MV Isocenter Research Articles

Implementing corrections of isocentric shifts for the stereotactic irradiation of cerebral targets: Clinical validation.

Purpose: Any Linac will show geometric imprecisions, including non‐ideal alignment of the gantry, collimator and couch axes, and gantry sag or wobble. Their angular dependence can be quantified and resulting changes of the dose distribution predicted (Wack, JACMP 20(5), 2020). We analyzed whether it is feasible to correct geometric shifts during treatment planning. The successful implementation of such a correction procedure was verified by measurements of different stereotactic treatment plans. Methods: Isocentric shifts were quantified for two Elekta Synergy Agility Linacs using the QualiForMed ISO‐CBCT+ module, yielding the shift between kV and MV isocenters, the gantry flex and wobble as well as the positions of couch and collimator rotation axes. Next, the position of each field's isocenter in the Pinnacle treatment planning system was adjusted accordingly using a script. Fifteen stereotactic treatment plans of cerebral metastases (0.34 to 26.53 cm3) comprising 9–11 beams were investigated; 54 gantry and couch combinations in total. Unmodified plans and corrected plans were measured using the Sun Nuclear SRS‐MapCHECK with the Stereophan phantom and evaluated using gamma analysis. Results: Geometric imprecisions, such as shifts of up to 0.8 mm between kV and MV isocenter, a couch rotation axis 0.9 mm off the kV isocente,r and gantry flex with an amplitude of 1.1 mm, were found. For eight, mostly small PTVs D98 values declined more than 5% by simulating these shifts. The average gamma (2%/2 mm, absolute, global, 20% threshold) was reduced from 0.53 to 0.31 (0.32 to 0.30) for Linac 1 (Linac 2) when including the isocentric corrections. Thus, Linac 1 reached the accuracy level of Linac 2 after correction. Conclusion: Correcting for Linac geometric deviations during the planning process is feasible and was dosimetrically validated. The dosimetric impact of the geometric imperfections can vary between Linacs and should be assessed and corrected where necessary.

Investigation of Elekta AQUA software for kilovoltage to megavoltage radiation isocenter coincidence.

Elekta AQUA v2.02 software (Gantry Runout isocenter test) was investigated as a tool for verification of kilovoltage to megavoltage gantry radiation isocenter coincidence. AQUA reported megavoltage (6 MV) isocenter was independent of field size over the range 5cm × 5cm to 20cm × 20cm. For the 10cm × 10cm field size, standard deviation in AQUA reported 3D megavoltage (6MV) isocenter over ten consecutive deliveries was less than 0.04mm for any axis. Compared to the full AQUA test delivery (Gantry Runout), the shorter AQUA test version (Gantry Runout short) gave a root mean square MV isocenter (± 1 SD) difference of 0.18mm ± 0.08. Across 7 machines, root mean square differences between AQUA and PIPS PRO reported MV isocenter (for 6 MV and 6 MV FFF beams) was 0.1mm ± 0.1mm, with most of the difference observed in the gun-target (Y-axis). AQUA 6 MV isocentre position was offset to gantry relative to Elekta XVI customer acceptance test (CAT) workflow by 0.25mm to 0.52mm. For 6 MV FFF beams, AQUA reported an MV isocenter of between 0.27 and 0.39mm offset to target relative to Elekta CAT.

Commissioning of a 1.5T Elekta Unity MR-linac: A single institution experience.

MR image‐guided radiotherapy has the potential to improve patient care, but integration of an MRI scanner with a linear accelerator adds complexity to the commissioning process. This work describes a single institution experience of commissioning an Elekta Unity MR‐linac, including mechanical testing, MRI scanner commissioning, and dosimetric validation. Mechanical testing included multileaf collimator (MLC) positional accuracy, measurement of radiation isocenter diameter, and MR‐to‐MV coincidence. Key MRI tests included magnetic field homogeneity, geometric accuracy, image quality, and the accuracy of navigator‐triggered imaging for motion management. Dosimetric validation consisted of comparison between measured and calculated PDDs and profiles, IMRT measurements, and end‐to‐end testing. Multileaf collimator positional accuracy was within 1.0 mm, the measured radiation isocenter walkout was 0.20 mm, and the coincidence between MR and MV isocenter was 1.06 mm, which is accounted for in the treatment planning system (TPS). For a 350‐mm‐diameter spherical volume, the peak‐to‐peak deviation of the magnetic field homogeneity was 4.44 ppm and the geometric distortion was 0.8 mm. All image quality metrics were within ACR recommendations. Navigator‐triggered images showed a maximum deviation of 0.42, 0.75, and 3.0 mm in the target centroid location compared to the stationary target for a 20 mm motion at 10, 15, and 20 breaths per minute, respectively. TPS‐calculated PDDs and profiles showed excellent agreement with measurement. The gamma passing rate for IMRT plans was 98.4 ± 1.1% (3%/ 2 mm) and end‐to‐end testing of adapted plans showed agreement within 0.4% between ion‐chamber measurement and TPS calculation. All credentialing criteria were satisfied in an independent end‐to‐end test using an IROC MRgRT phantom.

