images after gamma knife radiosurgery while avoiding the need for stereotactic definition.