Precision and accuracy of stereotactic convergent beam irradiations from a linear accelerator

Patient-specific geometrical distortion corrections of MRI images improve dosimetric planning accuracy of vestibular schwannoma treated with gamma knife stereotactic radiosurgery

PURPOSE

To investigate the impact of MRI patient-specific geometrical distortion (PSD) on the quality of Gamma Knife stereotactic radiosurgery (GK-SRS) plans of the vestibular schwannoma (VS) tumors.

METHODS AND MATERIALS

Three open access datasets including the MPI-Leipzig Mind-Brain-Body (318 patients), the slow event-related fMRI designs dataset (62 patients), and the VS dataset (242 patients) were used. We used first two datasets to train a 3D convolution network to predict the distortion map of third dataset that were then used to calculate and correct the PSD. GK-SRS plans of VS dataset were used to evaluate dose distribution of PSD-corrected MRI images. GK-SRS prescription dose of VS cases was 12 Gy. Geometric and dosimetric discrepancies were assessed between the dose distributions and contours before and after the PSD corrections. Geometry indices were center of the contours, Dice coefficient (DC), Hausdorff distance (HD), and dosimetric indices wereD μ ${D_\mu }$ ,D m a x ${D_{max}}$ ,D m i n ${D_{min}}$ , andD 95 % ${D_{95{\mathrm{\% }}}}$ doses, target coverage (TC), Paddick's conformity index (PCI), Paddick's gradient index (GI), and homogeneity index (HI).

RESULTS

Geometric distortions of about 1.2 mm were observed at the air-tissue interfaces at the air canal and nasal cavity borders. Average center of the targets was significantly distorted along the frequency encoding direction after the PSD-correction. Average DC and HD metrics were 0.90 and 2.13 mm. AverageD μ ${D_\mu }$ ,D 95 % , ${D_{95{\mathrm{\% ,}}}}$ andD m i n ${D_{min}}$ in Gy significantly increased after PSD correction from 16.85 to 17.25, 12.30 to 12.77, and from 8.98 to 9.92.D m a x ${D_{max}}$ did not significantly change after the correction. Average TC and PCI significantly increased from 0.97 to 0.98, and 0.94 to 0.96. Average GI decreased significantly from 2.24 to 2.15 after PSD correction. However, HI did not significantly change after the correction.

CONCLUSION

The proposed method could predict and correct the PSD that indicates the importance of PSD correction before GK-SRS plans of the VS patients.

Three-dimensional Winston-Lutz test using reusable polyvinyl alcohol-iodide (PVA-I) radiochromic gel dosimeter

Medical linear-accelerator-based stereotactic radiosurgery (SRS) using a stereotactic apparatus or image-guided radiotherapy system for intracranial lesions is performed widely in clinical practice. In general, Winston-Lutz (WL) tests using films or electric portal imaging devices (EPIDs) have been performed as pre-treatment and routine quality assurance (QA) for the abovementioned treatment. Two-dimensional displacements between the radiation isocentre and mechanical isocentre are analysed from the test; therefore, it is difficult to identify the three-dimensional (3D) isocentre position intuitively. In this study, we developed an innovative 3D WL test for SRS-QA using a novel radiochromic gel dosimeter based on a polyvinyl alcohol-iodide (PVA-I) complex that can be reused after annealing. A WL gel phantom that was consisted of the PVA-I gel dosimeter poured into a tall acrylic container and an embedded small tungsten sphere was used as a position detector. A flatbed scanner was used to analyse the isocentre position. The measured 3D isocentre accuracy from the gel-based WL test was within 0.1 mm compared with that obtained from the EPID-based WL test. Furthermore, excellent reusability of the WL gel phantom was observed in long-term SRS isocentre verification, in which clinical SRS cases involving repeated irradiation and annealing were analysed. These results demonstrate the high accuracy and reliable evaluation of the isocentre position using an innovative test. In addition, the clinical-based routine SRS-QA using the PVA-I gel dosimeter demonstrates a highly convenience while affording an easy and fast analysis process.

A closer look at the conventional Winston-Lutz test: Analysis in terms of dose

Aim

To investigate whether the target-isocenter deviations reported by a conventional Winston-Lutz (WL) test actually reflect the shifts of the measured prescription isodose line with respect to the target.

Background

A conventional WL test uses a metallic ball as a target that aims at several fields. But this test does not report information on the accuracy of the delivery in terms of dose.

Materials and methods

A conventional WL test using a metallic pointer as a target (Pointer-WL test) has been recreated in the Eclipse treatment planning system over an acrylic phantom containing a radiochromic film (Dose-WL test). After Dose-WL test delivery, the shift of the 80% prescription isodose line with respect to the target center (d80%-center) was measured using film dosimetry. The Pointer-WL and Dose-WL tests were performed in 10 different sessions. The isocenter deviation reported by the Pointer-WL test was compared to the d80%-center vector, according to the three patient's directions (Left-Right or LR; Anterior-Posterior or AP; and Superior-Inferior or SI).

Results

The deviations (mean ± SD) found for the Dose-WL tests (LR: 0.5 ± 0.4 mm; AP: 0.5 ± 0.4 mm; SI: 0.6 ± 0.2 mm) were in most cases less than 1 mm, and they were significantly smaller (all p < 0.05) than the maximum deviations reported by the Pointer-WL tests (LR: 1.3 ± 0.3 mm; AP: 1.2 ± 0.4 mm; SI: 1.1 ± 0.3 mm).

Conclusions

The Dose-WL test described in this study allows estimating the spatial accuracy of the prescription isodose line.

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.

The application of micro-vacuo-certo-contacting ophthalmophanto in X-ray radiosurgery for tumors in an eyeball

The large errors of routine localization for eyeball tumors restricted X-ray radiosurgery application, just for the eyeball to turn around. To localize the accuracy site, the micro-vacuo-certo-contacting ophthalmophanto (MVCCOP) method was used. Also, the outcome of patients with tumors in the eyeball was evaluated. In this study, computed tomography (CT) localization accuracy was measured by repeating CT scan using MVCCOP to fix the eyeball in radiosurgery. This study evaluated the outcome of the tumors and the survival of the patients by follow-up. The results indicated that the accuracy of CT localization of Brown-Roberts-Wells (BRW) head ring was 0.65 mm and maximum error was 1.09 mm. The accuracy of target localization of tumors in the eyeball using MVCCOP was 0.87 mm averagely, and the maximum error was 1.19 mm. The errors of fixation of the eyeball were 0.84 mm averagely and 1.17 mm maximally. The total accuracy was 1.34 mm, and 95% confidence accuracy was 2.09 mm. The clinical application of this method in 14 tumor patients showed satisfactory results, and all of the tumors showed the clear rims. The site of ten retinoblastomas was decreased significantly. The local control interval of tumors were 6 ∼ 24 months, median of 10.5 months. The survival of ten patients was 7 ∼ 30 months, median of 16.5 months. Also, the tumors were kept stable or shrank in the other four patients with angioma and melanoma. In conclusion, the MVCCOP is suitable and dependable for X-ray radiosurgery for eyeball tumors. The tumor control and survival of patients are satisfactory, and this method can effectively postpone or avoid extirpation of eyeball.

