Winston-Lutz Test Research Articles

DeepWL: Robust EPID based Winston-Lutz analysis using deep learning, synthetic image generation and optical path-tracing.

Radiation therapy requires clinical linear accelerators to be mechanically and dosimetrically calibrated to a high standard. One important quality assurance test is the Winston-Lutz test which localises the radiation isocentre of the linac. In the current work we demonstrate a novel method of analysing EPID based Winston-Lutz QA images using a deep learning model trained only on synthetic image data. In addition, we propose a novel method of generating the synthetic WL images and associated 'ground-truth' masks using an optical path-tracing engine to 'fake' mega-voltage EPID images. The model called DeepWL was trained on 1500 synthetic WL images using data augmentation techniques for 180 epochs. The model was built using Keras with a TensorFlow backend on an Intel Core i5-6500T CPU and trained in approximately 15h. DeepWL was shown to produce ball bearing and multi-leaf collimator field segmentations with a mean dice coefficient of 0.964 and 0.994 respectively on previously unseen synthetic testing data. When DeepWL was applied to WL data measured on an EPID, the predicted mean displacements were shown to be statistically similar to the Canny Edge detection method. However, the DeepWL predictions for the ball bearing locations were shown to correlate better with manual annotations compared with the Canny edge detection algorithm. DeepWL was demonstrated to analyse Winston-Lutz images with an accuracy suitable for routine linac quality assurance with some statistical evidence that it may outperform Canny Edge detection methods in terms of segmentation robustness and the resultant displacement predictions.

Towards zero radiation isocentre size: minimising radiation beam isocentricity on Elekta linear accelerators by means of optimising look-up tables.

The most important geometric characteristics of SRS/SBRT treatments are precise target localisation and precise aiming of the radiation beam at the target. The AAPM-RSS Medical Physics Practice Guideline 9.a. for SRS/SBRT recommends that the radiation isocentricity (i.e. beam deviation from the isocentre) should not exceed 1mm for SRS and 1.5mm for SBRT. Minimising the beam deviations from the treatment target, largely due to the gantry sag, can improve the accuracy of radiosurgery and stereotactic treatments and commonly beam steering parameters are optimised to achieve this objective. This study aims to investigate, as a proof of concept, if it is possible to eliminate the beam deviations on Elekta linear accelerators altogether by optimising gantry angle dependent beam steering parameters, as stored in look-up tables. The investigation used the EPID-based Winston-Lutz test at 13 gantry angles separated every 30° (from - 180° to + 180°). Elekta linacs have two look-up tables that can be customised explicitly for radial beam angle and transverse beam position. Modifications of the radial look-up table were limited by the radial beam asymmetry inhibit of more than 5%, as measured by the linac in-built ionisation chamber. Therefore, only small radial beam deviation reductions of 0.1mm were achieved (on average from 0.37 to 0.26mm) while radial beam symmetry changed significantly by up to ± 7%, depending on the gantry angle as measured by the IC Profiler™. The optimised transverse look-up table resulted in reduction of transverse beam deviations to almost zero (on average from 0.20 to 0.03mm), however, that changed the transverse beam symmetry by almost a constant value of 1%, as measured by the IC Profiler™. Ideally, two additional look-up tables are needed for effective beam steering, one for radial beam position and one for transverse beam angle. Four look-up tables in total would enable customising beam centre position and beam symmetry at any gantry angle that would minimize radiation isocentre size without compromising beam symmetry.

Development of a System for Evaluating the In-room-laser Alignment Including the Horizontality and Verticality Integrated the Light/Radiation Field Coincidence Test and the Winston-Lutz Test