A novel phantom and procedure providing submillimeter accuracy in daily QA tests of accelerators used for stereotactic radiosurgery*.

Stereotactic radiosurgery (SRS) places great demands on spatial accuracy. Steel BBs used as markers in quality assurance (QA) phantoms are clearly visible in MV and planar kV images, but artifacts compromise cone‐beam CT (CBCT) isocenter localization. The purpose of this work was to develop a QA phantom for measuring with sub‐mm accuracy isocenter congruence of planar kV, MV, and CBCT imaging systems and to design a practical QA procedure that includes daily Winston‐Lutz (WL) tests and does not require computer aid. The salient feature of the phantom (Universal Alignment Ball (UAB)) is a novel marker for precisely localizing isocenters of CBCT, planar kV, and MV beams. It consists of a 25.4 mm diameter sphere of polymethylmetacrylate (PMMA) containing a concentric 6.35 mm diameter tungsten carbide ball. The large density difference between PMMA and the polystyrene foam in which the PMMA sphere is embedded yields a sharp image of the sphere for accurate CBCT registration. The tungsten carbide ball serves in finding isocenter in planar kV and MV images and in doing WL tests. With the aid of the UAB, CBCT isocenter was located within 0.10±0.05 mm of its true positon, and MV isocenter was pinpointed in planar images to within 0.06±0.04 mm. In clinical morning QA tests extending over an 18 months period the UAB consistently yielded measurements with sub‐mm accuracy. The average distance between isocenter defined by orthogonal kV images and CBCT measured 0.16±0.12 mm. In WL tests the central ray of anterior beams defined by a 1.5×1.5 cm2 MLC field agreed with CBCT isocenter within 0.03±0.14 mm in the lateral direction and within 0.10±0.19 mm in the longitudinal direction. Lateral MV beams approached CBCT isocenter within 0.00±0.11 mm in the vertical direction and within ‐0.14±0.15 mm longitudinally. It took therapists about 10 min to do the tests. The novel QA phantom allows pinpointing CBCT and MV isocenter positions to better than 0.2 mm, using visual image registration. Under CBCT guidance, MLC‐defined beams are deliverable with sub‐mm spatial accuracy. The QA procedure is practical for daily tests by therapists.PACS number(s): 87.53.Ly, 87.56.Fc

SU‐F‐J‐36: Comparison of Ball Bearing and Iso‐Cube Phantoms for KV‐MV Iso‐Center Coincidence Check

Purpose:To compare two different quality assurance tools for kV‐MV isocenter coincidence check.Methods:Ball‐Bearing device (BBD) (Elekta) provided along with a CBCT equipped LINAC and isocenter cube phantom (CP) (a commercial product of Modus Medical Devices) are utilized to check the coincidence between the MV and kV beam isocenters. The microstepping meter of the BBD allows precision adjustment to better than 0.01mm in three directions. The BBD is aligned to the MV‐isocenter with the lasers and followed by taking MV‐images of at two collimator angles at each cardinal gantry, then its position adjusted. This process takes in iterative steps until 0.25mm tolerance is achieved. Four planar kV‐images are taken at cardinal angles and deviations of BBD position from kVisocenter are evaluated. CP is positioned on the couch with the lasers at known offset from the isocenter. CBCT is utilized to set the cube to its kV isocenter with the precision of linac's couch (1mm). MV‐images are taken at various gantry, collimator, and couch angles and evaluated with the CP manufacturer provided QA software to determine coincidence of the 6mm steel ball at cube center with MV‐isocenter. Three sets of measurements, one after another, were performed on the same day for each phantom on the same linac.Results:Assuming little variation between kV and MV isocenters in a short time, BBD measurements are more reproducible than CP. With comparable conditions both methods agree within 0.5 mm in each direction, while for BBD average standard deviation was 0.07mm and for iso‐cube 0.3mm.Conclusion:Since BBD is more reliable and its results are more reproducible, it should be used during the monthly QA. Since CP is a more efficient device, it should be used for daily QA. A comparison study between the two devices should be periodically performed.

