Gantry Sag Research Articles

SU-E-J-33: Geometric Agreement Check for Imaging System with Radiation Beam by KV and MV-CBCT.

The verification method of the geometry agreement between a light field and/or a laser coordinate and treatment beam should be easy and quick. In this presentation, we propose a novel QA method by using both kV- and MV-CBCT for kV-IGRT system. This method confirms the temporal unchanging the agreement of geometry in the kV-IGRT system with the treatment beam geometry. 1) MV-flexmap: Sequential MV-projection images were acquired during gantry rotation by iViewGT (Elekta) and MV-CBCT was reconstructed by in-house software with a flexmap correction. The flexmap is displacement of gantry and detector panel related with gantry sag. The geometric change affects the deranging reconstructed image. To evaluate how much displacement of EPID panel and gantry was detectable, the images of 8mm diameter ball-bearing (BB) located at the radiation isocenter were reconstructed with improper Flexmap.2) A comparison between the kV-CBCT and the MV-CBCT: The kV-CBCT was provided by X-ray Volume image (XVI) system (Elekta). To confirm the agreement for the geometry between kV-IGRT system and treatment beam, the kV-CBCTs of BB are compared with that of MV-CBCTs. The flexmaps were modified to (b)1mm / (c)3mm shifted to the rotation direction and (d)3mm to the rotation axis. The MV-CBCT were reconstructed with the correct flexmap and with incorrect flexmap (b), (c) and (d).　The geometric confirmation for MV-CBCT was done by comparison of the width and center of the BB on the MV-CBCT. The discrepancy of center between kV-CBCT and MV-CBCT was less than 1mm. Less than 1mm of the geometrical changing to rotation direction for MV-detector panel could be recognized by reconstructed images of BB. Using kV- and MV-CBCT enable us to perform the simple comparison for geometrical non-idealities between the kV-IGRT system and the treatment beam. Dr. K. Nakagawa received research grant from Elekta.

SU-E-T-133: Isocenter Measurements with the Winston-Lutz Test: Impact on Treatment Planning.

To assure high accuracy in radiation delivery during linear accelerator based stereotactic radiosurgery procedures, a Winston-Lutz test is typically performed to measure the effective isocenter wander over the wide range of gantry and couch angles employed. Ideally, this deviation is lower than 1-1.5 mm depending on institutional tolerances. Automation of this test with portal imaging allows a large amount of data to be taken and processed. This study explores the effect of various collimation techniques on beam-center stability, and addresses one technique of repositioning in response. Winston-Lutz shots were obtained using a Trilogy linear accelerator, BrainLab Winston-Lutz pointer, and electronic portal imaging from a full range of gantry and couch angles. The test was repeated using MLCs, a 10mm stereotactic cone from BrainLab, and jaws for beam collimation. All images were processed using an in-house MATLAB-based program which calculates the distance between the tungsten ball and aperture center with accuracy within ∼0.1mm. Ideal shifts were calculated by minimizing this variation from gantry rotation alone, and performed manually. The in-plane gantry sag was found to be on the order of ∼0.8mm, independent of collimation technique or setup. The MLC carriage had a ∼0.3mm cross-plane displacement to the inferior from the cone center, negated by a 90 degree collimator rotation. The total gantry misalignment was greatest near gantry 180 (∼1.5mm), varying by collimation method. Optimizing to minimize gantry error reduced errors to less than 0.7mm but increased the effect of couch rotation. Isocenter evaluation is dependent on collimation method used, and should be conducted with the same technique as delivery. It may be useful to consider the varying delivery accuracy from different couch/gantry angles during treatment planning. Repositioning methods after couch rotation may affect ideal laser positions.

Quantifying the gantry sag on linear accelerators and introducing an MLC‐based compensation strategy

Gantry sag is one of the well-known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. The objectives of this study were to quantify the gantry sag on multiple linear accelerators (linacs), to investigate a multileaf collimator (MLC)-based strategy to compensate for gantry sag, and to verify the gantry sag and its compensation with film measurements. The authors used the Winston-Lutz method to measure gantry sag on three Varian linacs. A ball bearing phantom was imaged with megavolt radiation fields at 10° gantry angle intervals. The images recorded with an electronic portal imaging device were analyzed to derive the radiation isocenter and the gantry sag, that is, the superior-inferior wobble of the radiation field center, as a function of the gantry angle. The authors then attempted to compensate for the gantry sag by applying a gantry angle-specific correction to the MLC leaf positions. The gantry sag and its compensation were independently verified using film measurements. Gantry sag was reproducible over a six-month measurement period. The maximum gantry sag was found to vary from 0.7 to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly (i.e., away from the gantry) when the gantry was rotated from 0° to 180°. After the MLC leaf position compensation was applied at 90° collimator angle, the maximum gantry sag was reduced to <0.2 mm. The film measurements at gantry angles of 0° and 180° verified the inferior shift of the radiation fields and the effectiveness of MLC compensation. The results indicate that gantry sag on a linac can be quantitatively measured using a simple phantom and an electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle-specific correction to MLC leaf positions at 90° collimator angle.

Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging

Electronic portal imaging devices (EPIDs) have been studied and used for pretreatment and in-vivo dosimetry applications for many years. The application of EPIDs for dosimetry in arc treatments requires accurate characterization of the mechanical sag of the EPID and gantry during rotation. Several studies have investigated the effects of gravity on the sag of these systems but each have limitations. In this study, an easy experiment setup and accurate algorithm have been introduced to characterize and correct for the effect of EPID and gantry sag during arc delivery. Three metallic ball bearings were used as markers in the beam: two of them fixed to the gantry head and the third positioned at the isocenter. EPID images were acquired during a 360° gantry rotation in cine imaging mode. The markers were tracked in EPID images and a robust in-house developed MATLAB code was used to analyse the images and find the EPID sag in three directions as well as the EPID + gantry sag by comparison to the reference gantry zero image. The algorithm results were then tested against independent methods. The method was applied to compare the effect in clockwise and counter clockwise gantry rotations and different source-to-detector distances (SDDs). The results were monitored for one linear accelerator over a course of 15 months and six other linear-accelerators from two treatment centers were also investigated using this method. The generalized shift patterns were derived from the data and used in an image registration algorithm to correct for the effect of the mechanical sag in the system. The Gamma evaluation (3%, 3 mm) technique was used to investigate the improvement in alignment of cine EPID images of a fixed field, by comparing both individual images and the sum of images in a series with the reference gantry zero image. The mechanical sag during gantry rotation was dependent on the gantry angle and was larger in the in-plane direction, although the patterns were not identical for various linear-accelerators. The reproducibility of measurements was within 0.2 mm over a period of 15 months. The direction of gantry rotation and SDD did not affect the results by more than 0.3 mm. Results of independent tests agreed with the algorithm within the accuracy of the measurement tools. When comparing summed images, the percentage of points with Gamma index <1 increased from 85.4% to 94.1% after correcting for the EPID sag, and to 99.3% after correction for gantry + EPID sag. The measurement method and algorithms introduced in this study use cine-images, are highly accurate, simple, fast, and reproducible. It tests all gantry angles and provides a suitable automatic analysis and correction tool to improve EPID dosimetry and perform comprehensive linac QA for arc treatments.

SU‐E‐T‐249: Evaluation of the Surface Rendering Patient Localization System for Proton Therapy Facility

Purpose: Proton therapy carries a number of dosimetric and radiobiological advantages for cancer patients and calls for rigorous quality assurance of patient registration. This work is a report of evaluation and use of surface rendering patient positioning system (AlignRTTM) in proton facility. Method and Materials: The AlignRT three‐camera system was installed in a specific non‐symmetric geometry, specific to the proton gantry room. Series of phantom shifts/rotations were done to assess the system performance versus shifts/rotations of the 6D PPS (Patient Positioning System) that accounts for proton gantry sag. 4D phantom (CIRS,inc.) was used to evaluate motion tracking capabilities of AlignRT. Finally, AlignRT installation was used for on‐line monitoring of thorax patients (emphasis on breathing motion). Retrospective analysis of breathing data was performed versus latest weekly 4DCT of these patients. Results: In case of the baseline images taken at specific PPS rotation angles, from which shifts were performed, the proton room‐specific 3‐camera AlignRTTM system shows satisfactory results with shifts/rotations within 0.5mm/0.3degrees maximum difference. The accuracy of SSD determination was within 1 mm. In case of treatment planning system‐generated baseline images and baseline images taken at PPS angle of zero degrees, the system performance depended on PPS rotation angle, the fact whether the AlignRT system had the nominal couch angle input, and the size of the shift. At shifts within +/−3cm from isocentric position, AlignRT system performed with accuracy of 2.0mm/0.3degrees maximum deviations for PPS rotation angles within 30degrees from the “golden standard” surface and for PPS rotations 90+/− 10degrees. At intermediate PPS angles (60+/−10degrees), accuracy shown: within 10mm longitudinal/lateral, sub‐millimeter vertical directions. Reproducibility of phantom position was within 0.5mm/0.3degree. The measured amplitude of the patient breathing motion was within 0.5mm from 4DCT. Conclusion: Installation of surface rendering patient positioning system (AlignRTTM) in proton room‐specific geometry was evaluated and used for patient positioning monitoring.