The in-room laser which is used for patient positioning in radiotherapy is generally projected on the radiation isocenter determined by the Winston-Lutz test and so on. In this study, a couch-mounted verification device was developed that could evaluate all in-room lasers' alignment including the horizontality and verticality at one time. The device has the function to perform the light/radiation field coincidence test and the Winston-Lutz test at the same time. The aim of this report was to introduce the verification procedure for two tests, using the newly developed software and device, and to present the tuning flow of the in-room laser. Moreover, the analysis accuracy of the developed software was evaluated in comparison with commercial software. First, the light/radiation field was evaluated by using tungsten markers on the central surface of the device. Next, after aligning the long-carved lines on the front and sides of the device with the in-room lasers, the Winston-Lutz test was carried out by using the tungsten sphere in the center of the device. The acquired images were collectively analyzed using the developed software equipped with the reporting function. Additionally, the result of this Winston-Lutz test was compared with the result from commercial software. A series of the light/radiation field coincidence test and the Winston-Lutz test were analyzed using the developed device and software. The results could be easily confirmed using the reporting function of the software. Regarding the result of the Winston-Lutz test, most of the analysis differences between the developed software and commercially available software were within the pixel size (0.22 mm). Since the accuracy of the radiation field affects the result of the Winston-Lutz test, the presented procedure of performing the light/radiation coincidence field test in advance facilitates the interpretation of the error of the Winston-Lutz test. Based on the results of the Winston-Lutz test, we were able to demonstrate the tuning flow of all in-room lasers including the horizontality and verticality by using the developed device. We have developed a couch-mounted verification device and software that can evaluate the light/radiation coincidence field test and the alignment including the horizontality and verticality of the in-room laser used for patient positioning in radiotherapy, and reported its usefulness. The analysis accuracy of the developed software was comparable to that of commercially available software. The use of this device and the developed software would contribute to not only the efficiency of adjusting all in-room lasers' alignment including the horizontality and verticality but also reflect accurately the result of the Winston-Lutz test.

Simultaneous Optimization of Radiation-Imaging Coincidence for a Multi-Energy Linac.

Introduction:Medical physics guidelines stress the importance of radiation-imaging coincidence, especially for stereotactic treatments. However, multi-energy linear accelerators may only allow a single imaging isocenter. A procedure was developed to simultaneously optimize radiation-imaging isocenter coincidence for all linac photon energies on a Versa HD.Materials and Methods:First, the radiation beam center of each energy was adjusted to match the collimator rotation axis using a novel method that combined ion chamber measurements with a modified Winston-Lutz (WL) test using images only at gantry, couch, and collimator angles of 0°. With all energies properly steered, an 8-field WL test was performed to determine average linac isocenter position across all energies, gantry, and collimator angles. Lasers and the kV imaging isocenter were calibrated to the average linac isocenter of all photon energies. Finally, A 12-field WL test consisting of gantry, couch, and collimator rotations was used to adjust the couch rotation axis to the average linac isocenter, thereby minimizing overall radiation-imaging isocentricity of the system.Results:Using this method, the beam centers were calibrated within 0.10 mm of collimator rotation axis, and linac isocenter coincidence was within 0.20 mm for all energies. Couch isocenter coincidence was adjusted within 0.20 mm of average linac isocenter. Average radiation-imaging isocentricity for all energies was 0.89 mm (0.80–0.98 mm) for a single imaging isocenter.Conclusion:This work provides a method to adjust radiation-imaging coincidence within 1.0 mm for all energies on Elekta's Versa HD.

Portal dosimetry scripting application programming interface (PDSAPI) for Winston-Lutz test employing ceramic balls.

PurposeStereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) treatments require a high degree of accuracy. Mechanical, imaging, and radiation isocenter coincidence is especially important. As a common method, the Winston‐Lutz (WL) test plays an important role. However, weekly or daily WL test can be very time consuming. We developed novel methods using Portal Dosimetry Scripting Application Programming Interface (PDSAPI) to facilitate the test as well as documentation.MethodsWinston‐Lutz PDSAPI was developed and tested on our routine weekly WL imaging. The results were compared against two commercially available software RIT (Radiological Imaging Technology, Colorado Springs, CO) and DoseLab (Varian Medical Systems, Inc. Palo Alto, CA). Two manual methods that served as ground truth were used to verify PDSAPI results. Twenty WL test image data sets (10 fields per tests, and 200 images in total) were analyzed by these five methods in this report.ResultsMore than 99.5% of WL PDSAPI 1D shifts agreed with each of four other methods within ±0.33 mm, which is roughly the pixel width of a‐Si 1200 portal imager when source to imager distance (SID) is at 100 cm. 1D shifts agreement for ±0.22 mm and 0.11 mm were 96% and 63%, respectively. Same trend was observed for 2D displacement.ConclusionsWinston‐Lutz PDSAPI delivers similar accuracy as two commercial applications for WL test. This new application can save time spent transferring data and has the potential to implement daily WL test with reasonable test time. It also provides the data storage capability, and enables easy access to imaging and shift data.