[Development of an automated method for analysis of Winston-Lutz test results using digital radiography and photostimulable storage phosphor]

Stereotactic radiosurgery (SRS) and radiotherapy (SRT) are intricate techniques that deliver a highly precise radiation dose to a localized target, usually a tumor. At our hospital, we perform SRS and SRT on brain tumors using a linear accelerator (linac) mounted with an external micro multi-leaf system. The Task Group TG-142 Report by the American Association of Physicists in Medicine recommends the coincidence of the radiation and mechanical isocenter to be within ±1 mm. The Winston-Lutz test is commonly used to verify the linac isocenter position: it has the advantages of being a simple method that uses a film or electronic portal imaging device (EPID). However, the film method requires a higher radiation dose, which makes it more time-consuming than the EPID method, and the results are highly dependent on the skills of the observer. The EPID method has certain advantages over the film method, but it has low resolution and can only be used for a few combinations of gantry and couch angles. This prompted us to develop an in-house-designed radiation receptor system based on digital radiography, using a photostimulable storage phosphor and automated analysis algorithm for Winston-Lutz test images using a template-matching technique based on cross-correlation coefficients. Our proposed method shows a maximum average absolute error of 0.222 mm (less than 2 pixels) for 0.5 mm and 1.0 mm displacement from the isocenter toward the inline and crossline directions. Our proposed method is thus potentially useful for verifying the Linac isocenter position with a small error and good reproducibility, as demonstrated by improved accuracy of evaluation.

[Comparison of image distortion between three magnetic resonance imaging systems of different magnetic field strengths for use in stereotactic irradiation of brain]

In this study, we evaluated the image distortion of three magnetic resonance imaging (MRI) systems with magnetic field strengths of 0.4 T, 1.5 T and 3 T, during stereotactic irradiation of the brain. A quality assurance phantom for MRI image distortion in radiosurgery was used for these measurements of image distortion. Images were obtained from a 0.4-T MRI (APERTO Eterna, HITACHI), a 1.5-T MRI (Signa HDxt, GE Healthcare) and a 3-T MRI (Signa HDx 3.0 T, GE Healthcare) system. Imaging sequences for the 0.4-T and 3-T MRI were based on the 1.5-T MRI sequence used for stereotactic irradiation in the clinical setting. The same phantom was scanned using a computed tomography (CT) system (Aquilion L/B, Toshiba) as the standard. The results showed mean errors in the Z direction to be the least satisfactory of all the directions in all results. The mean error in the Z direction for 1.5-T MRI at －110 mm in the axial plane showed the largest error of 4.0 mm. The maximum errors for the 0.4-T and 3-T MRI were 1.7 mm and 2.8 mm, respectively. The errors in the plane were not uniform and did not show linearity, suggesting that simple distortion correction using outside markers is unlikely to be effective. The 0.4-T MRI showed the lowest image distortion of the three. However, other items, such as image noise, contrast and study duration need to be evaluated in MRI systems when applying frameless stereotactic irradiation.

[Quality assurance procedure for assessing mechanical accuracy of a radiation field center in stereotactic radiotherapy]

Stereotactic radiotherapy requires a quality assurance (QA) program that ensures the mechanical accuracy of a radiation field center. We have proposed a QA method for achieving the above requirement by conducting the Winston Lutz test using an electronic portal image device (EPID). An action limit was defined as three times the standard deviation. Then, the action limits for mean deviations of the radiation field center during collimator rotation, gantry rotation, and couch rotation in clockwise and counterclockwise resulted in 0.11 mm, 0.52 mm, 0.37 mm, and 0.41 mm respectively. Two years after the QA program was launched, the mean deviation of the radiation field center during gantry rotation exceeded the above action limit. Consequently, a mechanical adjustment for the gantry was performed, thereby restoring the accuracy of the radiation field center. A field center shift of 0.5 mm was also observed after a micro multi-leaf collimator was unmounted.

Isocenter verification for linac-based stereotactic radiation therapy: review of principles and techniques

There have been several manual, semi-automatic and fully-automatic methods proposed for verification of the position of mechanical isocenter as part of comprehensive quality assurance programs required for linear accelerator-based stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. In this paper, a systematic review has been carried out to discuss the present methods for isocenter verification and compare their characteristics, to help physicists in making a decision on selection of their quality assurance routine.

Clinical results of a pilot study on stereovision-guided stereotactic radiotherapy and intensity modulated radiotherapy

Real-time stereovision-guidance has been introduced for efficient and convenient fractionated stereotactic radiotherapy (FSR) and image-guided intensity-modulated radiation therapy (IMRT). This first pilot study is to clinically evaluate its accuracy and precision as well as impact on treatment doses. Sixty-one FSR patients wearing stereotactic masks (SMs) and nine IMRT patients wearing flexible masks (FMs), were accrued. Daily target reposition was initially based-on biplane-radiographs and then adjusted in six degrees of freedom under real-time stereovision guidance. Mean and standard deviation of the head displacements measured the accuracy and precision. Head positions during beam-on times were measured with real-time stereovisions and used for determination of delivered doses. Accuracy ± ± precision in direction with the largest errors shows improvement from 0.4 ± 2.3 mm to 0.0 ± 1.0 mm in the inferior-to-superior direction for patients wearing SM or from 0.8 ± 4.3 mm to 0.4 ± 1.7 mm in the posterior-to-anterior direction for patients wearing FM. The image-guidance increases target volume coverage by >30% for small lesions. Over half of head position errors could be removed from the stereovision-guidance. Importantly, the technique allows us to check head position during beam-on time and makes it possible for having frameless head refixation without tight masks.

[Mechanical accuracy of a stereotactic irradiation system using a micro multi-leaf collimator]

Mechanical accuracy of a stereotactic irradiation system using a micro multi-leaf collimator (mMLC), Elekta DMLC, has been evaluated. Measurements were made to obtain transmission, leakage, penumbra, and positioning accuracy of the DMLC leaf for a 6 MV photon beam. Mechanical accuracy and long term stability of a linac isocenter was also evaluated. The resulting transmission, along a line perpendicular to the leaf movement, was 0.31±0.01%, and the leakage from the closed opposing leaf pairs was 0.39±0.01%. The measured penumbra, at a depth incurring maximum dose, was 2.37±0.16 mm toward the leaf end and 2.14±0.18 mm toward the leaf side for various field sizes. The leaf gap width error, of 0.10±0.08 mm, was obtained by analyzing picket fence test results. The maximum leaf positioning error, of 0.14±0.06 mm, was obtained by analyzing the log file for a various gantry angles during an arc delivery. The isocenter accuracy was within a radius of 1 mm, without any recalibration for two years. In conclusion, our stereotactic irradiation system using DMLC was capable of providing accurate stereotactic treatment.

On the accuracy of isocenter verification with kV imaging in stereotactic radiosurgery

BACKGROUND AND PURPOSE

Modern medical linear accelerators (linacs) are equipped with X-ray systems, which allow to check the patient's position just prior to treatment. Their usefulness for stereotactic radiosurgery (SRS) depends on how accurately they allow to determine the deviation between the actual and planned isocenter positions. This accuracy was investigated with measurements using two different phantoms (Figures 1 and 2).