SU‐F‐T‐480: Evaluation of the Role of Varian Machine Performance Check (MPC) in Our Daily QA Routine

Purpose:(A) To assess the role of Varian MPC in our daily QA routine, and (B) evaluate the accuracy and precision of MPC.Methods:The MPC was performed weekly, for five months, on a Varian TrueBeam for five photon (6x, 10x, 15x, 6xFFF, and 10xFFF) and electron (6e, 9e, 12e, 16e, and 20e) energies. Output results were compared to those determined with an ionization chamber (TN30001, PTW‐Freiburg) in plastic and a daily check device (DQA3, Sun Nuclear). Consistency of the Mechanical measurements over five months was analyzed and compared to monthly IsoCal results.Results:The MPC randomly showed large deviations (3–7%) that disappeared upon reacquisition. The MPC output closely matched monthly ion chamber and DQA3 measurements. The maximum and mean absolute difference between monthly and MPC was 1.18% and 0.28±0.21% for all energies. The maximum and mean absolute difference between DQA3 and MPC was 3.26% and 0.85±0.61%. The results suggest the MPC is comparable to the DQA3 for measuring output. The DQA3 provides wedge output, flatness, symmetry, and energy constancy checks, which are missing from the current implementation of the MPC. However, the MPC provides additional mechanical tests, such as size of the radiation isocenter (0.33±0.02 mm) and its coincidence with MV and kV isocenters (0.17±0.05 and 0.21±0.03 mm). It also provides positional accuracy of individual jaws (maximum σ, 0.33mm), all the MLC leaves (0.08mm), gantry (0.05°) and collimator (0.13°) rotation angles, and couch positioning (0.11mm) accuracy. MPC mechanical tests could replace our current daily on‐board imaging QA routine and provide some additional QA not currently performed.Conclusion:MPC has the potential to be a valuable tool that facilitates reliable daily QA including many mechanical tests that are not currently performed. This system can add to our daily QA, but further development would be needed to fully replace our current Daily QA device.

SU-E-T-204: Improving Multiple Isocenter Coincidence: Elekta Beam Modulator with HexaPOD Six Degrees Couchtop

Purpose: With evolution from frame-based to image-guided SRS/SRT technique, the coincidence of multiple isocenters must be within a tight tolerance: MV, kV, table mechanical, and, in this case, HexaPOD 6° patient positioning system. Reported here is a method for improving the effectiveness of an isocenter alignment procedure which ultimately led to isocenter coincidence on an Elekta Beam Modulator LINAC equipped with 4 mm MLC leaves and a fixed secondary jaw to within a radius of 1mm. Methods: Utilizing a self-leveling laser, the Elekta ball (BB) bearing phantom (8mm steel ball), the Modus QA Isocenter Cube software, and a colleague-provided isocenter alignment method, the kV, table mechanical, and HexaPOD isocenters were localized to the MV isocenter. Isocenter coincidence was tested using the Isocenter Cube from Modus QA, brought to kV isocenter using CBCT and the HexaPOD system. MV images were taken at various gantry, collimator, and couch angles and then analyzed with the aforementioned software to determine coincidence of the 5mm steel ball at cube center with MV isocenter. To improve overall coincidence, two errors were addressed iteratively: (1) LINAC mechanical and radiation isocenter coincidence allowing each isocenter volume to have similar variations during gantry and collimator rotation and (2) collimator rotational walkout which reduced the size of both mechanical and radiation isocenters. Beam steering was performed to bring MV isocenter closer to mechanical and collimator walkout was adjusted by physically shifting the MLC bank. Initial procedures were repeated to align all isocenters and perform regular checks for consistency. Results: Post-initial alignment revealed a maximum diameter of 1.8mm for MV isocenter and maximum isocenter alignment error of up to 1.22mm radius which improved to roughly 1.0mm MV isocenter diameter and <0.71mm radius alignment error with a consistency within 0.25mm. Conclusion: With the methods described, all isocenters were brought into alignment within a radius of 1mm.