Evaluation of the RUBY modular QA phantom for planar and non-coplanar VMAT and stereotactic radiations.

PurposeThis study evaluates the clinical use of the RUBY modular QA phantom for linac QA to validate the integrity of IGRT workflows including the congruence of machine isocenter, imaging isocenter, and room lasers. The results have been benchmarked against those obtained with widely used systems. Additionally, the RUBY phantom has been implemented to perform system QA (End‐to‐End testing) from imaging to radiation for IGRT‐based VMAT and stereotactic radiations at an Elekta Synergy linac.Material and MethodsThe daily check of IGRT workflow was performed using the RUBY phantom, the Penta‐Guide, and the STEEV phantom. Furthermore, Winston–Lutz tests was carried out with the RUBY phantom and a ball‐bearing phantom to determine the offsets and the diameters of the isospheres of gantry, collimator, and couch rotations, with respect to the room lasers and kV‐imaging isocenter. System QA was performed with the RUBY phantom and STEEV phantom for eight VMAT treatment plans. Additionally, the visibility of the embedded objects within these phantoms in the images and the results of CT and MR image fusions were evaluated.ResultsAll systems used for daily QA of IGRT workflows show comparable results. Calculated shifts based on CBCT imaging agree within 1 mm to the expected values. The results of the Winston–Lutz test based on kV imaging (2D planar and CBCT) or room lasers are consistent regardless of the system tested. The point dose values in the RUBY phantom agree to the expected values calculated using algorithms in Masterplan and Monte Carlo engine in Monaco within 3% of the clinical acceptance criteria.ConclusionAll the systems evaluated in this study yielded comparable results in terms of linac QA and system QA procedures. A system QA protocol has been derived using the RUBY phantom to check the IGRT‐based VMAT and stereotactic radiations workflow at an Elekta Synergy linac.

An integrated quality assurance phantom for frameless single-isocenter multitarget stereotactic radiosurgery

Brain stereotactic radiosurgery (SRS) treatments require multiple quality assurance (QA) procedures to ensure accurate and precise treatment delivery. As single-isocenter multitarget SRS treatments become more popular, the quantification of off-axis accuracy of the linear accelerator is crucial. In this study, a novel brain SRS integrated phantom was developed and validated to enable SRS QA with a single phantom to facilitate implementation of a frameless single-isocenter, multitarget SRS program. This phantom combines the independent verification of each positioning system, the Winston-Lutz, off-axis accuracy evaluation (i.e. off-axis Winston-Lutz), and the dosimetric accuracy utilizing both point dose measurements as well as film measurement, without moving the phantom. A novel 3D printed phantom, coined OneIso, was designed with a movable insert which can switch between the Winston-Lutz test target and dose measurement without moving the phantom itself. For dose verification, ten brain SRS clinical treatment plans with 10 MV flattening-filter-free beams were delivered on a Varian TrueBeam with a high-definition multileaf collimator (HD-MLC). Radiochromic film and pinpoint ion chamber comparison measurements were made between the OneIso and solid water (SW) phantom setups. For the off-axis Winston-Lutz measurements, a row of off-axis ball bearings (BBs) was integrated into the OneIso. To quantify the spatial accuracy versus distance from the isocenter, two-dimensional displacements were calculated between the planned and delivered BB locations relative to their respective MLC defined field border. OneIso and the SW phantoms agree within 1%, for both film and point dose measurements. OneIso identified a reduction in spatial accuracy further away from the isocenter. Differences increased as distance from the isocenter increased, exceeding recommended SRS accuracy tolerances at 7 cm away from the isocenter. OneIso provides a streamlined, single-setup workflow for single-isocenter multitarget frameless linac-based SRS QA. Additionally, with the ability to quantify off-axis spatial discrepancies, we can determine limitations on the maximum distance between targets to ensure a single-isocenter multitarget SRS program meets recommended guidelines.