MATERIAL AND METHODS

After precisely aligning a phantom onto the linac isocenter, two perpendicular X-rays or a cone-beam CT (CBCT) are taken, and the isocenter position is deduced from this data. The deviation of the thereby gained position from the setup isocenter is taken as a measure for the uncertainty of this method.

RESULTS

Isocenter verification with two orthogonal X-rays (Figure 4) achieves accuracies of better than 1 mm (Table 3). The distance between the isocenters of the CBCT and the linac (Figure 3) is in the order of 1 mm, but remains constant on the time scale of 1 week (Table 1) and may therefore be taken into account. The uncertainty after correction is below 0.2 mm.

CONCLUSION

kV imaging with the patient in treatment position allows to verify the isocenter position with submillimeter precision, and therefore offers a supplemental test, suitable for SRS, which takes all positional uncertainties into account.

Novalis frameless image-guided noninvasive radiosurgery: initial experience

OBJECTIVE

To evaluate our initial experience with Novalis (BrainLAB, Heimstetten, Germany) frameless image-guided noninvasive radiosurgery.

METHODS

The system combines the dedicated Novalis linear accelerator with ExacTrac X-Ray 6D, an infrared camera and a kilovolt stereoscopic x-ray imaging system, a noninvasive mask system, and ExacTrac robotics for patient positioning in six degrees of freedom. Reference cranial skeletal structures are radiographically imaged and automatically fused to digital reconstructed radiographs calculated from the treatment planning computed tomographic scan to find the target position and accomplish automatic real-time tracking before and during radiosurgery. We present the acceptance testing and initial experience in 15 patients with 19 intracranial lesions treated between December 2005 and June 2006 at the Charité by frameless image-guided radiosurgery with doses between 12 and 20 Gy prescribed to the target-encompassing isodose.

RESULTS

Phantom tests showed an overall system accuracy of 1.04 +/- 0.47 mm, with an average in-plane deviation of 0.02 +/- 0.96 mm for the x-axis and 0.02 +/- 0.70 mm for the y-axis. After infrared-guided patient setup of all patients, the overall average translational deviation determined by stereoscopic x-ray verification was 1.5 +/- 1.3 mm, and the overall average rotational deviation was 1.0 +/- 0.8 degree. The data used for radiosurgery, after stereoscopic x-ray verification and correction, demonstrated an overall average setup error of 0.31 +/- 0.26 mm for translation and 0.26 +/- 0.23 degree for rotation.

CONCLUSION

This initial evaluation demonstrates the system accuracy and feasibility of Novalis image-guided noninvasive radiosurgery for intracranial benign and malignant lesions.

Investigations of different kilovoltage x-ray energy for three-dimensional converging stereotactic radiotherapy system: Monte Carlo simulations with CT data

We are investigating three-dimensional converging stereotactic radiotherapy (3DCSRT) with suitable medium-energy x rays as treatment for small lung tumors with better dose homogeneity at the target. A computed tomography (CT) system dedicated for non-coplanar converging radiotherapy was simulated with BEAMnrc (EGS4) Monte-Carlo code for x-ray energy of 147.5, 200, 300, and 500 kilovoltage (kVp). The system was validated by comparing calculated and measured percentage of depth dose in a water phantom for the energy of 120 and 147.5 kVp. A thorax phantom and CT data from lung tumors (<20 cm3) were used to compare dose homogeneities of kVp energies with MV energies of 4, 6, and 10 MV. Three non-coplanar arcs (0 degrees and +/-25 degrees ) around the center of the target were employed. The Monte Carlo dose data format was converted to the XiO RTP format to compare dose homogeneity, differential, and integral dose volume histograms of kVp and MV energies. In terms of dose homogeneity and DVHs, dose distributions at the target of all kVp energies with the thorax phantom were better than MV energies, with mean dose absorption at the ribs (human data) of 100%, 85%, 50%, 30% for 147.5, 200, 300, and 500 kVp, respectively. Considering dose distributions and reduction of the enhanced dose absorption at the ribs, a minimum of 500 kVp is suitable for the lung kVp 3DCSRT system.

An MLC-based linac QA procedure for the characterization of radiation isocenter and room lasers’ position

We have designed and implemented a new stereotactic linac QA test with stereotactic precision. The test is used to characterize gantry sag, couch wobble, cone placement, MLC offsets, and room lasers' positions relative to the radiation isocenter. Two MLC star patterns, a cone pattern, and the laser line patterns are recorded on the same imaging medium. Phosphor plates are used as imaging medium due to their sensitivity to red light. The red light of room lasers erases some of the irradiation information stored on the phosphor plates enabling accurate and direct measurements for the position of room lasers and radiation isocenter. Using film instead of the phosphor plate as imaging medium is possible, however, it is less practical. The QA method consists of irradiating four phosphor plates that record the gantry sag between the 0 degrees and 180 degrees gantry angles, the position and stability of couch rotational axis, the sag between the 90 degrees and 270 degrees gantry angles, the accuracy of cone placement on the collimator, the MLC offsets from the collimator rotational axis, and the position of laser lines relative to the radiation isocenter. The estimated accuracy of the method is +/- 0.2 mm. The observed reproducibility of the method is about +/- 0.1 mm. The total irradiation/ illumination time is about 10 min per image. Data analysis, including the phosphor plate scanning, takes less than 5 min for each image. The method characterizes the radiation isocenter geometry with the high accuracy required for the stereotactic radiosurgery. In this respect, it is similar to the standard ball test for stereotactic machines. However, due to the usage of the MLC instead of the cross-hair/ball, it does not depend on the cross-hair/ball placement errors with respect to the lasers and it provides more information on the mechanical integrity of the linac/couch/laser system. Alternatively, it can be used as a highly accurate QA procedure for the nonstereotactic machines. Noteworthy is its ability to characterize the MLC position accuracy, which is an important factor in IMRT delivery.

The influence of head frame distortions on stereotactic localization and targeting

A strong attachment of a stereotactic head frame to the patient's skull may cause distortions of the head frame. The aim of this work was to identify possible distortions of the head frame, to measure the degree of distortion occurring in clinical practice and to investigate its influence on stereotactic localization and targeting. A model to describe and quantify the distortion of the Riechert-Mundinger (RM) head frame was developed. Distortions were classified as (a) bending and (b) changes from the circular ring shape. Ring shape changes were derived from stereotactic CT scans and frame bending was determined from intraoperative stereotactic x-ray images of patients with implanted 125I-seeds acting as landmarks. From the examined patient data frame bending was determined to be 0.74 mm+/-0.32 mm and 1.30 mm in maximum. If a CT-localizer with a top ring is used, frame bending has no influence on stereotactic CT-localization. In stereotactic x-ray localization, frame bending leads to an overestimation of the z-coordinate by 0.37 mm+/-0.16 mm on average and by 0.65 mm in maximum. The accuracy of patient positioning in radiosurgery is not affected by frame bending. But in stereotactic surgery with an RM aiming bow trajectory displacements are expected. These displacements were estimated to be 0.36 mm+/-0.16 mm (max. 0.74 mm) at the target point and 0.65 mm+/-0.30 mm (max. 1.31 mm) at the entry point level. Changes from the circularring shape are small and do not compromise the accuracy of stereotactic targeting and localization. The accuracy of CT-localization was found to be close to the resolution limit due to voxel size. Our findings for frame bending of the RM frame could be validated by statistical analysis and by comparison with an independent patient examination. The results depend on the stereotactic system and details of the localizers and instruments and also reflect our clinical practice. Therefore, a generalization is not possible. Preliminary experience with a new MR-compatible RM head frame made of ceramics shows no frame distortions as with the conventional frame made of an Al-Cu-Mg alloy.

