Geometrical accuracy of the Novalis stereotactic radiosurgery system for trigeminal neuralgia

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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.

Stereotactic radiosurgery and stereotactic radiotherapy are effectively used in the treatment of arteriovenous malformation, trigeminal neuralgia, and certain brain tumors. Over 50 years ago the first case of trigeminal neuralgia was treated radiosurgically by Lars Leksell.5 Stereotactic radiosurgery and stereotactic radiotherapy deliver high-dose radiation to a target volume in single and multiple fractions,4 respectively, while sparing the normal tissues.

The procedures involve placement of a stereotactic frame on the patient's skull, localization of the target coordinates with an imaging modality, treatment planning, and finally the treatment delivery. There are two advantages of the stereotactic radiosurgery technique: its submillimeter geometrical accuracy of the treatment delivery system and high conformity with steep dose gradients. Various stereotactic radiosurgery techniques have been developed and clinically used. These include gamma knife surgery, LINAC-based stereotactic radiosurgery, frameless stereotactic radiosurgery (Cyberknife), and charged-particle accelerators such as proton beam stereotactic radiosurgery.4

To deliver a precise target dose by means of stereotactic radiosurgery, comprehensive quality assurance is essential. The total spatial error is accumulated through the processes of target localization using medical imaging, image fusion, dose planning, mechanical errors, patient positioning, intraoperative movements, target positioning overlays, and radiation dose delivery system. There are numerous publications in which the spatial accuracy and quality assurance of the gamma knife unit7 and LINAC stereotactic radiosurgery systems have been assessed.3,9,10,12

The purpose of this study was to assess the geometrical accuracy of BrainLAB's Novalis system (BrainLAB, Heimstetten, Germany) in the treatment of trigeminal neuralgia. To measure the geometrical accuracy and the Novalis system in treating trigeminal neuralgia, we performed a simulation of the whole procedure. Novalis is a dedicated LINAC radiosurgery system with stringent isocentricity standards. The system has a maximum field size of 10 × 10 cm2 and is equipped with a micromultileaf collimator. The leaf width of the micromultileaf collimator is 3, 4.5, and 5.5 mm, respectively. In addition, cones of various sizes ranging from 4 to 20 mm can be attached to the gantry. The planned radiation dose is delivered using multiple arcs converged at the isocenter by means of either cones or a micromultileaf collimator. In addition, conformal static beams, intensity-modulated radiosurgery planning, and dose delivery are available.

Materials and Methods

Overall geometrical accuracy of our Novalis stereotactic system was evaluated using a commercially available Radionics Geometric Phantom Chamber. The phantom houses several MR imaging— and CT-friendly objects of various shapes (sphere, cylinder, cone, cube) and sizes (Fig. 1). The MR unit is a Signa 1.5-tesla system (General Electric Medical Systems, Waukesha, WI). The axial 1-mm MR images of the phantom were acquired using a T1-weighted 3D SPGR pulse sequence (TE 10 msec, TR 24 msec, FA 30°, bandwidth 15.63, field of view 25 cm, number of location 124, frequency 256, phase 256, NEX 1, phase field of view 1.0, frequency direction A/P, 256 × 256 matrix). Computerized tomography images of the phantom were acquired using a dual-slice system with 512 × 512 matrix and 1-mm slice thickness. The CT scans were acquired in sequential mode. We did not investigate the differences in CT images acquired in sequential and helical modes in this study; the MR and CT images were transferred to a planning computer (Brainscan; BrainLAB) and were coregistered using the autofusion option. Using the Brainscan tools, the dimensions of several objects in the phantom were measured from individual axial MR images, CT scans, and fused images. Knowing the exact physical dimensions of the objects in the phantom, the errors due to imaging geometrical distortions were calculated by subtracting the actual size of each object from the measured values. Finally the mean of all the errors from different objects and their standard deviations were computed. Localization of the targets in the phantom is done in the Brainscan (version 5.2) treatment planning system. The planning system produces the treatment parameters for seven circular arcs with a 4-mm cone and prints out four pages of the overlays with millimeter scales to be taped on the target localizer box. Target geometrical variations due to inaccurate printing of the overlays and errors due to inaccurate pasting on the localizer box were determined using a submillimeter scale and were averaged.

Fig. 1.
Fig. 1.