The potential of an optical surface tracking system in non-coplanar single isocenter treatments of multiple brain metastases.

To evaluate the accuracy of a commercial optical surface tracking (OST) system and to demonstrate how it can be implemented to monitor patient positioning during non‐coplanar single isocenter stereotactic treatments of brain metastases. A 3‐camera OST system was used (Catalyst HD™, C‐RAD) on a TruebeamSTx with a 6DoF couch. The setup accuracy and agreement between the OST system, and CBCT and kV‐MV imaging at couch angles 0° and 270°, respectively, were examined. Film measurements at 3 depths in the Rando‐Alderson phantom were performed using a single isocenter non‐coplanar VMAT plan containing 4 brain lesions. Setup of the phantom was performed with CBCT at couch 0° and subsequently monitored by OST at other couch angles. Setup data for 7 volunteers were collected to evaluate the accuracy and reproducibility of the OST system at couch angles 0°, 45°, 90°, 315°, and 270°. These results were also correlated to the couch rotation offsets obtained by a Winston‐Lutz (WL) test. The Rando‐Alderson phantom, as well as volunteers, were fixated using open face masks (Orfit). For repeated tests with the Rando‐Alderson phantom, deviations between rotational and translational isocenter corrections for CBCT and OST systems are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the film and TPS doses in the center of the 4 lesions were −1.2%, −0.1%, −0.0%, and −1.9%. Local gamma evaluation criteria of 2%/2 mm and 3%/1 mm yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for the 4 lesions. Regarding the volunteers, the mean translational and rotational isocenter shift values were (0.24 ± 0.09) mm and (0.15 ± 0.07) degrees. Largest isocenter shifts were found for couch angles 45˚ and 90˚, confirmed by WL couch rotation offsets. Patient monitoring during non‐coplanar VMAT treatments of brain metastases is feasible with submillimeter accuracy.

Comparison of three different phantoms used for Winston-Lutz test with Artiscan software.

One of the most important test in every quality assurances process of medical linear accelerators is the Winston-Lutz test, allowing an evaluation of the treatment isocentre in the light of uncertainty of the position of the collimator, the gantry and the couch. The purpose of this work was analysis of the results of the Winston-Lutz test performed with three different phantoms for two different accelerators. Measurements were performed on two Varian machines: TrueBeam equipped with aS1200 EPID and TrueBeam equipped with aS1000 EPID. During the study three different phantoms dedicated for verification of the radiation isocentre were used: PTW Isoball, AQUILAB Isocentre Phantom and Varian Isocentre Cube. Analysis of the DICOM images was performed in Artiscan software. For TrueBeam with as1200 EPID, gantry MV isocentre was about 0.18 mm larger for Varian Isocentre Cube than for two other phantoms used in this study. The largest variability of this parameter was observed for the couch. The results differed to 1.16 mm. For TrueBeam with as1000 EPID, results for collimator isocentre with PTW Isoball phantom were about 0.10 mm larger than for two other phantoms. For the gantry, results obtained with Varian Isocentre Cube were 0.21 mm larger. The obtained results for all three phantoms are within the accepted tolerance range. The largest differences were observed for treatment couch, which may be related to the phantom mobility during couch movement.