Evaluation of a new IR‐guided system for mechanical QA of linear acceleratorsa)

The authors report the development of a new procedure for mechanical quality assurance of linear accelerators using an infrared-guided system. The system consists of an infrared (IR) camera and an IR-reflective marker that can be attached to a gantry, a collimator, or a treatment table. The trace of this marker can be obtained in three dimensions (3D) for a full or partial rotation of the mechanical devices. The software is written to localize rotational axes of the gantry, collimator, and the treatment table based on the marker traces. The separation of these axes characterizes the size of the sphere defining the mechanical isocenter. Additional information on anomalies in gantry movement such as degree of gantry sag and hysteresis can also be obtained. An intrinsic uncertainty of the system to localize rotational axis is 0.35 mm or less. Tests on a linear accelerator demonstrated the ability of this system to detect the separation between rotational axes of less than 1 mm and to confirm orthogonality of the planes of gantry, collimator, and table rotation.

Linac mechanic QA using a cylindrical phantom

Precise mechanical operation of a linear accelerator (linac) is critical for accurate radiation therapy dose delivery. Quantitative procedures for linac mechanical quality assurance (QA) used in the standard of care are time consuming and therefore conducted on a relatively infrequent basis. We present a method for evaluating the mechanical performance of a linac based on a series of projection portal images of a prototype cylindrical phantom with embedded radiopaque fiducial markers. The marker autodetection process included modeling imager response to the radiation beam where the projected cylinder attenuation yielded a non-uniform image background. The linac mechanical characteristics were estimated based on nonlinear multi-objective optimization of the projected marker locations. The estimated geometry parameters for the tested commercial model were gantry angle deviation 0.075 ± 0.076° (1 SD), gantry sag 0.026 ± 0.02°, source-to-axis distance SAD 998.3 ± 1.7 mm, source-to-detector distance SDD 1493 ± 5.0 mm, couch vertical motion 0.6 ± 0.45 mm, couch rotation 0.154 ± 0.1° and average linac rotation center (1.02, −0.27, −0.37) ± (0.36,0.333,1.20) mm relative to the laser intersection. The imager shift was [−0.44, 2.6] ± [0.20, 1.1] mm and the imager orientation was in-plane rotation 0.05 ± 0.03°, roll −0.14 ± 0.09° and pitch −0.9 ± 0.604°. The performance of this procedure concerning marker detection and optimization was examined by comparing the detected set of marker coordinates to its back-calculated counterpart for three subgroups of markers: central, wall and intermediate relative to the center of the phantom. The maximum difference was less than 0.25 mm with a mean of 0.146 mm and a standard deviation of 0.07 mm. The clinical use of this automated procedure will allow more efficient, more thorough, and more frequent mechanical linac QA.

WE-D-351-06: Automated Quality Assurance Test for MLC-Based Stereotactic Radiosurgery

Purpose: We present an automated method of Linac mechanical quality assurance (QA) for MLC‐based Stereotactic Radiosurgery. The test is designed to determine 3D deviation of the Linac beam central axis defined by the center of the collimator aperture defined by the MLC leaves from the machine isocenter defined by the small tungsten sphere target. Method and Materials: The automation is achieved using Portal Imager as a 2D detector. Based on the approach of Winston‐Lutz Alignment Test, we monitor the alignment of the isocentrically‐placed tungsten sphere target, mounted on a metal stem, with the center of the MLC‐defined small square field. Multiple projection images are acquired for a specified set of gantry angles. After acquisition, the EPIDimages are saved in DICOM format by the imager's software for subsequent automated analysis on a Windows‐based PC running MATLAB®. Results: The center of the sphere deviated by 0.75 ±0.17 mm (1SD) from the MLC defined field center. The lateral, longitudinal and vertical sphere displacements were [0.66±0.17, 0.20±0.18, 0.217±0.13] mm, respectively. The average extent of the MLC motion in the direction parallel to the leaf travel was within 0.2 mm maximum span, while the carriage motion in the perpendicular direction ranged from 0.2 mm for 5×5cm2 field to 1 mm motion for 2×2cm2 field. All results were automatically processed using GUI. Conclusion: Our test is capable of sub‐millimeter sensitivity to the MLC leaf position and carriage position, as well as gantry sag and carriage shift due to the effect gravity at multiple gantry angles. The system employs a graphic interface, which is intended to facilitate the frequent clinical use of the presented QA technique. Conflict of Interest: This work is partially supported by Varian Medical Systems (PaloAlto, CA).

