Isocenter Accuracy Research Articles

Patient position reproducibility in fractionated stereotactically guided conformal radiotherapy using the BrainLab mask system.

Dedicated mask systems nowadays allow the use of stereotactic radiotherapy in fractionated regimes, therefore combining the advantages of high precision radiotherapy with the biological benefit of fractionation. Therefore the knowledge of institution specific isocenter accuracy is essential for decision-making about margins to be allowed to form the planning target volume. Measurements of isocenter deviations during fractionated treatments were performed in 33 patients using the simulator Simulix-xy (Oldelft) in connection with the BrainLab angiographic localizer-box as well as port-films. In both cases repeated images were overlaid by use of anatomical landmarks with a methodical accuracy in the order of 0.5 mm. Both methods yield random isocenter deviations of less then 2 mm (standard deviation) in all three directions and no significant systematic deviations. These values are in the order of the accuracy of the method, obtained by comparison of two independent investigators, as well as they are comparable with the literature. The accuracy of less than 2 mm indicates safety margins of 3-4 mm as sufficient for clinical routine to cover the target in 95.5% of all set-ups (2 SD).

On isocentre adjustment and quality control in linear accelerator basedradiosurgery with circular collimators and room lasers

We have developed a densitometric method for measuring the isocentric accuracyand the accuracy of marking the isocentre position for linear acceleratorbased radiosurgery with circular collimators and room lasers. Isocentric shotsare used to determine the accuracy of marking the isocentre position with roomlasers and star shots are used to determine the wobble of the gantry and tablerotation movement, the effect of gantry sag, the stereotactic collimatoralignment, and the minimal distance between gantry and table rotation axes.Since the method is based on densitometric measurements, beam spot stabilityis implicitly tested. The method developed is also suitable for qualityassurance and has proved to be useful in optimizing isocentric accuracy. Themethod is simple to perform and only requires a film box and film scanner forinstrumentation. Thus, the method has the potential to become widely availableand may therefore be useful in standardizing the description of linearaccelerator based radiosurgical systems.

Two-Staged Radiosurgical Treatment of Large Arteriovenous Malformations

When the size of an arteriovenous malformation (AVM) in any one direction is more than 4 cm, an attempt is made to treat it in two stages, separated by 6 months. The first stage treats one-half of the AVM and the remainder is treated in the second stage, thus giving rise to a higher prescription dose for each stage. Therefore, the AVM, as a whole, could be given a higher dose than if the entire AVM were treated in a single fraction. In order to reproduce, in each stage, the isocenter coordinates determined from a pair of orthogonal films, two techniques are applied. One is the use of BrainLAB's mask system, which maintains the isocenter accuracy within ± 1 mm between the two stages. The other is the use of four 3-mm long titanium screws (fiducial markers) embedded in the patient's skull. The coordinate transformation matrix established for the fiducial markers between the first- and second-treatment stages can be applied to the respective isocenter coordinates. Thus, the original isocenter can be reestablished in the coordinate system of the second-treatment stage. Application of this method was first tested with four 5-mm diameter aluminum balls (BBs) attached as fiducial markers to the external surface of the Rando head phantom and three 3-mm tungsten balls embedded as isocenters inside the skull. One patient case was also studied in terms of predicted accuracy, since the isocenters cannot be pinpointed in terms of anatomical structure (like the three tungsten balls). The two-staged radiosurgical approach with small screws embedded in the skull maintains the accuracy required for stereotactic radiosurgery, thereby facilitating the treatment of large AVMs. Since rigid fixation of the head, as is used with traditional stereotactic radiosurgery, is not used with this two-staged approach, treatment does not have to be delivered shortly after diagnostic images are acquired for treatment planning. This gives the physicist, radiation oncologist, and neurosurgeon additional time to optimize the treatment plan, if necessary. By dividing the treatment into two stages, it is hoped that a higher dose of radiation can be safely delivered to large AVMs, thereby increasing the likelihood of cure.

Design and development of new collimator cones for fractionated stereotactic radiation therapy in Samsung medical center

Purpose: In stereotactic radiotherapy using X-Knife system, the commercially supplied collimator cone system had a few mechanical limitations. The authors have developed new collimator cones to overcome these limitations and named them “SMC type” collimator cones. Methods: We made use of cadmium-free cerrobend alloy within the stainless steel cylinder housing. We made nine cones of relatively larger sizes (3.0 cm to 7.0 cm in diameter) and of shorter length with bigger clearance from the isocenter than the commercial cones. The cone housing and the collimator cones were designed to insert into the wedge mount of the gantry head to enable double-exposure linac-gram taking. Results: The mechanical accuracy of pointing to the isocenter was tested by ball test and cone rotation test, and the dosimetric measurements were performed, all of which were with satisfactory results. A new innovative quality assurance procedure using linac-grams on the patients at the actual treatment setup was attempted after taking 10 sets of AP and lateral linac-grams and the overall mechanical isocenter accuracy was excellent (average error = 0.4 ± 0.2 mm). Conclusions: We have developed the SMC type collimator cone system mainly for fractionated stereotactic radiation therapy use with our innovative ideas. The new cones’ mechanical accuracy and physical properties were satisfactory for clinical use, and the verification of the isocenter accuracy on the actual treatment setup has become possible.