SU‐E‐J‐138: An IGRT QA Device for Measuring with Tenths‐Millimeter Accuracy KV and MV Isocenter Congruence, Couch Travel and Laser Alignment of Accelerators Used for SRS and SBRT

Purpose:To develop a practical device having sufficient accuracy for daily QA tests of accelerators used for SRS and SBRT.Methods:The UAB (Universal Alignment Ball) consists of a 6.35 mm (1/4 inch) diameter tungsten sphere located concentrically within a 25.4 mm (1 inch) diameter acrylic plastic (PMMA) sphere. The spheres are embedded in polystyrene foam, which, in turn, is surrounded by a cylindrical PMMA shell. The UAB is placed on the couch and aligned with wall lasers according to marks that have known positions in relation to the center of the spheres. Using planar and cone beam images the couch is shifted till the surface of the PMMA sphere matches Eclipse‐generated circular contours. Anterior and lateral MV images taken with small MLC openings allow measurement of distance between kV and MV isocenter, laser and MLC alignment. Measurements were taken over a one‐month period.Results:Artifacts from the tungsten sphere were confined within the PMMA sphere and did not affect cone beam localization of the sphere boundary, allowing 0.1 mm precise alignment with a computer‐generated circle centered at kV isocenter. In tests extending over a one‐month period, the distance between kV and MV isocenters along the vertical, longitudinal and lateral directions was 0.125 +/−0.06, 0.19 +/−0.08, and 0.02 +/−0.08 mm, respectively. Laser misalignment along these directions was 0.34 +/‐ 0.15, 0.74 +/−0.29, and 0.49 +/−0.22 mm. Automated couch shifts moved the spheres to within 0.1 mm of the selected position. The center of a 1cmx1cm MLC‐defined field remained within +/−0.2 mm of the tungsten sphere center as the gantry was rotated.Conclusion:The UAB is practical for daily end‐to‐end QA tests of accelerator alignment. It provides tenths‐mm accuracy for measuring agreement of kV and MV isocenters, couch motions, gantry flex and laser alignment.

SU‐E‐J‐47: Development of a High‐Precision, Image‐Guided Radiotherapy, Multi‐ Purpose Radiation Isocenter Quality‐Assurance Calibration and Checking System

Purpose:To develop a system that can define the radiation isocenter and correlate this information with couch coordinates, laser alignment, optical distance indicator (ODI) settings, optical tracking system (OTS) calibrations, and mechanical isocenter walkout.Methods:Our team developed a multi‐adapter, multi‐purpose quality assurance (QA) and calibration device that uses an electronic portal imaging device (EPID) and in‐house image‐processing software to define the radiation isocenter, thereby allowing linear accelerator (Linac) components to be verified and calibrated. Motivated by the concept that each Linac component related to patient setup for image‐guided radiotherapy based on cone‐beam CT should be calibrated with respect to the radiation isocenter, we designed multiple concentric adapters of various materials and shapes to meet the needs of MV and KV radiation isocenter definition, laser alignment, and OTS calibration. The phantom's ability to accurately define the radiation isocenter was validated on 4 Elekta Linacs using a commercial ball bearing (BB) phantom as a reference. Radiation isocenter walkout and the accuracy of couch coordinates, ODI, and OTS were then quantified with the device.Results:The device was able to define the radiation isocenter within 0.3 mm. Radiation isocenter walkout was within ±1 mm at 4 cardinal angles. By switching adapters, we identified that the accuracy of the couch position digital readout, ODI, OTS, and mechanical isocenter walkout was within sub‐mm.Conclusion:This multi‐adapter, multi‐purpose isocenter phantom can be used to accurately define the radiation isocenter and represents a potential paradigm shift in Linac QA. Moreover, multiple concentric adapters allowed for sub‐mm accuracy for the other relevant components. This intuitive and user‐friendly design is currently patent pending.

Improved accuracy for noncoplanar radiotherapy: an EPID‐based method for submillimeter alignment of linear accelerator table rotation with MV isocenter

Accurate alignment of linear accelerator table rotational axis with radiation isocenter is critical for noncoplanar radiotherapy applications. The purpose of the present study is to develop a method to align the table rotation axis and the MV isocenter to submillimeter accuracy. We developed a computerized method using electronic portal imaging device (EPID) and measured alignment stability over time. Mechanical and radiation isocenter coincidence was measured by placing a steel ball bearing at radiation isocenter using existing EPID techniques. Then, EPID images were acquired over the range of table rotation. A MATLAB script was developed to calculate the center of rotation, as well as the necessary adjustment to move the table rotational axis to MV isocenter. Adjustment was applied via torque to screws at the base of the linac table. Stability of rotational alignment was measured with 49 measurements over 363 days on four linacs. Initial rotational misalignment from radiation isocenter ranged from 0.91−2.11 mm on the four tested linacs. Linac‐A had greatest error (>2 mm) and was adjusted with the described method. After adjustment, the error was significantly decreased to 0.40±0.12 mm. The adjustment was stable over the course of 15 measurements over 231 days. Linac‐B was not adjusted, but tracked from time of commissioning with 27 measurements over 363 days. No discernible shift in couch characteristics was observed (mean error 1.40±0.22 mm). The greater variability for Linac‐B may relate to the interchangeable two‐piece couch, which allows more lateral movement than the one‐piece Linac‐A couch. Submillimeter isocenter alignment was achieved by applying a precision correction to the linac table base. Table rotational characteristics were shown to be stable over the course of twelve months. The accuracy and efficiency of this method may make it suitable for acceptance testing, annual quality assurance, or commissioning of highly‐conformal noncoplanar radiotherapy programs.PACS number: 87