SU-GG-J-20: A Trajectory Estimation Method of Moving Target From Cone Beam CT Projection Images

Purpose: The aim of this work was to develop and investigate a method for trajectory estimation of a moving target from cone beam CT projection images. It can be useful for assessment of respiratory tumor motion just before or during treatment, and for developing further respiratory compensation strategies. Method and Materials: Six lungtumor trajectories with range of motion larger than 10 mm, acquired using a CyberKnife system, were used to simulate CBCT acquisition. To reconstruct 3D positions from projected positions, the two nearest angular projection images with the same SI‐projection positions were used for triangulation. With the triangulated positions, a linear correlation was assumed for tumor motion. LR and AP positions were then derived from SI positions using the linear model. The estimated 3D tumor positions were compared with the known positions. To demonstrate the real simulation, CBCT projection images for a seed‐embedded moving phantom were acquired using a Varian OBI system. A 3D motion platform was used to drive the phantom with one of six trajectories. The marker position from each projection image was extracted using Varian RPM‐Fluoro application. Results: The mean estimation errors were within 0.3±1.8 mm while errors of up to 4 mm were observed. In general, higher correlation coefficients with SI‐motion provide better estimation of AP and LR positions. Applying the center offset of OBI imager (xOffset=−2 pixel, yOffset=0 pixel), the remaining uncertainty associated with gantry sag and marker extraction (∼1 pixel) was within ∼0.5 mm. Conclusion: The proposed method provides useful information for tumor motion characteristics such as the mean position, range, and principal direction of motion which can be useful for respiratory compensation treatments. Conflict of Interest: Research supported by Varian Medical Systems.

SU‐GG‐T‐441: A New Method of Measuring the Accuracy of Stereotactic Cones and MLC with Gantry and Collimator Rotation

Precision and accuracy are vital for stereotactic radiosurgery and radiotherapy. We set up the patients with lasers and we treat with radiation fields defined by cones or MLC, therefore the lasers must be accurately aligned with the radiation fields. In addition, the manner in which the beam shapers move with gantry and collimator angles must be reproducible and reliable. The standard method of measuring laser alignment and rotational shifts at the Tom Baker Cancer Centre, the Winston‐Lutz test, is only semi‐quantitative, and the results are subject to inter‐observer variation. We have developed a method of digitally detecting therapy X‐Rays and lasers using a 512 by 1024 photodiode array with a 48μm pixel pitch, the Radeye1 from the Rad‐icon Imaging Corporation to which we added optical density filters and an intensifying screen. We took images of the lasers and the radiation fields at various gantry and collimator angles and analyzed them with in‐house developed Matlab tools. For the 4mm diameter cone we measured an apparent head tilt of 0.14 (±0.02) mm and a gantry sag of 0.39 (±0.02) mm. The lasers were within 0.25 (±0.05) mm of the compromise radiation isocentre. For a 6mm by 6mm MLC we found the center of the radiation field varied by 0.58 (±0.02) mm with collimator rotation. The ceiling laser was 0.52 (±0.02) mm from the centre of the MLC rotation. This system can be used to both monitor and improve machine performance, which may lead to improved outcomes for patient treatment.

MO‐E‐AUD‐04: Automated Linear Accelerator Quality Assurance Using a Commercial Cylindrical Calibration Phantom