Comprehensive radiation and imaging isocenter verification using NIPAM kV‐CBCT dosimetry

To develop and demonstrate a comprehensive method to directly measure radiation isocenter uncertainty and coincidence with the cone-beam computed tomography (kV-CBCT) imaging coordinate system that can be carried out within a typical quality assurance (QA) time slot. An N-isopropylacrylamide (NIPAM) three-dimensional (3D) dosimeter for which dose is observed as increased electron density in kV-CBCT is irradiated at eight couch/gantry combinations which enter the dosimeter at unique orientations. One to three CBCTs are immediately acquired, radiation profile is detected per beam, and displacement from imaging isocenter is quantified. We performed this test using a 5mm diameter MLC field, and 7.5 and 4mm diameter cones, delivering approximately 16Gy per beam. CBCT settings were 1035-4050mAs, 80-125kVs, smooth filter, 1mm slice thickness. The two-dimensional (2D) displacement of each beam from the imaging isocenter was measured within the planning system, and Matlab code developed in house was used to quantify relevant parameters based on the actual beam geometry. Detectability of the dose profile in the CBCT was quantified as the contrast-to-noise ratio (CNR) of the irradiated high-dose regions relative to the surrounding background signal. Our results were compared to results determined by the traditional Winston-Lutz test, film-based "star shots," and the vendor provided machine performance check (MPC). The ability to detect alignment errors was demonstrated by repeating the test after applying a 0.5mm shift to the MLCs in the direction of leaf travel. In addition to radiation isocenter and coincidence with CBCT origin, the analysis also calculated the actual gantry and couch angles per beam. Setup, MV irradiation, and CBCT readout were carried out within 38min. After subtracting the background signal from the pre-CBCT, the CNR of the dosimeter signal from the irradiation with the MLCs (125kVp, 1035mAs, n=3), 7.5mm cone (125kVp, 1035mAs, n=3), and 4mm cone (80kVp, 4050mAs, n=1) was 5.4, 5.9, and 2.9, respectively. The minimum radius that encompassed all beams calculated using the automated analysis was 0.38, 0.48, and 0.44mm for the MLCs, 7.5mm cone, and 4mm cone, respectively. When determined manually, these values were slightly decreased at 0.28, 0.41, and 0.40mm. For comparison, traditional Winston-Lutz test with MLCs and MPC measured the 3D isocenter radius to be 0.24mm. Lastly, when a 0.5mm shift to the MLCs was applied, the smallest radius that intersected all beams increased from 0.38 to 0.90mm. The mean difference from expected value for gantry angle was 0.19±0.29°, 0.17±0.23°, and 0.12±0.14° for the MLCs, 7.5mm cone, and 4mm cone, respectively. The mean difference from expected for couch angle was -0.07±0.28°, -0.08±0.66°, and 0.04±0.25°. This work demonstrated the feasibility of a comprehensive isocenter verification using a NIPAM dosimeter with sub-mm accuracy which incorporates evaluation of coincidence with imaging coordinate system, and may be applicable to all SRS cones as well as MLCs.

12. MLC QA and Winston-Lutz Test by Electric Portal Imaging Device Using ImageJ

12. MLC QA and Winston-Lutz Test by Electric Portal Imaging Device Using ImageJ

A new concept for verifying the isocentric alignment of the proton-rotational gantry for radiation control.

The purpose of this study was to introduce the modified Winston-Lutz (mWL) test and to evaluate its feasibility. This is a new method to completely absorb the proton beam around the isocenter inside a phantom for radiation control. The mWL test was performed using a 14-cm-diameter acrylic Lucy 3D QA Phantom for a passive-scattering proton beam gantry. The energy of the unmodulated proton beam was adjusted such that the residual range was < 130mm, and the energy of the proton beam was completely lost around the isocenter. The radiation field was formed with a multi-leaf collimator at 8.6 × 8.6mm2 in the isocenter plane. The phantom was loaded with a 4-mm-diameter tungsten ball, and the EBT3 was set up at the isocenter. The proton beam was irradiated at gantry angles with 45° steps, and the isocenter deviation of the proton beam was measured and subsequently analyzed. Although the radiation field penumbra was blurred under the influence of scattered radiation due to placement in the phantom compared to the traditional WL test placed in the air, evaluation of the beam axis accuracy was possible. The results confirmed that the maximum total displacement was less than 0.9mm, and the specifications of the device were met. The mWL test is feasible and effective to reduce the building activation in proton beam treatment facilities. Thus, it can be considered a useful method that sufficiently satisfies the shielding calculation conditions.

A sparse orthogonal collimator for small animal intensity-modulated radiation therapy. Part II: hardware development and commissioning.