Winston-lutz test and acquisition of flexmap using rotational irradiation

IGRT (image guided radiation therapy) is a useful technique for implementing precisely targeted radiation therapy. Quality assurance and quality control (QA/QC) medical linear accelerators with a portal imaging system (electronic portal imaging device: EPID) are the key to ensuring safe IGRT. The Winston-Lutz test (WLT) provides an evaluation of the MV isocenter, which is the intersection of radiation, collimator, and couch isocenters. A flexmap can indicate a displacement of EPID from the beam center axis as a function of gantry angles which can be removed from the images. The purpose of this study was to establish a novel method for simultaneously carrying out WLT and acquiring a flexmap using rotational irradiation. We also observed long-term changes in flexmaps over a period of five months. We employed rotational irradiation with a rectangular field (30×30 mm). First, the displacement of EPID from the beam center axis, indicated by the ball bearing (BB) center, was evaluated using an in-house program. The location of the BB center was then modified according to WLT. Second, a second irradiation was used to acquire a flexmap. We performed this examination regularly and evaluated long-term changes in the flexmap. It proved feasible to perform WLT and flexmap measurements using our proposed methods. The precision of WLT using rotational irradiation was 0.1 mm. In flexmap analysis, the maximum displacement from the mean value for each angle was 0.4 mm over five months. We have successfully established a novel method of simultaneously carrying out WLT and flexmap acquisition using rotational irradiation. Maximum displacement from the mean in each angle was 0.4 mm over five months.

SU‐E‐J‐14: Evaluation of MV and KV Isocenter Displacement for An IGRT Linac with and Without An Add‐On Micro‐Multileaf Collimator

Purpose: The impact of the add‐on micro MLC (mMLC) on the MV and kV isocenters was investigated. Methods: Winston‐Lutz tests was performed with an add‐on mMLC, and a ball bearing (BB) was placed at the isocenter given by localization lasers. A square MV field was shaped by the mMLC, and the MV field center position relative to the BB position were measured by a portal imager with different gantry angles. A desirable BB position that minimizes the maximum deviation was obtained as an MV isocenter, and the BB center was aligned onto the revised MV isocenter. Using an on‐board kV imaging system, two‐dimensional BB image was acquired during gantry rotation. The displacement of the kV projection image center from the BB for each gantry angle was recorded as ‘flexmap’. After unmounting the mMLC, Winston‐Lutz test and the flexmap measurement were performed again. A phantom was placed on a couch and cone‐beam CT (CBCT) images were acquired with the two different flexmap data. The set‐up errors were calculated and the differences of required couch translations with each of the flexmap data were obtained. Results: After unmounting the mMLC, the MV field center and the kV image center were shifted approximately 0.4 mm and 0.05 mm upward, respectively. Assuming a perfect MV and kV isocenter coincidence with mMLC using the flexmap for CBCT reconstruction, the MV/kV isocenter discrepancy without the mMLC could be calculated by the difference of the above two values, 0.35 mm which satisfies AAPM TG179 guideline of 1 mm. The largest difference of required couch translations between the two flexmap data was 0.07 mm toward the vertical direction, which agreed well with the kV image center shift of 0.05 mm. Conclusion: The MV and kV isocenters defined with the add‐on mMLC are also applicable to image‐guided VMAT without the mMLC.