Stereotactic radiosurgery XVI: Isodosimetric comparison of photon stereotactic radiosurgery techniques (gamma knife vs. micromultileaf collimator linear accelerator) for acoustic neuroma—and potential clinical importance

PURPOSE

Two stereotactic photon radiation therapy methods are currently in practice for the treatment of acoustic neuroma. In the 1990s, our data and those of others demonstrated isodosimetric advantages for gamma knife technology over linear accelerator methodology. Since then, the introduction of micromultileaf collimator technology has improved the conformity of the linear accelerator method such that the isodosimetric differences between the two techniques have narrowed.

MATERIALS AND METHODS

In this study, modern gamma knife isodosimetry was compared to that of modern linac technology (conformal fixed fields and dynamic arcs) for the therapy of acoustic neuroma. This is an unusual target in that a special sensory nerve (holding the key to hearing preservation) frequently runs through the targeted volume, unlike the majority of other stereotactic radiation therapy targets. This was a single-dose prescription comparison; the perceived extra benefit of fractionation (a technique not routinely available to the gamma knife) was thereby abrogated.

RESULTS

Although the gamma knife technique maintained a slight, but statistically significant, advantage with regard to dose conformity (p < 0.02) (at the debatable cost of a lower minimum target dose), the much higher internal dose gradient (high maximum dose to prescription dose [MD:PD] ratio) could be interpreted as a disadvantage with respect to hearing preservation, although advantageous with regard to tumor ablation. Of the two linac methods, the dynamic arc method gave a statistically significant advantage over the fixed-field method as regards conformity (p < 0.05), at the expense of a slightly higher brainstem dose (an average of 12.4 Gy, cf. 11.7 Gy for fixed fields), but this result was not statistically significant. No significant difference was seen in the MD:PD ratio for the two single-isocenter linac techniques.

CONCLUSIONS

Gamma knife methodology remains well validated, with very good isodosimetry, but when hearing preservation is important, the improving linac technologies will compete with the gamma knife for optimal therapy. In these circumstances, the minor differences in isodosimetry between the two techniques will become important.

Geometrical and dosimetrical characterization of the photon source using a micro-multileaf collimator for stereotactic radiosurgery

A micro-multileaf collimator (microMLC) for stereotactic radiosurgery is used for determination of the spatial intensity distribution of the photon source of a linear accelerator. The method is based on grid field dose measurements using film dosimetry and is easy to perform. Since the microMLC does not allow 'direct' imaging of the photon source, special software has been developed to analyse grid field measurements. Besides the source-density function, grid field analysis yields the position of the focal spot in the room laser coordinate system of the linear accelerator and the position of the treatment head rotation axis and the inclination angle of the leaf bank. Thus the method can be used for base dosimetry and for quality assurance in radiosurgery using a microMLC.

Attenuation and scatter correction for in-beam positron emission tomography monitoring of tumour irradiations with heavy ions

An in-beam dual-head positron camera is used to monitor the dose application in situ during the tumour irradiation with carbon ion beams at the experimental heavy ion therapy facility at GSI Darmstadt. Therefore, a positron emission tomograph has been mounted directly at the treatment site. A fully 3D reconstruction algorithm based on the maximum likelihood expectation maximization (MLEM) algorithm has been developed and adapted to this spatially varying imaging situation. The scatter and attenuation correction are included in the forward projection step of the maximum likelihood image reconstruction. This requires an attenuation map containing the information on the material composition and densities. This information is derived from the x-ray computed tomograms of the patient and the patient fixation system including the head-rest. The normalization of scattered events relative to the unscattered events is done by a global scatter fraction factor which is estimated by means of a Monte Carlo simulation. The feasibility of the proposed algorithm is shown by means of computer simulations, phantom measurements as well as patient data.

Assessment of the accuracy of a conventional simulation for radiotherapy of head and skull base tumors

In this prospective study we investigated the absolute accuracy of the conventional simulation in head and skull base tumors. 41 isocenters in 40 consecutive patients with tumors of the head and skull base were included. In all cases a rigid stereotactic mask system was used for non-invasive fixation. The stereotactic ("calculated") coordinates of the isocenter were defined by the treatment planning computer. Each patient underwent a physical simulation using exclusively anatomical reference points to define the "preliminary" isocenter. The displacement between its coordinates and those of the stereotactic target point was recorded in X-, Y- and Z-direction with help of the targeting device, and the spatial error was calculated. Additionally, the patients were stratified by basal or calvarial tumor site to estimate the importance of the basal bone structures in the simulation accuracy. The influence of the learning effect on simulation accuracy was also determined. The results showed an accuracy of set-up at the linac within 1 mm in all three directions as calculated from orthogonal portal films. Mean shift of the isocenter coordinates obtained from physical simulation compared to the calculated stereotactic coordinates was 2.15 mm, 2.54 mm, and 2.69 mm for X-, Y-, and Z-direction, respectively. Mean spatial displacement amounted 5.06 mm, and the median was 4.50 mm. No significant difference could be noted between basal and calvarial location of the isocenter. A significant "learning effect" was observed with a decrease in spatial shift with increasing patient numbers. This effect was stronger in basal lesions, whereas calvarial lesions showed only a minor, insignificant effect. In conclusion, a physical simulation requires a safety margin of 5 mm in PTV definition in addition to other factors, e.g. organ movement.

Comprehensive quality assurance for stereotactic radiosurgery treatments

We have used a commercially available high precision Lucy phantom to perform comprehensive quality assurance for stereotactic radiosurgery treatments. The quantitative evaluation of system uncertainties included imaging, planning and treatment delivery systems. The quality assurance tests showed that the well-defined targets were identified to within +/-1 mm in all the imaging modalities. The pre-known target volumes were reproduced within 2 cm3 in both MR and CT. The planned target was delivered within 2% of the prescribed dose and to within 2 mm accuracy. The inaccuracy in the isocentre position at the Linac was less than 1.2 mm. The maximum error observed in the depth helmet was 0.5 mm and the overall uncertainty was within 0.23 mm. We have also established a quality assurance program based on the study and proposed the tolerance and the frequency of the tests required to be carried out. The tests were carried out using a Radionics planning system and delivered on a Varian Clinac 2100 linear accelerator machine. These tests also established a base line for future comparisons.