A dose-modulation device for small animal radiotherapy is required to use clinically analogous treatment techniques, which will likely increase the translatability of preclinical research results. Because the clinically used multileaf collimator (MLC) is impractical for miniaturization, we have developed a simpler, better-suited sparse orthogonal collimator (SOC) for delivering small animal intensity-modulated radiation therapy (IMRT) using a rectangular aperture optimization (RAO) treatment planning system. The SOC system was modeled in computer-aided design software and fabricated with machined tungsten leaves and three-dimensional (3D) printed leaf housing. A graphical user interface was developed for controlling and calibrating the SOC leaves, which are driven by Arduino-controlled stepper motors. A Winston-Lutz test was performed to assess mechanical alignment, and abutting field and grid dose patterns were created to analyze intra- and intercalibration leaf positioning error. Leaf transmission and penumbra were measured over the full range of gantry angles and leaf positions, respectively. Three SOC test plans were delivered, and film measurements were compared to the intended dose distributions. The differences in maximum, mean, and minimum, as well as pixelwise absolute dose differences, were compared for each structure, and a gamma analysis was performed for the target structures using criteria of 4% dose difference and 0.3mm distance to agreement. The Winston-Lutz test revealed maximum directional offsets between the SOC and primary collimator axes of 0.53mm at 0° and 0.68mm over the full 360°. Upper and lower abutting field patterns had maximum dose deviations of 18.8±3.1% and 15.5±2.9%, respectively, and grid patterns showed intra- and intercalibration repeatability of 93% and 91%, respectively. Extremely low midleaf (0.15±0.05%) and interleaf (0.27±0.22%) transmission was measured, with no significant rotational variation. The average penumbra was ~0.8mm for all leaves at field center, with a range of 0.17mm for all leaf positions. A highly concave test plan was delivered with a~95% gamma analysis pass rate, and a realistic mouse phantom liver irradiation plan achieved a pass rate of ~98%. A highly complex dose distribution was also created with 551 SOC apertures averaging 2.4mm in size. A sparse orthogonal collimator was developed and commissioned, with promising preliminary dosimetry results. The SOC design, with its limited moving components and high dose-modulation resolution, is ideal for delivering high-quality small animal IMRT with our RAO-based treatment planning system.

Feasibility of virtual starshot analysis providing submillimeter radiation isocenter accuracy: A long-term multi-institutional analysis.

PurposeWe developed a technique to calculate the offset between room lasers and the radiation isocenter using a digital Winston–Lutz (WL) test with a starshot technique. We have performed isocenter localization quality assurance (QA) with submillimeter accuracy for a long period. Here we evaluated the feasibility and accuracy of this virtual starshot (VS) analysis for isocenter localization QA.MethodsA 6‐MV photon beam with a square multileaf collimator field was used to irradiate a WL sphere positioned at the intersection of the room lasers. Images were acquired using an electronic portal imaging device. A four‐field WL test was performed, and the path of each beam was calculated from the offset between the beam and sphere. Virtual starshot analysis was used to analyze the radiation isocenter, which calculates the center of the beam paths by using a least‐squares method, similar to the starshot analysis. Then, eight coplanar and 12 noncoplanar beams were irradiated to evaluate isocenter localization accuracy.ResultsSeveral VS analyses, using different WL spheres, were performed at three institutions, and the calculated accuracies were within 0.1 mm at all institutions. Long‐term analysis showed that the isocenter localization accuracy was appropriately managed with three‐dimensional accuracy within ± 0.5 mm for 90 months after the first laser adjustments. The offset between each beam and the room laser was within 0.6 mm and within 1.0 mm for eight coplanar and 12 noncoplanar beams, respectively, for 90 months. Cone‐beam computed tomography images, acquired after verification beams, showed that the offset between the radiation isocenter and the imaging center was within 0.66 mm for 90 months. The isocenter localization accuracy within 1 mm was kept for long period at other four institutions.ConclusionsLong‐term analysis showed the feasibility of VS analysis for isocenter localization QA, including room laser re‐alignment, noncoplanar irradiation verification, and image guidance accuracy.