Photograph showing the Radionics Geometric Phantom Chamber with several objects of various shapes (sphere, cylinder, cone, and cube) and sizes.

The isocentric accuracy of the Novalis gantry (Fig. 2) as well as the patient support system (Fig. 3) were measured with the Winston—Lutz test. The details of the test have been described elsewhere.4,6 Given an acceptable mechanical isocentricity, the lasers are then set to converge at the isocenter. The cumulative deviations of the lasers not coinciding at the isocenter were determined with a submillimeter ruler for a 1-week period, and its means and standard deviations were calculated.

Fig. 2.
Fig. 2.

Winston—Lutz test of the gantry rotation at 270, 0, and 90° positions.

Fig. 3.
Fig. 3.

Winston—Lutz test of the couch rotation at 0, 90, 63, 36, and 10° couch positions.

Because inaccuracies are cumulative, we calculated the system's overall spatial accuracy by using the RMS of all the errors in the chain. 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 with a 4-mm cone to treat the target on the film. The geometrical accuracy of the target treatment was determined by measuring the length of the spherical marker position from the maximum optical density on the film in x and y directions with a ×7 lens with a 0.1-mm precision scale. The calculated system RMS value was then correlated with the position of the target and the highest density on the radiochromic film.

Results

Table 1 summarizes the mean and standard deviation values of the geometrical errors due to MR and CT imaging and fusion, as well as the mechanical isocentricity of the Novalis system. The system's overall RMS values were 0.9 and 0.6 mm, with and without the couch errors included, respectively. The isocenter variations due to couch rotation were microadjusted between couch positions. Figure 4 shows the spherical marker, and the radiochromic film treated with seven arcs with a 4-mm cone at ×7 magnification with a 0.1-mm precision scale. 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.

TABLE 1

Summary of the geometrical errors

MR (mm)CT (mm)Auto Fusion (mm)Gantry Isocenter (mm)Couch (mm)Laser (mm)Target Position (mm)Total RMS W/ & W/O Couch (mm)
0.22 ± 0.100.12 ± 0.140.41 ± 0.300.30 ± 0.100.60 ± 0.150.20 ± 0.100.20 ± 0.100.90 ± 0.40/0.60 ± 0.39

Fig. 4.
Fig. 4.

Graph demonstrating bimonthly MR imaging external magnetic field homogeneity as a function of the test dates.

The mean overall system deviation was 0.32 ± 0.42 mm. The positional error due to lasers not converging precisely at the isocenter was 0.2 ± 0.1 mm. The printer connected to the BrainLAB planning system for printing the target positioning overlays is routinely calibrated, and pasting the overlays on the target positioning box was measured to be accurate within 0.2 ± 0.1 mm.

Discussion

Each step of the way in the stereotactic radiosurgery treatments may introduce an error. The overall precision of the technique cannot be better than the poorest accuracy of the single links in the chain of an optimal treatment.7 Stringent geometrical accuracy and precision are required when conducting stereotactic radiosurgery. Accurate targeting is especially important when treating a patient with a single fraction of very high radiation dose (90 Gy) to a small target volume such as in trigeminal neuralgia (3 to 4—mm diameter). The spatial errors are best estimated using a systematic approach to isolate independent contributing factors.12 Therefore, we determined the inaccuracies in each step of the procedure including imaging, fusion, planning, and finally the treatment. As shown by our results and those reported in the literature,1,2,11 the weakest links in the entire stereotactic radiosurgery technique involve fusion inaccuracies, gantry isocentricity, and MR imaging distortions. Medical imaging techniques including 3D MR imaging used for contouring the target and the critical structures, CT scans used for dose calculations, and target localization are essential pieces of the stereotactic radiosurgery technique. Magnetic resonance images are prone to artifacts and distortion. General causes of geometrical distortion include inhomogeneous magnetic field, ferromagnetic materials, spatial variations in magnetic susceptibility, and nonlinear magnetic field gradient.11 It has been demonstrated that the T1-weighted image sequence is more prone to distortion than the T1-weighted 3D SPGR or its equivalent protocol on a non—General Electric MR unit.7 In our experience, CT scanning provides higher spatial resolution, less or negligible distortion, and accurate target localization. Image fusion programs can potentially introduce errors in the target positioning because of misregistration. In this study we evaluated the effects of medical imaging and image fusion on the overall spatial accuracy of stereotactic radiosurgery.