Verification of the dose to the isocentre in stereotactic plans

BACKGROUND AND PURPOSE

The aim of the study was: (a) to develop a simple, reproducible, technique to verify the dose to the isocentre, in a typical stereotactic treatment plan, for collimators from 12.5 to 40 mm in diameter; (b) to investigate a variety of detectors to compare different approaches; and (c) to introduce the technique into a quality assurance programme.

MATERIAL AND METHODS

The symmetry, directional response and stability of calibration of a small 0.125 cm(3) ion chamber, a diamond and three types of diode (photon, electron and stereotactic) were tested. Correction factors were calculated to account for directional dependence, where appropriate and calibration factors were obtained to convert each reading to absorbed dose in water. Single arcs and typical four arc treatments were planned on XKnife and the dose to the isocentre verified in phantom with each usable detector.

RESULTS

The ion chamber showed no asymmetry, the stereotactic diodes exhibited 4% and the others 1-2%. Maximum directional dependence was 1% for the ion chamber and diamond and 7-20% for the diodes. Correction factors were calculated to account for this. Only the response of the diodes decreased with cumulative dose; the response of the other detectors remained constant. The ion chamber, electron diode and diamond measured the dose in single arcs to within 1.5% of calculation, in the 40 and 12.5 mm collimators. The photon diode was within 3.5 and 2.5% in the largest and smallest collimators, respectively.

CONCLUSION

A simple method of verification was developed. The ion chamber, the diamond and the electron diode were found to be the best detectors to verify the dose to the isocentre in a typical multiple arc treatment for collimators between 40 and 12.5 mm in diameter. The technique has been incorporated into a quality assurance programme, using the ion chamber and diamond, on a twice yearly basis.

Dose-response curves and tolerance doses for late functional changes in the normal rat brain after stereotactic radiosurgery evaluated by magnetic resonance imaging: influence of end points and follow-up time

Late reaction of normal tissue is still a limiting factor in radiotherapy and radiosurgery of patients with brain tumors. Few quantitative data in terms of dose-response curves are available. In the present study, 99 animals were irradiated stereotactically at the right frontal lobe using a linear accelerator and single doses between 26 and 50 Gy. The diameter of the spherical dose distribution was 4.7 mm (80% isodose). Dose-response curves for late changes in the normal brain at 20 months were measured using T1- and T2-weighted magnetic resonance imaging (MRI). The dependence of the dose-response curves on the follow-up time and the definition of the biological end point were determined. Tolerance doses were calculated at several effect probability levels and times after irradiation. The MRI changes were found to be dependent on dose and progressive in time. At 20 months, the tolerance doses at a 50% effect probability level were 39.6 +/- 1.0 Gy and 42.4 +/- 1.4 Gy for changes in T1- and T2-weighted images, respectively. These dose-response curves can be used for further quantitative investigations on the influence of various treatment parameters, such as the application of charged particles, radiopharmaceuticals or the variation of tissue oxygenation.

Quality control of the stereotactic radiosurgery procedure with the polymer-gel dosimetry

PURPOSE

To assess the entire geometric and dosimetric (relative) uncertainties of the radiosurgery procedure with the Leksell gamma knife.

MATERIALS AND METHODS

The entire Leksell gamma knife stereotactic radiosurgery treatment procedure was simulated with the use of a special water filled head phantom and polymer-gel dosimeter evaluated by nuclear magnetic resonance (NMR). A test vessel filled with the polymer-gel dosimeter was fixed in the head phantom. The phantom underwent stereotactic NMR imaging, treatment planning and then irradiation according to the treatment plan prepared exactly the same way as in the ordinary treatment procedure for a patient. The treatment plan was represented by one isocenter positioned approximately centrally in the head phantom. This procedure was subsequently repeated for all four collimators (4, 8, 14, 18mm) used on the Leksell gamma knife. Evaluation of dosimeters was performed on a Siemens EXPERT 1T NMR scanner. Dose profiles in X, Y and Z axes through the ellipsoidal shaped dose distribution were obtained to compare experimental results from the irradiated phantom with the treatment planning system calculations.

RESULTS

Reasonable agreement was observed between the treatment planning system calculations of relative dose distribution and the measured data. The maximum observed deviation in the spatial position between the center of the measured and calculated dose profiles was 0.6mm. The maximum observed difference in full width of half maximum between calculated and measured profiles was 1.2mm.

CONCLUSIONS

The use of polymer-gel dosimetry for a verification of stereotactic procedures has some unique advantages that can be summarized as follows: the dosimeter itself is tissue equivalent, three-dimensional dose distributions can be measured and the dosimeter allows simulation of the patient's procedures without any limitations.

Quantitative analysis of errors in fractionated stereotactic radiotherapy

Fractionated stereotactic radiotherapy (FSRT) offers a technique to minimize the absorbed dose to normal tissues; therefore, quality assurance is essential for these procedures. In this study, quality assurance for FSRT of 58 cases, between August 1995 and August 1997 are described, and the errors for each step and overall accuracy were estimated. Some of the important items for FSRT procedures are: accuracy in CT localization, transferred image distortion, laser alignment, isocentric accuracy of linear accelerator, head frame movement, portal verification, and various human errors. A geometric phantom, that has known coordinates was used to estimate the accuracy of CT localization. A treatment planning computer was used for checking the transferred image distortion. The mechanical isocenter standard (MIS), rectilinear phantom pointer: (RLPP), and laser target localizer frame (LTLF) were used for laser alignment and target coordinates setting. Head-frame stability check was performed by a depth confirmation helmet (DCH). A film test was done to check isocentric accuracy and portal verification. All measured data for the 58 patients were recorded and analyzed for each item. 4-MV x-rays from a linear accelerator, were used for FSRT, along with homemade circular cones with diameters from 20 to 70 mm (interval: 5 mm). The accuracy in CT localization was 1.2+/-0.5 mm. The isocentric accuracy of the linear accelerator, including laser alignment, was 0.5+/-0.2 mm. The reproducibility of the head frame was 1.1+/-0.6 mm. The overall accuracy was 1.7+/-0.7 mm, excluding human errors.

Risk analysis of Leksell Gamma Knife Model C with automatic positioning system

PURPOSE

This study was conducted to evaluate the decrease in risk from misadministration of the new Leksell Gamma Knife Model C with Automatic Positioning System compared with previous models.

METHODS AND MATERIALS

Elekta Instruments, A.B. of Stockholm has introduced a new computer-controlled Leksell Gamma Knife Model C which uses motor-driven trunnions to reposition the patient between isocenters (shots) without human intervention. Previous models required the operators to manually set coordinates from a printed list, permitting opportunities for coordinate transposition, incorrect helmet size, incorrect treatment times, missing shots, or repeated shots.

RESULTS

A risk analysis was conducted between craniotomy involving hospital admission and outpatient Gamma Knife radiosurgery. A report of the Institute of Medicine of the National Academies dated November 29, 1999 estimated that medical errors kill between 44,000 and 98,000 people each year in the United States. Another report from the National Nosocomial Infections Surveillance System estimates that 2.1 million nosocomial infections occur annually in the United States in acute care hospitals alone, with 31 million total admissions.