A Daily QA Phantom for Linear Accelerator with Image-Guided Radiosurgery Capability

Image guided radiosurgery and stereotactic body radiotherapy (IG-SRS/SBRT) have been widely adopted on linear accelerators. Ideally, a daily quality assurance program on such machines should provide the targeting accuracy from image guidance quantitatively and efficiently. A suitable phantom for such test requires sufficient landmarks for robust online image registration and a high contrast ball bearing (BB) to provide clear images for Winston Lutz (WL) test. Such phantoms, however, are not widely available. In this study, we investigated a prototype daily QA phantom that allows all major components of the IG-SRS/SBRT process to be tested. The phantom is a 12 cm cube with 3 pairs of AL rods at 3 corners, and 4 embedded AL ceramic BBs. The BB in the center of phantom is 5/16 inch in diameter, while the others are ¼ inch in size. On the treatment table, the phantom sits atop a holder with build-in pitch, roll, and pitch. The holder can be index to the couch at a preset starting position. A high resolution planning CT was acquired of the phantom on a level surface. A plan was then generated with isocenter placed precisely in the central BB of the phantom. The plan was delivered on a medical linear accelerator and a radiosurgery system, using cone beam CT (CBCT) and stereoscopic x-ray system for image guidance, respectively. Both imaging systems provide 6DoF deviations that were subsequently applied to the treatment couch. Afterwards, verification imaging was acquired with an on-board imaging (OBI) KV/MV pair on a medical linear accelerator and a 2nd pair of stereotactic x-ray images on a radiosurgery system, and WL tests were performed to measure the agreement between central BB and machine isocenter. The plan was delivered 12 times each on the two machines. The initial correction, deviation from verification imaging, and WL test results (μ ± σ) are provided in the table below, where R represents 3D vector length.The verification imaging indicated that all 6DoF corrections were applied correctly, and WL test showed that both systems are able to localize well within 1 mm accuracy on a phantom. The phantom incorporates necessary features to enable daily QA with the goal of simulating IG-SRS/SBRT workflow and quantitatively evaluating localization accuracy. Accuracy determined from this daily QA procedure should represent the upper limit of targeting accuracy for ideal conditions.Abstract 3613; Table 1Medical Linear Accelerator (n=12)Radiosurgery System (n=12)CorrectionVerificationWL testCorrectionVerificationWL testVRT (mm)-8.0±0.10.0±0.10.02±0.04-8.6±0.30.0±0.10.30±0.15LNG (mm)6.8±0.10.2±0.20.25±0.087.0±0.40.0±0.10.18±0.10LAT (mm)6.3±0.2-0.1±0.2-0.20±0.146.0±0.40.1±0.10.20±0.13R (mm)12.2±0.20.4±0.10.34±0.0912.6±0.40.2±0.10.45±0.11Yaw (o)1.4±0.1-0.2±0.1-1.4±0.20.1±0.1-Pitch (o)-1.8±0.10.1±0.1--1.7±0.10.0±0.1-Roll (o)-1.0±0.10.1±0.1--0.9±0.10.0±0.1- Open table in a new tab

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

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

Semi-automated IGRT QA using a cone-shaped scintillator screen detector for proton pencil beam scanning treatments