The LINAC device was tested daily by using the Winston—Lutz test for accurate isocentricity as part of the quality-assurance program.4,6 The gantry isocentricity can drift over time (we saw it ranging from 0.1 to 0.6 mm) over our 0.3-mm tolerance. In addition, we have a daily quality-assurance program in place for our MR imaging system that includes imaging a phantom and showing the resolution and distortion. Comprehensive tests of the magnetic field uniformity, as well as gradient field measurements, are performed by a General Electric serviceperson bimonthly or as needed. Figures 5 and 6 show the external magnetic field homogeneity and the variation of x, y, and z gradient fields measured in each plane, respectively, at various dates as part of our routine quality assurance. Table 2 outlines our quality-assurance frequency and tolerance limit data. The overall RMS values of all the tolerances with and without the couch included were less than 1 and 0.66 mm, respectively. The recommended tests and their frequencies and the tolerances in Table 2 may be used as a quality-assurance guide. Adherence to these levels will ensure higher-quality treatments.

Fig. 5.
Fig. 5.

Graph showing bimonthly MR imaging gradient calibration as a function of the test dates.

Fig. 6.
Fig. 6.

The spherical marker and the radiochromic film treated with seven arcs and a 4-mm cone shown at × 7 magnification with a 0.1-mm precision scale

TABLE 2

Quality-assurance frequency and tolerance

Quality-Assurance FactorFrequencyTolerance (mm)
MR resolution & distortion phantomdaily<0.30
MR external magnetic field homogeneitybimonthly
MR gradient x, y, & z plane calibrationbimonthly<0.30
CT resolution phantomweekly<0.30
gantry isocentricity, Winston—Lutz testdaily<0.30
couch isocentricity, Winston—Lutz testmonthly<0.75
laser convergence at isocenterdaily<0.20
printer calibration (target position; overlays)monthly<0.20
image fusioneach case<0.40
total RMS of tolerances including the couch<1.08
total RMS of tolerances excluding the couch<0.78

Conclusions

Our highest spatial errors were due to image fusion, gantry isocentricity, and image distortions; this finding is in line with published data.1–3,10,12 A comprehensive quality-assurance program is outlined in this paper to ensure high-quality stereotactic radiosurgery treatments and to provide possibly better clinical outcomes. The presented data shows that dedicated LINAC stereotactic radiosurgery systems possess adequate geometrical accuracy to treat trigeminal neuralgia.

References

  • 1.

    ACR practice guideline for the performance of stereotactic radiosurgery. ACR Practice Guideline:5675722002ACR practice guideline for the performance of stereotactic radiosurgery. ACR Practice Guideline:

  • 2.

    Brommeland THennig R: Mechanical accuracy of a new stereotactic guide. Acta Neurochir (Wien) 142:4494542000Acta Neurochir (Wien) 142:

  • 3.

    Drzymala REKlein EESimpson JRet al: Assurance of high quality linac-based stereotactic radiosurgery. Int J Radiat Oncol Biol Phys 30:4594721994Int J Radiat Oncol Biol Phys 30:

  • 4.

    Khan Faiz M: Physics of Radiation Therapyed 3. Philadelphia: Lippincott Williams & Wilkins2003507520Khan Faiz M:

  • 5.

    Leksell L: The stereotactic method and radiosurgery of the brain. Acta Chir Scand 102:3163191951Leksell L: The stereotactic method and radiosurgery of the brain. Acta Chir Scand 102:

  • 6.

    Lutz WAWinston KRMaleki N: A system for stereotactic radiosurgery with a linear accelerator. Int J Radiat Oncol Biol Phys 14:3731988Int J Radiat Oncol Biol Phys 14:

  • 7.

    Mack ACzempiel HKreiner HJet al: Quality assurance in stereotactic space. A system test for verifying the accuracy of aim in radiosurgery. Med Phys 29:5615682002Med Phys 29:

  • 8.

    Malatesta TLandoni VCanne S delleet al: Dosimetric, mechanical, and geometric verification of conformal dynamic arc treatment. J Appl Clin Med Phys 4:1952032003J Appl Clin Med Phys 4:

  • 9.

    Ramaseshan RHeydarian M: Comprehensive quality assurance for stereotactic radiosurgery treatments. Phys Med Biol 48:1992052003Phys Med Biol 48:

  • 10.