SU-E-T-61: Towards An End-To-End Test for Image-Guided Stereotactic Radiotherapy Using the ArcCHECK Phantom

Purpose: To develop a reliable and fast end‐to‐end test for image‐guided stereotactic radiotherapy using the ArcCHECK cylindrical phantom from SunNuclear. The test aims to quantify the geometric and dosimetric discrepancies with a single measurement procedure. Methods: A stereotactic treatment plan with eight fields, incorporating four table rotations and four gantry positions, is created on the basis of a planning CT with isocenter at the position marked on the ArcCHECK's surface. Placing the phantom at the laser isocenter and registering the cone‐beam with the planning CT yields the discrepancy between the laser and kV frame. Subsequently, the phantom is moved to the determined kV‐cone‐beam isocenter, where the plan is delivered and cylinder and isocenter dose are measured. Results: The image registration using Elekta's XVI system suggested positional corrections that were in a sub‐millimeter range. The dose in the isocenter was found to be within 2% of the planned dose. Comparing the measured and planned dose distribution in the cylinder using the SNC‐Patient software with a gamma‐criterion (1.5mm/10%; hence an emphasis on positional error) validates the agreement between kV and MV isocenter: three points failed the gamma‐test. The sensitivity was evaluated by applying increasingly larger translocations of the phantom away from the isocenter, which caused the number of failed points to increase monotonically: 29 points failed for a 3mm lateral translation. Conclusion: The implemented test is capable of quantifying the agreement between lasers and kV isocenter, with sub‐millimeter accuracy. Over that, the agreement between kV and MV isocenters could be validated. The pass rate of the gamma‐criterion was well correlated to set‐up errors. A bespoke analysis of the ArcCheck dose distribution is currently being implemented in order to quantify the caustic of the isocenter.

SU-E-T-130: Improved Accuracy for Stereotactic Radiotherapy: An Image-Based Method for Submillimeter Alignment of Linear Accelerator Table Rotation About MV Isocenter

Purpose: Accurate alignment of the linear accelerator table with radiation isocenter is critical for several radiotherapy applications, including stereotactic radiotherapy. We developed a computerized image-based method to align the table rotation axis and the MV isocenter to sub-millimeter accuracy, and observed that alignment was stable over time. Methods: An 8 mm ball-bearing was placed within 0.2 mm of average radiation isocenter using an existing digital star shot alignment methodology. Electronic portal imaging device (EPID) images were acquired between table rotation of −90° to 90° in 30° intervals. A Matlab script was developed to calculate the center of rotation as well as the necessary adjustment to move the table rotational axis to MV isocenter. Adjustment was applied via torque to screws at the linac base. Stability of rotational alignment was measured with 37 measurements over 320 days on two linacs (Linac-A, Elekta Synergy S; Linac-B, Elekta Synergy). Results: Linac-A initially had table alignment inaccuracy greater than 2 mm with respect to MV isocenter. After adjustment, error was significantly decreased from 2.11+/−0.11 to 0.40+/−0.12 mm. This adjustment was stable over the course of 15 measurements over 231 days. Linac-B was not adjusted but tracked from time of commissioning with 20 measurements over 320 days. No discernible shift in couch characteristics was observed (mean error 1.40+/−0.25 mm). The greater variability for Linac-B may relate to the interchangeable two-piece couch, which allows more lateral movement than the one-piece Linac-A couch. Conclusion: Sub-millimeter isocenter alignment was achieved by applying a precision correction to the linac table base. The alignment on two linear accelerators was durable over the course of ten months as assessed by our computerized EPID method. Further investigation on other radiotherapy tables is warranted as this method can significantly improve the quality of stereotactic radiation and other precision techniques.

SU-E-J-12: Flex Compensation for a KV Cone Beam Computed Tomography System Integrated with Linear Accelerator: A Comparison with and without Add-On Micro Multi-Leaf Collimator

Purpose: Due to the weight of the accelerator head and the additional weight of the kV x‐ray tube along with its flat panel detector, flex compensation is required to match the kV cone beam CTimage center to the MV isocenter. However, the flex compensation has not been compared with and without an add‐on micro multi‐leaf collimator. The purpose of this study is to evaluate the impact of the add‐on micro multi‐leaf collimator on the flex compensation or flex map calculation. Methods: Before attaching an add‐on micro multi‐leaf collimator to Elekta Synergy, MV radiation center was determined by a ball bearing and the flex map was obtained. Subsequently, a micro multi‐leaf collimator was attached to the Synergy head and Winston‐Lutz test was performed to locate the most reasonable MV isocenter. Then a ball bearing was positioned at the MV isocenter., and another flex map was obtained and compared to the previously obtained one acquired before attaching the micro multi‐leaf collimator. Results: MV isocenter discrepancy with and without the micro multi‐leaf collimator was 0.5 mm in GT direction which is less than our tolerance limit. The flex map discrepancy with and without the micro multi‐leaf collimator was not significant, either. Conclusions: We could not detect significant flex map discrepancy with and without the micro multi‐leaf collimator. However, flex map measurement needs to be repeated after fully unifying the method for determining the MV isocenter between the two configurations.