CONCLUSIONS

All medical procedures have attendant risks of morbidity and possibly mortality. Each patient should be counseled as to the risk of adverse effects as well as the likelihood of good results for alternative treatment strategies. This paper seeks to fill a gap in the existing medical literature, which has a paucity of data involving risk estimates for stereotactic radiosurgery.

Multi-isocenter stereotactic radiotherapy: implications for target dose distributions of systematic and random localization errors

PURPOSE

This investigation examined the effect of alignment and localization errors on dose distributions in stereotactic radiotherapy (SRT) with arced circular fields. In particular, it was desired to determine the effect of systematic and random localization errors on multi-isocenter treatments.

METHODS AND MATERIALS

A research version of the FastPlan system from Surgical Navigation Technologies was used to generate a series of SRT plans of varying complexity. These plans were used to examine the influence of random setup errors by recalculating dose distributions with successive setup errors convolved into the off-axis ratio data tables used in the dose calculation. The influence of systematic errors was investigated by displacing isocenters from their planned positions.

RESULTS

For single-isocenter plans, it is found that the influences of setup error are strongly dependent on the size of the target volume, with minimum doses decreasing most significantly with increasing random and systematic alignment error. For multi-isocenter plans, similar variations in target dose are encountered, with this result benefiting from the conventional method of prescribing to a lower isodose value for multi-isocenter treatments relative to single-isocenter treatments.

CONCLUSIONS

It is recommended that the systematic errors associated with target localization in SRT be tracked via a thorough quality assurance program, and that random setup errors be minimized by use of a sufficiently robust relocation system. These errors should also be accounted for by incorporating corrections into the treatment planning algorithm or, alternatively, by inclusion of sufficient margins in target definition.

System validation and work practice efficiency gains of a new localization method for stereotactic radiotherapy

The increased procedural demands of stereotactic localization techniques when compared with conventional treatment practices reduces machine efficiency, an outcome likely to be greatly magnified by the introduction of fractionation to stereotactic techniques. Currently in Australia and New Zealand there are no guidelines for the definition of efficiency. We sought to devise a system to simultaneously validate the accuracy and efficiency of the technique. The frameless relocation methods employed in the Medtronic Sofamor Danek (MSD) stereotactic radiotherapy (SRT) system were studied in the clinical setting. Accuracy has been determined according to the accumulation of errors throughout the planning and treatment process. The clinical demands of the system (staffing and resources) were analysed relative to conventional treatment approaches. Timing studies indicate a mean time of 19.7 min for treatment of a daily SRT fraction (4-5 arcs, single isocentre). Cost and staffing requirements are similar to those for conventional radiotherapy. It is concluded that with the system used, SRT is efficient for routine clinical implementation, with the level of efficiency increasing with increasing patient numbers. It is recommended that a common acceptance standard be developed to allow cross-institutional comparison of the clinical efficiency of new treatment techniques.

Three-dimensional conformal radiation treatment planning and delivery for low- and intermediate-grade gliomas

Three-Dimensional conformal radiation treatment (3D-CRT) planning and delivery is an external beam radiation therapy modality that has the general goal of conforming the shape of a prescribed dose volume to the shape of a 3-dimensional target volume, simultaneously limiting dose to critical normal structures. 3-Dimensional conformal therapy should include at least one volumetric imaging study of the patient. This image should be obtained in the treatment position for visualizing the target and normal anatomic structures that are potentially within the irradiated volume. Most often, computed tomography (CT) and/or magnetic resonance imaging (MRI) are used; however, recently, other imaging modalities such as functional MRI, MR spectroscopy, and positron emission tomography (PET) scans have been used to visualize the clinically relevant volumes. This article will address the clinically relevant issues with regard to low- and intermediate-grade gliomas and the role of 3D-CRT planning. Specific issues that will be addressed will include normal tissue tolerance, target definition, treatment field design in regard to isodose curves and dose-volume histograms, and immobilization.

An algorithm for stereotactic localization by computed tomography or magnetic resonance imaging

Stereotactic localization of an intracranial lesion by computed tomography or magnetic resonance imaging requires the use of a head frame that is fixed to the skull of the patient. To such head frames are attached either N-shaped or V-shaped localization rods. Because of patient positioning, the transverse imaging slices may not be parallel to the frame base; a coordinate transformation algorithm that takes this possibility into consideration is crucial. Here we propose such an algorithm for a head frame with V-shaped localization rods. Our algorithm determines the transformation matrix between the image coordinate system of a transverse image and the frame coordinate system. The determining procedure has three steps: (a) calculation of the oblique angles of a transverse image relative to the head frame and calculation of the image magnification factor; (b) determination of the coordinates of four central markers in both coordinate systems; and (c) determination of the 3 x 3 transformation matrix by using the coordinates of the four markers. This algorithm is robust in principle and is useful for improving the accuracy of localization.

A method for determining the alignment accuracy of the treatment table axis at an isocentric irradiation facility

At an isocentric irradiation facility, the rotation axis of the treatment table has to be accurately aligned in vertical orientation to the isocentre, which is usually marked by three perpendicular laser planes. In particular, high precision radiotherapy techniques, such as radiosurgery or intensity modulated radiotherapy, require a higher alignment accuracy of the table axis than routinely specified by the manufacturers. A simple and efficient method is presented to measure the direction and the size of the displacement of the table axis from the isocentre as marked by the lasers. In addition, the inclination of the table axis against the vertical direction can be determined. The measured displacement and inclination provide the required data to correct for possible misalignments of the treatment table axis and to maintain its alignment. Measurements were performed over a period of two years for a treatment table located at the German heavy ion therapy facility. The mean radial distance between the table axis and the isocentre was found to be 0.25 +/- 0.25 mm. The mean inclination of the table axis in the XZ- and YZ-planes was measured to be -0.03 +/- 0.02 degrees and -0.04 +/- 0.01 degrees, respectively. The measurements demonstrate the good alignment of the treatment table over the analysed time period. The described method can be applied to any isocentric irradiation facility, especially including isocentric linear accelerators used for radiosurgery or other high precision irradiation techniques.

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

We have developed a densitometric method for measuring the isocentric accuracy and the accuracy of marking the isocentre position for linear accelerator based radiosurgery with circular collimators and room lasers. Isocentric shots are used to determine the accuracy of marking the isocentre position with room lasers and star shots are used to determine the wobble of the gantry and table rotation movement, the effect of gantry sag, the stereotactic collimator alignment, and the minimal distance between gantry and table rotation axes. Since the method is based on densitometric measurements, beam spot stability is implicitly tested. The method developed is also suitable for quality assurance and has proved to be useful in optimizing isocentric accuracy. The method is simple to perform and only requires a film box and film scanner for instrumentation. Thus, the method has the potential to become widely available and may therefore be useful in standardizing the description of linear accelerator based radiosurgical systems.

Commissioning an image-guided localization system for radiotherapy

PURPOSE

To describe the design and commissioning of a system for the treatment of classes of tumors that require highly accurate target localization during a course of fractionated external-beam therapy. This system uses image-guided localization techniques in the linac vault to position patients being treated for cranial tumors using stereotactic radiotherapy, conformal radiotherapy, and intensity-modulated radiation therapy techniques. Design constraints included flexibility in the use of treatment-planning software, accuracy and precision of repeat localization, limits on the time and human resources needed to use the system, and ease of use.