    Verellen DLinthout NBel Aet al: Assessment of the uncertainties in dose delivery of a commercial system for LINAC-based stereotactic radiosurgery. Int J Radiat Oncol Biol Phys 2:4214331999Int J Radiat Oncol Biol Phys 2:

  • 11.

    Wood MLHenkelman RM: ArtifactsStark DDBradley WG (eds): Magnetic Resonance Imaginged 3. Baltimore: Mosby19991215230

  • 12.

    Yeung DPalta JFontanesi Jet al: Systematic analysis of errors in target localization and treatment delivery in stereotactic radiosurgery (SRS). Int J Radiat Oncol Biol Phys 28:4934981994Int J Radiat Oncol Biol Phys 28:

Article Information

Address reprint requests to: Javad Rahimian, Ph.D., Department of Radiation Oncology, Southern California Permanente Medical Group, 4950 Sunset Boulevard, Los Angeles, California 90027. email: Javad.X.Rahimian@KP.org.

© AANS, except where prohibited by US copyright law."

Headings

Figures

  • View in gallery

    Photograph showing the Radionics Geometric Phantom Chamber with several objects of various shapes (sphere, cylinder, cone, and cube) and sizes.

  • View in gallery

    Winston—Lutz test of the gantry rotation at 270, 0, and 90° positions.

  • View in gallery

    Winston—Lutz test of the couch rotation at 0, 90, 63, 36, and 10° couch positions.

  • View in gallery

    Graph demonstrating bimonthly MR imaging external magnetic field homogeneity as a function of the test dates.

  • View in gallery

    Graph showing bimonthly MR imaging gradient calibration as a function of the test dates.

  • View in gallery

    The spherical marker and the radiochromic film treated with seven arcs and a 4-mm cone shown at × 7 magnification with a 0.1-mm precision scale

References

1.

ACR practice guideline for the performance of stereotactic radiosurgery. ACR Practice Guideline:5675722002ACR practice guideline for the performance of stereotactic radiosurgery. ACR Practice Guideline:

2.

Brommeland THennig R: Mechanical accuracy of a new stereotactic guide. Acta Neurochir (Wien) 142:4494542000Acta Neurochir (Wien) 142:

3.

Drzymala REKlein EESimpson JRet al: Assurance of high quality linac-based stereotactic radiosurgery. Int J Radiat Oncol Biol Phys 30:4594721994Int J Radiat Oncol Biol Phys 30:

4.

Khan Faiz M: Physics of Radiation Therapyed 3. Philadelphia: Lippincott Williams & Wilkins2003507520Khan Faiz M:

5.

Leksell L: The stereotactic method and radiosurgery of the brain. Acta Chir Scand 102:3163191951Leksell L: The stereotactic method and radiosurgery of the brain. Acta Chir Scand 102:

6.

Lutz WAWinston KRMaleki N: A system for stereotactic radiosurgery with a linear accelerator. Int J Radiat Oncol Biol Phys 14:3731988Int J Radiat Oncol Biol Phys 14:

7.

Mack ACzempiel HKreiner HJet al: Quality assurance in stereotactic space. A system test for verifying the accuracy of aim in radiosurgery. Med Phys 29:5615682002Med Phys 29:

8.

Malatesta TLandoni VCanne S delleet al: Dosimetric, mechanical, and geometric verification of conformal dynamic arc treatment. J Appl Clin Med Phys 4:1952032003J Appl Clin Med Phys 4:

9.

Ramaseshan RHeydarian M: Comprehensive quality assurance for stereotactic radiosurgery treatments. Phys Med Biol 48:1992052003Phys Med Biol 48:

10.

Verellen DLinthout NBel Aet al: Assessment of the uncertainties in dose delivery of a commercial system for LINAC-based stereotactic radiosurgery. Int J Radiat Oncol Biol Phys 2:4214331999Int J Radiat Oncol Biol Phys 2:

11.

Wood MLHenkelman RM: ArtifactsStark DDBradley WG (eds): Magnetic Resonance Imaginged 3. Baltimore: Mosby19991215230

12.

Yeung DPalta JFontanesi Jet al: Systematic analysis of errors in target localization and treatment delivery in stereotactic radiosurgery (SRS). Int J Radiat Oncol Biol Phys 28:4934981994Int J Radiat Oncol Biol Phys 28:

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