Geometrical accuracy of the Novalis stereotactic radiosurgery system for trigeminal neuralgia

Abstract

Object. Stringent geometrical accuracy and precision are required in the stereotactic radiosurgical treatment of patients. Accurate targeting is especially important when treating a patient in a single fraction of a very high radiation dose (90 Gy) to a small target such as that used in the treatment of trigeminal neuralgia (3 to 4—mm diameter). The purpose of this study was to determine the inaccuracies in each step of the procedure including imaging, fusion, treatment planning, and finally the treatment. The authors implemented a detailed quality-assurance program. Methods. Overall geometrical accuracy of the Novalis stereotactic system was evaluated using a Radionics Geometric Phantom Chamber. The phantom has several magnetic resonance (MR) and computerized tomography (CT) imaging—friendly objects of various shapes and sizes. Axial 1-mm-thick MR and CT images of the phantom were acquired using a T1-weighted three-dimensional spoiled gradient recalled pulse sequence and the CT scanning protocols used clinically in patients. The absolute errors due to MR image distortion, CT scan resolution, and the image fusion inaccuracies were measured knowing the exact physical dimensions of the objects in the phantom. The isocentric accuracy of the Novalis gantry and the patient support system was measured using the Winston—Lutz test. Because inaccuracies are cumulative, to calculate the system's overall spatial accuracy, the root mean square (RMS) of all the errors was calculated. To validate the accuracy of the technique, a 1.5-mm-diameter spherical marker taped on top of a radiochromic film was fixed parallel to the x–z plane of the stereotactic coordinate system inside the phantom. The marker was defined as a target on the CT images, and seven noncoplanar circular arcs were used to treat the target on the film. The calculated system RMS value was then correlated with the position of the target and the highest density on the radiochromic film. The mean spatial errors due to image fusion and MR imaging were 0.41 ± 0.3 and 0.22 ± 0.1 mm, respectively. Gantry and couch isocentricities were 0.3 ± 0.1 and 0.6 ± 0.15 mm, respectively. The system overall RMS values were 0.9 and 0.6 mm with and without the couch errors included, respectively (isocenter variations due to couch rotation are microadjusted between couch positions). The positional verification of the marker was within 0.7 ± 0.1 mm of the highest optical density on the radiochromic film, correlating well with the system's overall RMS value. The overall mean system deviation was 0.32 ± 0.42 mm. Conclusions. The highest spatial errors were caused by image fusion and gantry rotation. A comprehensive quality-assurance program was developed for the authors' stereotactic radiosurgery program that includes medical imaging, linear accelerator mechanical isocentricity, and treatment delivery. For a successful treatment of trigeminal neuralgia with a 4-mm cone, the overall RMS value of equal to or less than 1 mm must be guaranteed.

Technique for Targeting Arteriovenous Malformations Using Frameless Image-Guided Robotic Radiosurgery

“…Indeed, when assessed with the hidden target, Winston-Lutz test (18) in anthropomorphic phantoms, modern image-guided radiosurgery systems demonstrate targeting accuracy from 0.4 mm to 1.1 mm depending on the system and resolution of the CT scan used for target localization (19)(20)(21). Comparable targeting accuracy in the range of 0.3 mm to 1.3 mm has been reported with frame-based approaches (22,23). Furthermore, frequent intrafractional imaging combined with skull-based tracking can reduce random targeting errors from patient motion down to 1 mm (24), which is similar to the motion-induced error with frame-based radiosurgery (21).…”

Dosimetric Comparison of Helical Tomotherapy and Dynamic Conformal Arc Therapy in Stereotactic Radiosurgery for Vestibular Schwannomas

“…23 There are three different leaf widths in the m 3 -mMLC, which has 26 pairs of tungsten alloy (95% W, 3.4% Ni and 1.6% Fe) leaves (14 ϫ 3, 6 ϫ 4.5, and 6 ϫ 5.5 mm, with a maximum useful field of 9.8 ϫ 9.8 cm 2 ). In addition, the system is combined with ExacTrac® X-Ray 6D (BrainLAB, Heimstetten, Germany), an infrared (IR) camera, a kV stereoscopic x-ray imaging system, a relocatable stereotactic frame system, a noninvasive mask system, and ExacTrac® Robotics, for patient positioning in all 6D of freedom.…”

“…The dosimetric comparisons of PTV between HT and DCAT plans for each patient SD 1.51 Ϯ 0 23. 1.94 Ϯ 0.34 Ϫ22.22 Ϯ 14.32 † 1.08 Ϯ 0.03 1.09 Ϯ 0.02 Ϫ0.81 Ϯ 1.8* 40.25 Ϯ 10.90 64.92 Ϯ 13.58 38.58 Ϯ 7.9 †Abbreviations: PTV ϭ planning target volume; HT ϭ helical tomotherapy; DCAT ϭ dynamic conformal arc therapy; CI ϭ conformity index; HI ϭ homogeneity index; GSI ϭ gradient score index; ⌬CI (%) ϭ the CI improvement ratio; ⌬HI (%) ϭ The HI improvement ratio; ⌬GSI (%) ϭ the GSI…”

Filmless Evaluation of the Mechanical Accuracy of the Isocenter in Stereotactic Radiotherapy

“…Here, a metal ball is positioned at the isocenter and a film behind the ball is exposed by a small beam, using different combinations of the angles for gantry and treatment table [18,26,30]. The small beam is larger than the metal ball.…”

“…The evaluation of the geometric accuracy of the isocenter is mostly based on films [10,18,31,33,34], though other methods like a laser application are described [14]. A common test for a fast evaluation of an overall isocenter accuracy based on film is known as the Winston-Lutz test or joint-center test.Here, a metal ball is positioned at the isocenter and a film behind the ball is exposed by a small beam, using different combinations of the angles for gantry and treatment table [18,26,30]. The small beam is larger than the metal ball.…”