SU-E-T-84: Development of a Modified Winston-Lutz Test for Evaluating Errors in IGRT Based Setups

Purpose: To develop a modified Winston‐Lutz test for determining Image Guided Radiotherapy(IGRT) setup accuracy relative to the radiation isocenter. Methods: A Winston‐Lutz cube (Modus Medical Devices) was placed directly on the treatment table and offset from isocenter by 5.0 mm in the longitudinal, lateral, and vertical dimensions. A high resolution Cone‐Beam CT(CBCT) was acquired and aligned to the reference CTimage. The table was shifted and Anterior‐Posterior (AP) and Lateral (LAT) Electronic Portal Imaging Device(EPID)images were acquired, with radiation to cube isocenter agreement determined for each image. This procedure was repeated ten times to determine reproducibility, and then was repeated daily for 51 days. In addition, this test was repeated daily with a random cube shift. Results: The reproducibility test yielded a mean 3D vector isocenter displacement of 0.7 mm ± 0.23 and 0.26 for the AP and LAT images, respectively. Maximum deviation was 1.09 mm for the AP images and 1.1 mm for the LAT images. The daily 5.0 mm offset test yielded a mean displacement of 0.92 mm ± 0.38 with a maximum of 1.7 mm for the AP images and 0.95 mm ± 0.47 with a maximum of 2.0 mm for the LAT images. The random offset test yielded a mean displacement of 0.68 mm ± 0.31 with a maximum of 1.54 mm for the AP images and 0.89 mm ± 0.38 with a maximum of 2.0 mm for the LAT images. Conclusions: A modified Winston‐Lutz test was developed that helps to quantify the maximum error to be expected for IGRT based setup patients. The test took into account inaccuracies due to IGRT isocenter definition, MV isocenter variation, IGRT software alignment, IGRT couch motions, TPS centroid isocenter definition, CT slice width effects, ROI contouring accuracy, and MLC positioning.

SU‐GG‐J‐81: Geometric Accuracy of Imaging Systems on Trilogy MX Using an Automated Geometric Test Tool

Purpose: To characterize the geometric accuracy of the imaging systems of a newly designed accelerator (Trilogy MX, Varian Medical Systems, Palo Alto, CA) using a fully automated, geometric calibration tool. Methods: A geometric calibration tool ‐ consisting of a phantom containing 16 tungsten carbide ball‐bearings and a MV collimator insert with a central pin ‐ has been used to characterize the geometric coincidence of MV and kV imaging equipment with the radiation isocenter of Trilogy MX. Three sets of data are acquired: MV images at multiple MV collimator rotations, and both MV and kV images at multiple gantry angles. The MV collimator images are used to identify the central axis of the MV beam, and the MV images at different gantry angles are then used to identify the MV radiation isocenter. Calculations then determine the corrections needed to move both the MV and kV imagers so that their central pixels exactly align with the projection of the radiation isocenter. The Trilogy MX employs a new control system so corrections can be downloaded to the control system and used to physically correct the imager positions while the gantry is being rotated. The positions of the imagers have been verified using the geometric calibration tool and a Winston‐Lutz test. Results: Coincidence between the MV isocenter ‐ measured using the geometric tool ‐ and the central imager pixels of both imagers was less than ±0.1mm. The coincidence between the central pixels of both imagers and the projection of the Winston‐Lutz BB was better than ±0.3mm. Conclusions: An automated tool for calibrating the geometry of MV and kV imaging equipment has been developed and tested. The tool gives similar results to conventional Winston‐Lutz measurements. The geometric accuracy of the imaging systems of Trilogy MX is better than ±0.3mm.

SU‐GG‐T‐466: Quality Assurance and An Assessment of Using Cone‐Beam CT with Extra‐Cranial Frameless‐Stereotactic Radiotherapy