Quality assurance for the Leksell gamma unit: considering magnetic resonance image-distortion and delineation failure in the targeting of the internal auditory canal.

Our aim in this study was to distinguish quantitatively between the localization accuracy of a commercially available stereotactic fixation device as claimed by the manufacturer and the target accuracy as measured by a user, applying neuroradiologic imaging in Gamma Knife planning and phantom irradiation. Missing the target is the most serious possible failure in Gamma Knife and Linac therapy. To reduce this risk, we developed a quality control algorithm and designed a phantom. To evaluate the accuracy of the targeting procedure with a Leksell Gamma unit, and to experience the possible errors in all procedural steps, irradiations of phantoms were performed, using the so-called "unknown" targeting method. Accuracy is defined by the extent of spatial deviation of the irradiated target from the calculated target. Digital imaging was used for therapy planning. GafChromic films, which had been irradiated while affixed to a specially developed phantom, were used for measuring the precision of the radiation unit. A series of MR images (in two plains: transverse and coronal) was acquired sequentially to image the three-dimensional (3-D) volume of the phantom. The results obtained for isocentric accuracy of the Leksell Gamma unit, model B, were in good agreement to the calculated position. The observed spatial deviations between calculated and irradiated targets is less than 1 mm. The newly designed phantom and quality control algorithm are useful in quality assurance measurements of stereotactic radiation therapy.

Localization and System Integration Tests as Quality Assurance for the Leksell γ-Unit

Quality control in stereotactic radiotherapy of brain lesions with the Leksell γ-unit or the LINAC-facility is a must, as missing the target is the most serious error that can occur in that kind of treatment. We developed a quality assurance program to reduce this risk. To evaluate the accuracy of the procedure, which defines the target, including all possible errors of the therapy chain, irradiations of phantoms were performed, using the so-called “known” and “unknown” target method. Accuracy is defined by the deviation of the irradiated target from the calculated target with digital images used for therapy planning. GafChromic MD-55 films, which have been irradiated by means of a special author-developed phantom, were applied for measuring the precision of the radiation unit. The results obtained for isocentric accuracy of the Leksell γ-unit were in good agreement with the measurements of the manufacturing company. The deviation measured with the 4-mm collimator helmet was up to 0.3 mm, including the CT images and the stereotactic frame less than 0.85 mm, respectively. The phantom designed for this purpose is useful in quality assurance measurements of this stereotactic therapy chain.

Technical Aspects of LINAC Radiosurgery for the Treatment of Small Functional Targets

Recently the use of stereotactic radiosurgery to treat functional disorders such as Parkinson's disease, epilepsy, and intractable pain has been reported in the literature. In such applications, a large single dose is typically delivered to an extremely small (<0.05 cm3) target volume. The purpose of this work is to investigate whether the dosimetric and imaging characteristics of radiosurgery treatment planning provide sufficient accuracy to allow efficacious therapy of functional disorders. We have begun treating trigeminal neuralgia using our linear accelerator-based radiosurgery system: 70 Gy is prescribed to the maximum dose in the volume (in our case the 100% isodose level) and delivered to the base of the fifth nerve in a single fraction using a 5 mm collimator, with the standard Brown–Roberts–Wells (BRW) radiosurgery accessories employed for fixation and localization. Because the fifth nerve cannot be visualized on x-ray computed tomography (CT), our radiosurgery treatment planning system was modified to use magnetic resonance images for localization, though dose calculations are still performed using CT. Isocentric accuracy of our original radiosurgery system, consisting of a floor stand and isocentric subunit, and our new couch mount system, was evaluated using the Winston–Lutz film test method. In order to evaluate the spatial accuracy of magnetic resonance (MR) treatment planning, eight 4 mm sections of a 7 French catheter were filled with CuSO4 contrast material and attached rigidly to the stereotactic fixation posts of our BRW frame, four each in orientations parallel and perpendicular to the axial plane. The position of the externally placed fiducial markers, as well as internal anatomical structures, were then compared with CT. Monte Carlo calculations were compared with those from a commercial radiosurgery treatment planning system in order to investigate the effects of tissue heterogeneities on the resulting dose distributions. While commercial radiosurgery systems assume tissue homogeneity, the Monte Carlo calculations were performed in a patient-specific CT geometry accounting for all tissue inhomogeneities. The resulting 128 × 128 Monte Carlo dose grid was superimposed on the original CT data for analysis and comparison with identical treatment plans from the commercial system. The ability of our LINAC-based systems to accurately target a desired point in space has been effectively demonstrated: 0.32 ± 0.32 mm (N = 556) for our floor stand system and 0.34 ± 0.23 mm (N = 50) for our newer couch-mounted system. Inaccuracies introduced by tomographic imaging devices are significantly greater. The use of gel-filled fiducial markers in magnetic resonance imaging (MRI) guided radiosurgery produces significant spatial distortion, resulting in Euclidean root-mean-square deviations of 2.32 ± 0.96 mm (N = 31) and 3.64 ± 1.28 mm (N = 27) at the center and periphery (extracranial) of the field of view respectively, as compared with CT. Use of water of CuSO4 filled rods had a minimal effect on these deviations: 2.51 ± 1.25 mm (N = 31) and 3.37 ± 1.28 mm (N =27) for central and peripheral targets respectively. Magnetic susceptibility artifacts in the frequency encoding (AP) direction produce a systematic posterior shift. This together with axial slice spacing accounts for the majority of the deviation. Tissue heterogeneities such as bone and air cavities produce a lateral spreading of the dose from small photon beams, resulting in a prescription dose volume smaller than predicted by conventional treatment planning systems. For a typical are configuration designed to produce a spherical dose volume, Monte Carlo calculations show the 90% dose volume to be significantly smaller than that predicted by the commercial system when either 5 mm or 10 mm collimators are used. Use of a LINAC-based system does not preclude accurate treatment of small functional targets. Isocentric uncertainty for either of two LINAC systems that we evaluated is small compared to imaging and dosimetric factors. However, chemical shifts and object-induced magnetic susceptibility artifacts can produce systematic spatial distortions in magnetic resonance images; thus, MR imaging may not possess the inherent accuracy necessary for stereotactic localization and targeting of small cranial structures. In addition, both CT and MR possess an inherent inaccuracy of at least one-half of the axial slice thickness; thus, for localization purposes, a slice spacing as small as possible should be used when treating small targets. Tissue heterogeneities decrease the volume covered by the higher isodose lines. As a result, the target may be only partially covered by the intended dose level, with the remainder lying in the high gradient region. This same lateral spreading may also increase the risk to adjacent normal structures. Imaging and dosimetric considerations are not unique to linear accelerator systems but apply equally to all stereotactic photon irradiation. Until spatial and dosimetric errors can be accounted for, use of a larger collimator will ensure better coverage of small targets.