METHODS AND MATERIALS

A commercially marketed, stereotactic radiotherapy system, based on a system designed at the University of Florida, Gainesville, was adapted for use at the University of Washington Medical Center. A stereo pair of cameras in the linac vault were used to detect the position and orientation of an array of fiducial markers that are attached to a patient's biteblock. The system was modified to allow the use of either a treatment-planning system designed for stereotactic treatments, or a general, three-dimensional radiation therapy planning program. Measurements of the precision and accuracy of the target localization, dose delivery, and patient positioning were made using a number of different jigs and devices. Procedures were developed for the safe and accurate clinical use of the system.

RESULTS

The accuracy of the target localization is comparable to that of other treatment-planning systems. Gantry sag, which cannot be improved, was measured to be 1.7 mm, which had the effect of broadening the dose distribution, as confirmed by a comparison of measurement and calculation. The accuracy of positioning a target point in the radiation field was 1.0 +/- 0.2 mm. The calibration procedure using the room-based lasers had an accuracy of 0.76 mm, and using a floor-based radiosurgery system it was 0.73 mm. Target localization error in a phantom was 0.64 +/- 0.77 mm. Errors in positioning due to couch rotation error were reduced using the system.

CONCLUSION

The system described has proven to have acceptable accuracy and precision for the clinical goals for which it was designed. It is robust in detecting errors, and it requires only a nominal increase in setup time and effort. Future work will focus on evaluating its suitability for use in the treatment of head-and-neck cancers not contained within the cranial vault.

Treatment planning algorithm corrections accounting for random setup uncertainties in fractionated stereotactic radiotherapy

A number of relocatable head fixation systems have become commercially available or developed in-house to perform fractionated stereotactic radiotherapy (SRT) treatment. The uncertainty usually quoted for the target repositioning in SRT is over 2 mm, more than twice that of stereotactic radiosurgery (SRS) systems. This setup uncertainty is usually accounted for at treatment planning by outlining extra target margins to form the planning target volume (PTV). It was, however, shown by Lo et al. [Int. J. Radiat. Oncol., Biol., Phys. 34, 1113-1119 (1996)] that these extra margins partly offset the radiobiological advantages of SRT. The present paper considers dose calculations in SRT and shows that the dose predictions could be made at least as accurate as in SRS with no extra margins required. It is shown that the dose distribution from SRT can be calculated using the same algorithms as in SRS, with the measured off-axis ratios (OARs) replaced by "effective" OARs. These are obtained by convolving the probability density distribution of the isocenter positions (assumed to be normal) and the original OARs. An additional output correction factor has also been introduced accounting for the isocenter dose reduction (2.4% for a 7 mm collimator) due to the OARs "blurring." Another correction factor accommodates for the reduced (by 1% for 6 MV beam) dose rate at the isocenter due to x-ray absorption in the relocatable mask. Mean dose profiles and the standard deviations of the dose (STD) were obtained through simulating SRT treatment by a combination of normally distributed isocenters. These dose distributions were compared with those calculated using the convolution approach. Agreement of the dose distributions was within 1%. Since standard deviation reduces with the number of fractions, N, as STD/square root(N), the planning predictions in fractionated stereotactic radiotherapy can be made more accurate than in SRS by increasing N and using "effective" OARs along with corrected dose output.

Setup verification in linac-based radiosurgery

A semi-automatic technique for the direct setup alignment of radiosurgical circular fields from an isocentric linac to treatment room laser cross-hairs is described. Alignment is achieved by acquiring images of the treatment room positioning laser cross-hairs superimposed on the radiosurgical circular field image. An alignment algorithm calculates the center of the radiosurgical field image as well as the intersection of the laser cross-hairs. This determines any alignment deviations and the information is then used to translate the radiosurgical collimator to its correct aligned position. Two detectors, each being sensitive to the lasers and ionizing radiation, were used to acquire the radiation/laser images. The first detector consists of a 0.3-mm-thick layer of photoconducting a-Se deposited on a 1.5-mm-thick copper plate and the second is film. The algorithm and detector system can detect deviations with a precision of approximately 0.04 mm. A device with gyroscopic degrees of freedom was built in order to firmly hold the detector at any orientation perpendicular to the radiosurgical beam axis. This device was used in conjunction with our alignment algorithm to quantify the isocentric sphere relative to the treatment room lasers over all gantry and couch angles used in dynamic stereotactic radiosurgery.

A simple method to verify in vivo the accuracy of target coordinates in linear accelerator radiosurgery

PURPOSE

A simple method that verifies the coincidence of the isocenter with the center of the target volume in radiosurgery treatment conditions is described. The accuracy is compared to that of accepted computerized procedures employing fiducial markers.

METHODS AND MATERIALS

The center of the beam is identified by a cylindrical localizer, fixed to the plate of the supplemental collimator, with a 2 x 50 mm tungsten rod coincident with the beam axis and is projected onto the x-ray portal verification films. Prior to irradiation, the coordinates of the intersection of the beams axes, which is in a known spatial relationship with the isocenter, are read directly on portal x-ray films and their coincidence with the coordinates set during patient positioning, is checked.

RESULTS

The mean displacement in AP, Lat, and Vert coordinates respectively, over 84 patients, between the coordinates calculated by the computerized procedure employing fiducial markers and the coordinates calculated by using the rulers was 0.3 +/- 0.4 mm.

CONCLUSIONS

From the results obtained with the two methods we can conclude that rulers method can be used as a fast indirect control of the position of the radiation isocenter. Moreover, the dimensions of the radiation field and the correct alignment of the tertiary circular collimator can be also documented.

IRLED-based patient localization for linac radiosurgery

PURPOSE

Currently, precise stereotactic radiosurgery delivery is possible with the Gamma Knife or floor-stand linear accelerator (linac) systems. Couch-mounted linac radiosurgery systems, while less expensive and more flexible than other radiosurgery delivery systems, have not demonstrated a comparable level of precision. This article reports on the development and testing of an optically guided positioning system designed to improve the precision of patient localization in couch-mounted linac radiosurgery systems.

METHODS AND MATERIALS

The optically guided positioning system relies on detection of infrared light-emitting diodes (IRLEDs) attached to a standard target positioner. The IRLEDs are monitored by a commercially available camera system that is interfaced to a personal computer. An IRLED reference is established at the center of stereotactic space, and the computer reports the current position of the IRLEDs relative to this reference position. Using this readout from the computer, the correct stereotactic coordinate can be set directly.

RESULTS

Bench testing was performed to compare the accuracy of the optically guided system with that of a floor-stand system, that can be considered an absolute reference. This testing showed that coordinate localization using the IRLED system to track translations agreed with the absolute to within 0.1 +/- 0.1 mm. As rotations for noncoplanar couch angles were included, the inaccuracy was increased to 0.2 +/- 0.1 mm.