Purpose: Precise mechanical operation of a linear accelerator is critical for accurate dose delivery. Available quantitative procedures for the linac mechanical quality assurance (QA) are time consuming and therefore conducted on a relatively infrequent basis. We present a method for evaluating the mechanical performance of a linac based on a series of projection portal images of a prototype cylindrical calibration phantom with embedded markers.Method and Materials: We used non‐linear multiobjective optimization of information extracted from the images to determine a number of geometric parameters of interest. The markers detection included modeling the imager response to radiation beams where significantly non‐uniform background was expected. Results: The average results for the parameters of our geometric linac model were: gantry angle deviation 0.066 ± 0.085° (1 SD), gantry sag 0.026 ± 0.02°, imager in‐plane rotation 0.026 ± 0.055°, roll — 0.082 ± 0.16° and pitch −0.9 ± 0.604°, SDD 1489.7 ± 5 mm, SAD 998 .3 ± 1.7 mm, and the imager shift [− 0.66,3.9]±[0.30,1.6] mm. The results were corrected for the phantom center shift relative to the linac rotational center. The average rotational center was . The average couch height and angle variations were 0.15±0.9 mm and 0.154±0.1°, respectively. The image analysis quality was examined by comparing the detected set of marker coordinates to its simulated counterpart for three regions of the phantom image: central, near the edge and the intermediate region (relative to the central line of the cylinder). The upper limit of the mean difference was less than 0.25 mm with the cumulative mean of 0.146 mm and SD of 0.07 mm. The results of the primary optimization of directly detected marker coordinates virtually coincided with their counterparts based on the simulated coordinates for all the geometric parameters of the model.Conclusion: This procedure is accurate and automated, which allows precise mechanic QA to be performed more frequently.Conflict of Interest: partially supported by Varian Medical Systems.

MO‐E‐AUD‐02: A Novel Infrared Camera System for Mechanical QA of Radiation Treatment Accelerators

Purpose: To develop a fast and accurate procedure for mechanical QA of linear accelerators that can be used during installation and routine mechanical QA. Method and Materials: A system was developed which consists of an IR‐camera and an IR‐marker attached to the gantry, treatment table or collimator. Software was written to process signals from the IR‐camera for the duration of gantry rotation and reconstruct the trace of the marker in three orthogonal planes. A circular fit of the trace in the plane of gantry rotation provides 3D coordinates of mechanical isocenter for gantry rotation. A linear fit of this trace in orthogonal planes provides information on gantry sag and rotational hysteresis. A similar procedure was developed for table mechanical QA. Orthogonality of the planes of table and gantry rotation is determined by the angle between linear fits for the gantry and table traces. Results: Mechanical QA tests were performed in two different treatment rooms equipped with IR‐camera systems. The accuracy of mechanical alignment of the axis of rotation of the gantry and couch was demonstrated to be within 1mm of the nominal isocenter defined by the room lasers. In each treatment room, the tests were repeated three times and reproducibility of the tests was better than 1mm. The tests revealed both the known effect of gantry sag and also rotational hysteresis. Orthogonality of the planes of gantry and table rotation was also determined. Conclusions: An IR‐based procedure for mechanical QA of radiation treatment accelerators was developed and tested. This procedure accurately detects the location of mechanical isocenter and provides additional information such as the extent of gantry sag and rotational hysteresis. The system provides quantitative 3D displacement of mechanical isocenter that can be effectively used in the initial installation of accelerators as well as in an ongoing quality assurance program.

SU-FF-T-153: Development of An Electron Multileaf Collimator Performing Computer-Controlled Beam Collimation and Isocentric Dose Delivery with Electrons

Purpose: To develop an add-on multileaf collimator for electrons (eMLC) and to verify its efficiency according to the design goals. Method and Materials: The design parameters were determined such that a compact and light-weight eMLC can be manufactured and stable mounted on a conventional accelerator. The source-to-collimator distance (SCD) was optimized to avoid patient realignments during gantry rotation and with consideration of the required maximum field size and minimum beam penumbra. To verify the efficiency of the eMLC, the stability of the mounting was examined at all gantry angles. Additionally, at 6–14 MeV and field sizes of 3×3–20×20 cm, the dose distribution and the optimal jaw setting were determined with a pinpoint ionization chamber. The radiation leakage was measured with radiochromic films. Results: A prototype was constructed consisting of 2×24 motorized brass leaves with 1.8 cm height. Since the necessary distance between eMLC and patient surface can significantly vary, the eMLC can be placed at 84 and 72 cm SCD implemented by interchangeable distance holders. At these SCDs, the maximum field sizes are 17×17 and 20×20 cm, respectively. The prototype weights 23 kg and thus causes no gantry sag. Additionally, the sag of the distance holder is less than 0.5 mm at all gantry angles. Compared to the dose distributions of the applicator at 6–14 MeV, the penumbra of the eMLC at 84 and 72 cm SCD are 0.8–0.6 and 1.8–1.0 cm greater respectively and the depth-dose curves show a larger build-up effect. Due to the tongue-and-groove leaf shape, interleaf leakage is negligible. The leaf-end leakage could be eliminated by adjoining the leaf-ends off-axis. At 14 MeV, bremsstrahlung results in 2 % intraleaf leakage. Conclusion: The eMLC facilitates computer-controlled beam collimation and isocentric dose delivery at all gantry angles necessary to expand the IMRT techniques to electrons.