To promote accurate image-guided radiotherapy (IGRT) for a proton pencil beam scanning (PBS) system, a new quality assurance (QA) procedure employing a cone-shaped scintillator detector has been developed for multiple QA tasks in a semi-automatic manner. The cone-shaped scintillator detector (XRV-124, Logos Systems, CA) is sensitive to both x-ray and proton beams. It records scintillation on the cone surface as a 2D image, from which the geometry of the radiation field that enters and exits the cone can be extracted. Utilizing this feature, QA parameters that are essential to PBS IGRT treatment were measured and analyzed. The first applications provided coincidence checks of laser, imaging and radiation isocenters, and dependencies on gantry angle and beam energies. The analysis of the Winston–Lutz test was made available by combining the centricity measurements of the x-ray beam and the pencil beam. The accuracy of the gantry angle was validated against console readings provided by the digital encoder and an agreement of less than 0.2° was found. The accuracy of the position measurement was assessed with a robotic patient positioning system (PPS) and an agreement of less than 0.5 mm was obtained. The centricity of the two onboard x-ray imaging systems agreed well with that from the routinely used Digital Imaging Positioning System (DIPS), up to a consistent small shift of (−0.5 mm, 0.0 mm, −0.3 mm). The pencil beam spot size, in terms of σ of Gaussian fitting, agreed within 0.2 mm for most energies when compared to the conventional measurements by a 2D ion-chamber array (MatriXX-PT, IBA Dosimetry, Belgium). The cone-shaped scintillator system showed advantages in making multi-purpose measurements with a single setup. The in-house algorithms were successfully implemented to measure and analyze key QA parameters in a semi-automatic manner. This study presents an alternative and more efficient approach for IGRT QA for PBS and potentially for linear accelerators.

EP-2071 Redefining the classical Winston-Lutz test for cranial radiosurgery in terms of dose

EP-2071 Redefining the classical Winston-Lutz test for cranial radiosurgery in terms of dose

Winston-Lutz-Gao Test on the True Beam STx Linear Accelerator

In the linear accelerator-based stereotactic radio surgery (SRS) and stereotactic body radiotherapy (SBRT) programs, single isocenter-multiple metastases’ treatment has become more and more popular due to their high efficiency in treatment time. However, the absence of a comprehensive quality assurance program is still the challenge for medical physicists. The Winston-Lutz-Gao test, which we developed two years ago, was performed for the first time on a True Beam STx (Varian Medical System) linear accelerator in this study. Beams were designed by Eclipse with gantry, collimator, and couch full rotations, and a 200-pound weight was placed on the couch to mimic real treatment. The “frameless SRS QA target pointer” from the Brainlab company, with a 3.5-mm metallic ball embedded in the center, was used as a phantom. Images were acquired by the portal imager built-in linear accelerator and analyzed directly by the Image browser in ARIA. We found that the farther the metastases were from the linac isocenter, the worse the congruence was between the beam mechanical and the radiation center. The farthest metastases should be within 6 cm from the linac isocenter per the AAPM TG-142 and American Society for Radiation Oncology (ASTRO) white paper criteria. To the best of our knowledge, this is the first off-isocenter Winston-Lutz test performed on a True Beam STx linear accelerator.

A novel-integrated quality assurance phantom for radiographic and nonradiographic radiotherapy localization and positioning systems.

Various localization and positioning systems utilizing radiographic or nonradiographic methods have been developed to improve the accuracy of radiation treatment. Each quality assurance (QA) procedure requires its own phantom and is independent from each other, so the deviation between each system is unavailable. The purpose of this work is to develop and evaluate a single-integrated QA phantom for different localization and positioning systems. The integrated phantom was designed in three-dimensional (3D) CAD software and 3D printed. The phantom was designed with laser alignment marks, a raised letter "S" on the anterior surface for optical surface monitoring system registration, a core for radiofrequency (RF) tracking system alignment, eight internal fiducials for image alignment, and an isocentric bearing for Winston-Lutz test. Tilt legs and rotational stage were designed for rotational verification of optical surface mapping system and RF tracking system, respectively. The phantom was scanned using a CT scanner and a QA plan was created. This prototype phantom was evaluated against established QA techniques. The QA result between the proposed procedure and established QA technique are 1.12 ± 0.31 and 1.14 ± 0.31 mm, respectively, for RF tracking system and 0.18 ± 0.06 and 0.18 ± 0.05 mm for Winston-Lutz test. There is no significant difference for the QA results between the established QA and proposed procedure (P > 0.05, t test). The accuracy of rotational verification for surface mapping system and RF tracking system are less than 0.5 and 1° compared the predefined value. The isocenter deviation of each location system is around l mm. We have designed and evaluated a novel-integrated phantom for radiographic and nonradiographic localization and positioning systems for radiotherapy. With this phantom, we will reduce the variation in measurements and simplify the QA procedures.