CONCLUSIONS

IRLED technology improves the accuracy of patient localization relative to the linac isocenter in comparison with conventional couch-mounted systems. Further, the patient's position can be monitored in real time as the couch is rotated for all treatment angles. Thus, any errors introduced by couch inaccuracies can be detected and corrected.

[Technical evolution of irradiation in stereotactic conditions: dose fractionation]

The development of non-invasive head fixation systems, allowing 3D determination of the target coordinates, has lead to the increased use of fractionated stereotactic irradiation. These systems have been checked for accuracy and the mean precision of repositioning has been evaluated to +/- 1 mm. With the mean geometrical accuracy set at +/- 1 mm, a 2 mm safety margin is usually added to the clinical target volume in order to define the planning target volume. Quality assurance procedures must conform to the required precision of the technique while remaining realistic in day-to-day use relative to planned conventional treatments. Biologically different from single dose irradiation, the fractionated stereotactic irradiation completes the range of techniques used in the treatment of intra-cerebral lesions.

Dosimetry studies with TLDs for stereotactic radiation techniques for intraocular tumours

Between March 1993 and January 1997, stereotactic radiation techniques were used to irradiate 66 intraocular tumour patients with the Gamma Knife (Leksell Gamma Knife, model B unit) at the University of Vienna, Austria. This study investigates the dosimetry for stereotactic irradiation of ocular structures. For the dosimetry program KULA 4.4, Gamma Knife stereotactic irradiation of the eye represents an extreme frontal skull position. In addition, irradiation of the eye may be performed in the usual supine position in exceptional cases only. With the patient in the prone position, the dose planning program has to calculate with a significantly large number of single-beam extrapolations. In our first experiment we measured the isocentre dose for eight different gamma-angle positions, both in prone and supine positions, using TLD measurements in an Alderson head phantom. We found a maximum deviation of +/- 1.6% using these individually calibrated TLDs. In the second experiment we examined the dose cross profiles for the two most frequently used treatment positions (supine position, gamma = 65 degrees, and prone position, gamma = 140 degrees). For this purpose we implanted a specially designed TLD array into the orbit of a human cadaver head. We found excellent agreement of the dose values measured for the isocentre as well as the posterior part of the eye with orbit with deviations of less than -2.7%. However, for the anterior part of the eye, deviations between computer-generated calculations and the TLD measurements were found to range up to -30%. These differences were noticed both for supine and prone positions. For the Gamma Knife stereotactic irradiation of ocular tumours or pathologies, precautions should be taken to avoid significant underdosage in the anterior part of the radiation field.

Quality assurance system to correct for errors arising from couch rotation in linac-based stereotactic radiosurgery

PURPOSE

The purpose of this project was the development of a quality assurance (QA) system that would provide geographically accurate targeting for linac-based stereotactic radiosurgery (LBSR).

METHODS AND MATERIALS

The key component of our QA system is a novel device (Alignment Tool) for expedient measurement of gantry and treatment table excursions (wobble) during rotation. The Alignment Tool replaces the familiar pencil-shaped pointers with a ball pointer that is used with the field light of the accelerator to indicate alignment of beam and target. Wobble is measured prior to each patient treatment and analyzed together with the BRW coordinates of the target by a spreadsheet. The corrections required to compensate for any imprecisions are identified, and a printout generated indicating the floor stand coordinates for each couch angle used to place the target at isocenter.

RESULTS

The Alignment Tool has an inherent accuracy of measurement better than 0.1 mm. The overall targeting error of our QA method, found by evaluating 177 target simulator films of 55 foci in 40 randomly selected patients, was 0.47 +/- 0.23 mm. The Alignment Tool was also valuable during installation of the floor stand and a supplemental collimator for the accelerator.

CONCLUSIONS

The QA procedure described allows accurate targeting in LBSR, even when couch rotation is imprecise. The Alignment Tool can facilitate the installation of any stereotactic irradiation system, and can be useful for annual QA checks as well as in the installation and commissioning of new accelerators.

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).

Verification of radiosurgery target point alignment with an electronic portal imaging device (EPID)

A procedure has been developed using an electronic portal imaging device (EPID) to verify that the center of a patient's lesion is aligned with the center of a treatment cone prior to treatment in a linac-based stereotactic radiosurgery procedure. The coordinates of the lesion center are set on the Brown-Roberts-Wells phantom base using a target simulator. A 3 mm tungsten ball, mounted on the target simulator, is used as the reference point for the planned isocenter. The target simulator is then attached to an adapter mounted on the linac couch, and an EPID image of the simulated target is acquired. The center of the circular-shaped radiation field is calculated from the centroid of the segmented EPID image, and the center of the tungsten ball is identified by an automated computer search algorithm. A summation filter is used to find the position of the lowest radiation intensity coincident with the center of the ball. The alignment error is defined as the difference between the center of the radiation field and the center of the ball. The accuracy of this method was tested and found to be within 0.2 mm. The advantage of the EPID-based procedure is that it can give quantitative offset values quickly for immediate readjustment. We have found that the method is also a convenient tool for testing room laser alignment and the accuracy of the treatment cones.

Analyses of multi-irradiation film for system alignments in stereotactic radiotherapy (SRT) and radiosurgery (SRS)

In stereotactic radiosurgery, a seven-irradiation film was used to define any discrepancy between the beam and target centres. A mathematical model based on the linac alignment and target set-up was developed to diagnose the discrepancies of the seven-irradiation film between the beam and simulation target centres. From the measured data of the multi-irradiation film, this mathematical model leads to five parameters in seven equations. Twin computer codes were employed to solve the five parameters from the seven equations. By feeding the discrepancy data into the two computer codes, the sources of the target-to-beam discrepancy were revealed. From these decoded sources, the target coordinates were adjusted and then the seven-irradiation film procedure was repeated. This discrepancy thus obtained was found to be drastically reduced. Some decoded parameters were consistently verified by direct measurements. This demonstrates that the present mathematical model and computer code do reveal the causes of the target-to-beam misalignment and gantry sag. In a further effort to test the feasibility of the mathematical model and the computer codes, the target's lateral coordinate was deliberately offset by 1.5 mm and then another seven-irradiation film was taken. By inserting these discrepancies into the computer codes, it was found that the deviation was consistent with the intentional offset. In addition, the mathematical model and computer codes are applicable to any multi-irradiation technique.

A system for stereotactic irradiation and magnetic resonance evaluations in the rat brain

PURPOSE

A stereotactic fixation and localization device developed for small animal stereotactic radiosurgery is described.

METHODS AND MATERIALS

Irradiated volumes of spherical shape down to 1.7 mm in diameter at the 80% isodose level are attainable. The fixation device can also be used for magnetic resonance imaging (MRI) and allows target localization during magnetic resonance (MR) image content measurement. The capabilities of the entire system were investigated using a phantom that permitted measurement and localization of the three-dimensional dose distribution. Localization of target isocenter coordinates in MR images was also checked with the phantom.

RESULTS

An overall spatial error of about 1 mm for subsequent stereotactic irradiation was obtained.

CONCLUSIONS

The accuracy of the fixation and localization techniques is adequate to investigate radiation-induced changes in the rat brain.