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.

SU‐FF‐T‐175: A New Linac QA Procedure for the Characterization of Gantry Radiation Isocenter

Purpose: A new linac QA method is developed to characterize the size and shape of gantry radiation isocenter. The method accurately measures gantry sag as a function of gantry angles and the size of gantry radiation isocenter in the gantry rotation plane. Method and Materials: In order to determine both sag and radiation isocenter size in the gantry rotation plane we wrap a film (or a phosphor imaging plate) on a cylinder of circumference equal to the film length. We designed a static MLC “dashed line” pattern consisting of regions of radiation along a line. The pattern is designed such that the center of the irradiation line can be easily determined. The cylinder is positioned such that its axis is the same as the gantry rotation axis. The film is exposed to the MLC line pattern for multiple gantry angles. Both entry and exit patterns are seen on the film. A special MLC pattern was designed to accurately characterize the gap between the two ends of the film. An automated analysis program determines the MLC line angles and centers on the film, characterizes the cylinder position and generates the desired gantry radiation isocenter information. Results: The gantry sag is observed to have a smooth dependence on gantry angle with extreme values for 0° and 180°. The smallest circle crossing or tangent to all rays in the gantry rotation plane determines the gantry radiation isocenter size in this plane. The inherent accuracy of the method is estimated at ±/− 0.1 mm. The total irradiation time is about 15 min. The data analysis is less than 5 min. Conclusion: The designed method characterizes both gantry sag and wobble in one simple and accurate test.

SU-FF-T-390: A New Linac QA Procedure for the Characterization of Radiation Isocenter and Room Lasers Position

Purpose: We have designed and implemented a new stereotactic machine QA test. The method is used to characterize gantry sag, couch wobble, cone placement, MLC offset and room lasers position relative to radiation isocenter. An image containing a series of test patterns is generated in a direct and integrated fashion. Method and Materials: Two MLC star patterns, a cone pattern and the laser lines are recorded on the same imaging medium, enabling 0.1 mm accuracy measurements.Phosphor plates are used as the imaging medium due to their unique property that the red light of wall laser erases the radiation information stored on phosphor plates. The room lasers position relative to the radiation isocenter can be measured. The developed QA method consists of four images that measure the gantry sag between 00 and 1800 gantry angles, the position and stability of couch rotational axis, the sag between 900 and 2700 gantry angles, the accuracy of cone placement on the collimator and the position of laser lines relative radiation isocenter. Results: The inherent precision of the numerical algorithms developed is +/− 0.05mm. The inherent accuracy of the method as a whole is +/− 0.1mm. The total irradiation/illumination time is about 10 min per image. Automating the generation of collimator star patterns can reduce this time. The data analysis (including the phosphor plate scanning) is less than 5min. Conclusion: The presented method reproducibly characterizes the radiation isocenter geometry with the high accuracy required for stereotactic surgery. It can replace the standard ball test and it can provide a highly accurate QA procedure for the non‐stereotactic machines.

A block design for split-field tests of accelerator alignment.

We present a block design that facilitates split-field tests, a technique frequently employed to assess several potential misalignments in linear accelerators. Two tests are possible with this device using: (1) opposed collimator angles and (2) opposed gantry angles. The first test is sensitive to jaw symmetry only. In the second test, the alignment of one field edge indicates gantry sag, while the other field edge is sensitive to the combined effects of jaw asymmetry, misalignment of the beam spot, and misalignment of the gantry and collimator rotation axes. The block arrangement consists of a pattern of three polygons cast in lead or Cerrobend. The assembly mounts in the accessory tray of the accelerator and allows all necessary exposures to be taken in a single room entry. Any field size up to 24 cm can be tested with the assembly constructed here. A simultaneous test of light field-radiation field coincidence can also be accommodated. Exposed films with a prototype device show that offsets of +/-1 mm are readily visible. The block design facilitates the required measurements for split-field tests and makes this a practical technique for routine quality assurance on a linear accelerator.

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.

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

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

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.