Accuracy in target localization in stereotactic radiosurgery

The accuracy in target localization of CT, MRI, and digital angiography and the isocentric deviation of linear accelerator were investigated for stereotactic radiosurgery. Twenty-five slice images using CT and MRI were obtained out of geometrical phantom which was designed to produce exact 3-dimensional coordinates of several points within a 0.1 mm error range. These diagnostic images were transferred to a 3-D treatment planning system through ethernet. Measured 3-D coordinates of these images from the planning system were compared to known values by geometrical phantom. Anterior-posterior and lateral films were taken by digital angiography for measurement of spatial accuracy. The accuracy of gantry isocenter aligned by laser localizer was also measured. Lead ball was located at the isocenter of linear accelerator and x-ray films were taken by 4-MV photon beam with gantry angles of 0, 90, 180 and 270 degrees. Overall procedure from CT scanning to treatment was carried out using the geometrical phantom. The target accuracy was verified by high energy x-ray portal film. The accuracy of diagnostic machines were within 2.1 mm except MR-axial images. In case of linear accelerator, the deviation of isocenter was within 0.7 mm. Finally, the total isocentric deviation of overall procedure was 1.3 ± 0.5 mm (using CT localization).

Measurement of mechanical accuracy of isocenter in conventional linear-accelerator-based radiosurgery

Five Varian linear accelerators were studied to determine whether their mechanical isocentric accuracies were sufficient for radiosurgery and, if not, if the observed errors were sufficiently consistent and predictable to be correctable by some form of secondary collimator steering device to maintain isocentric alignment. A 0.3 mW 670 nm diode laser was mounted in the secondary collimator insert of a radiosurgery extended collimator assembly. A cylindrical lens was used to create a laser fan beam that passed through isocenter and could be oriented parallel or perpendicular to the plane of rotation. A position sensitive photo-diode having an electrical output that varied with the portion of its surface illuminated was mounted at isocenter in a rotational mount. This mount tracked the accelerator gantry such that the surface of the photo-diode remained perpendicular to the laser beam during gantry rotation. An X/Y recorder was connected to the gantry-angle potentiometer of the accelerator and to the photo-diode and plotted the positional variation from isocenter with gantry rotation. The root-mean-square error for the five machines was +/- 0.06 to +/- 0.08 mm in the plane of rotation and +/- 0.17 to +/- 0.35 mm out of (perpendicular to) the plane of rotation. The in-plane-of-rotation errors tended to be maximal near the diagonal gantry angles and the out-of-plane-of-rotation errors were maximal in the over and under vertical positions. Both types of errors were predictable but only the out-of-plane-of-rotation errors were considered large enough to warrant consideration of correction (although the need is debatable). On all the tested machines, the out-of-plane-of-rotation error curve was a relatively smooth bell-shaped function that would be readily amenable to correction. The diode laser/photo-detector system used should prove useful in accurately defining isocenter and facilitating the precise adjustment of the laser isocenter lights.