Purpose: Standard quality assurance of Varian's trilogy frameless‐stereotactic radiotherapy(SRT) relies on the a match between the system's optical isocenter and room lasers. It does not match the static kV isocenter of the On‐Board‐Imaging(OBI) or the dynamic kV isocenter of the CBCT, nor does it QA the MV isocenter. Our purpose was to establish a quality assurance(QA) procedure for all isocenters including the MV isocenter for frameless‐SRT. Method and Materials: An acrylic phantom with dimensions of 15×15×15 cm3 was constructed. The phantom included a removable slab that contains two machined holes designed to hold a stainless‐steel ball‐bearing(BB). One machined hole is at the center of the cube, and another is off to one side to established phantom orientation during scanning. The BB is removed during CBCT imaging to minimize artifacts and is replaced for kV and MV images. The Trilogy frameless‐SRT array is mounted atop of the phantom and it is positioned at the LINAC isocenter. Repeated CB‐3D, 2D‐OBI datasets were acquired to determine the system's ability to detect and correct for known table shifts. All shifts observed and recovered by the SRT system were tabulated. Each recovered shift was verified by a Winston‐Lutz(WL) test. The differences between known shift, OBI, CBCT, SRT and the WL test were recorded and tabulated. The couch's ability to recover to a known initial position was also tested and tracked by SRT camera system. Results and Conclusion: Initial results showed the couch had a precision of &lt;1mm. The CBCT, SRT‐array and OBI correctly predicted the movement within 0.13±0.35mm, 0.14±0.24 mm and 0.21±0.38mm, respectively. The mean agreement between the kV, MV and SRT‐optical isocenters was 0.4 mm±0.6 mm via WL. Initial results show that this QA of four isocenters has the potential to allow the use of CBCT and OBI during extra‐cranial frameless‐SRT as an additional modality during treatment.

MO‐E‐AUD‐05: Validating a Video‐Based 3‐D Surface Imaging System and Testing Its Use to Set‐Up CBCT QA Device

Purpose: To validate resolution of a video‐based 3‐D surface imaging system before its clinical use and to test feasibility of using system in setting‐up a cone beam CT (CBCT) QA device to accelerate the total QA process. Method and Materials: A CBCT QA alignment tool called Ball‐Bearing device (BBD) (Elekta Inc.) is designed to check the coincidence between the MV and kV beams' isocenters. The micro‐stepping meter of the BBD allows precision adjustment to better than 0.01 mm in three directions. A foam phantom was added to the BBD in order to generate a large 3‐D surface image which can be detected by a video‐based 3‐D surface imaging system (AlignRT system, VisionRT Ltd., London UK). The BBD‐phantom has been imaged by the AlignRT system with changes in position (0.0 to 8.0 mm) in all three directions independently. The phantom shifts were determined by AlignRT and compared to the preset ones. After this validation test, AlignRT has been used in the initial set‐up of the CBCT device in replacement of the iterative process required to set the BBD to the MV isocenter. By carefully setting‐up the BBD using the iterative imaging method to place the ball‐bearing to MV beam isocenter, a reference image of BBD‐phantom was taken using AlignRT. Subsequently, AlignRT was used in BBD set‐up; multiple tests have been performed and testing continues. Results: AlignRT can detect 0.1 mm change in both lateral and longitudinal directions, 0.3 mm in vertical direction. AlignRT reduced set‐up time for CBCT QA device by a factor of two compared to traditional iterative method. Conclusions: AlignRT can detect fractional mm shifts in the BBD‐phantom; vertical resolution is not as sensitive as horizontal. AlignRT can be used in setting‐up the CBCT QA device (providing appropriate phantom added) and significantly reduces set‐up time.

TH‐C‐ValB‐03: Quality Assurance Procedure for a KV Cone‐Beam Device

Purpose: To establish a quality assurance procedure for kV cone‐beam (CB) devices, and use the procedure to assess the accuracy and precision of a particular CB unit. Introduction: The introduction of the Elekta CB system has created a need for a quality assurance (QA) procedure to test the system's reliability and accuracy and it's alignment with the MV treatment beam isocenter. Materials and Methods: An acrylic phantom with dimensions of 18×19×18 cm3 was used for this work. The phantom included a central slab made of polystyrene that contained two drill holes. One was drilled to the center of the cube, and the other was off to one side and established phantom orientation during scanning. The holes were left open during CT and CB imaging to minimize artifacts. The phantom was first positioned with the center hole at the accelerator mechanical isocenter. Repeated cone‐beam datasets were acquired to determine the system's ability to detect and correct for known table shifts. Finally, the phantom was positioned with CB guidance and the beebee was placed in the center hole. The Winston‐Lutz (WL) test in the phantom was preformed using the electronic portal imager (EPID). The differences between known and CB determined shifts and the WL test analysis were tabulated. Results: and Conclusion: The results showed that the cone‐beam system was capable of determining the shifts applied to the QA phantom to within 0.5 ± 0.4 mm with the largest difference between the known and calculated shift of 1.9 mm. The mean agreement between the kV and MV isocenters was 0.5 mm ± 0.4 mm with the largest deviation of 1.3 mm. This QA study supports a conclusion that the Elekta CB system can be reliably used for positioning of patients with the accuracy in a 1 to 2 mm range.