Couch Positioning Research Articles

MR-linac daily semi-automated end-to-end quality control verification.

Adaptive radiation therapy (ART) on the integrated Elekta Unity magnetic resonance (MR)-linac requires routine quality assurance to verify delivery accuracy and system data transfer. In this work, our objective was to develop and validate a novel automated end-to-end test suite that verifies data transfer between multiple software platforms and quantifies the performance of multiple machine subcomponents critical to the ART process. We designed and implemented a software tool to quantify the MR and megavoltage (MV) isocenter coincidence, treatment couch positioning consistency, isocenter shift accuracy for the adapted plan as well as the MLC and jaw position accuracy following the beam aperture adaptation. Our tool employs a reference treatment plan with a simulated isocenter shift generated on an MR image of a readily available phantom with MR and MV visible fiducials. Execution of the test occurs within the standard adapt-to-position (ATP) clinical workflow with MV images collected of the delivered treatment fields. Using descriptive statistics, we quantified uncertainty in couch positioning, isocentre shift as well as the jaw and MLC positions of the adapted fields. We also executed sensitivity measurements to evaluate the detection algorithm's performance. We report the results of 301 daily testing instances. We demonstrated consistent tracking of the MR-to-MV alignment with respect to the established value and to detect small changes on the order of 0.2mm following machine service events. We found couch position consistency relative to the test baseline value was within 95% CI [-0.31, 0.26mm]. For phantom shifts that form the basis for the plan adaptation, we found agreement between MV-image-detected phantom shift and online image registration, within±1.5mm in all directions with a 95% CI difference of [-1.29, 0.79mm]. For beam aperture adaptation accuracy, we found differences between the planned and detected jaw positions had a mean value of 0.27mm and 95% CI of [-0.29, 0.82mm] and -0.17mm and 95% CI of [-0.37, 0.05mm] for the MLC positions. Automated fiducial detected accuracy was within 0.08±0.20mm of manual localization. Introduced jaw and MLC position errors (1-10mm) were detected within 0.55mm (within 1mm for 15/256 instances for the jaws). Phantom shifts (1.3 or 5mm in each cardinal direction) from a reference position were detected within 0.26mm. We have demonstrated the accuracy and sensitivity of a daily end-to-end test suite capable of detecting errors in multiple machine subcomponents including system data transfer. Our test suite evaluates the entire treatment workflow and has captured system communication issues prior to patient treatment. With automated processing and the use of a standard vendor-provided phantom, it is possible to expand to other Unity sites.

Designing Robust and Automated Treatment Plans Utilizing Dynamic Couch Motion With Automated Treatment Planning: Optimizing Couch Motion for Precise Delivery ― Part A

<h3>Purpose/Objective(s)</h3> This study finds a relationship among couch speed, dose rate, and MLC, allowing gradual start and stop positioning of the couch for treatments utilizing dynamic couch-based dose delivery at static gantry positions. <h3>Materials/Methods</h3> We introduce a gantry static couch motion (GsCM) based technique, which delivers the dose using a dynamic couch and static gantry positions. It is imperative to study couch motion behavior for techniques like GsCM, where the patient is a moving target throughout the dose delivery. Sudden changes in couch speed can result in a shift in position and discomfort for the patient. Therefore, it is necessary to implement a smooth start, a constant speed in the middle, and deceleration at the couch's stopping positioning. We studied the relationship among the following parameters: dose rate, MLC speed, and couch motion. To fulfill this, we simulated a single treatment arc manually in the treatment planning system that includes multiple couch positions at static gantry positions. All the fields were merged into a single deliverable arc while exporting into the Extensible Markup Language (XML) format using the TPS scripting API. Developer mode on a Linac performed the treatment delivery. The second part of this study looked into the various ways to control the couch motion. Methods tried included a fixed couch speed versus controlling the couch speed by limiting the dose rate. Trajectory log files helped analyze the couch positional error (RMSE). <h3>Results</h3> We found couch speed optimization by limiting the dose rate is better than the MLC speed or couch speed. A slow ramping up and down of the speed with a constant speed in the middle of an arc field is achievable. Linac log file analysis showed minimum couch position error (RMSE: median 0.035⁰, range 0.02-0.0.045⁰) for 1° couch speed as compared to system defined couch speed (RMSE: median 0.07⁰, range 0.05-0.12⁰), and (RMSE: median 0.07⁰, range 0.06-0.09⁰) for 2° couch speed respectively. Even though 1° couch speed provided a minimum couch RMSE error but at the expense of a longer treatment time which is not suitable for keeping the patient longer on the treatment table. For the dose rate limiting option, couch positioning error at the start and stop location (RMSE: median 0.008⁰, range 0.006-0.02⁰) showed a gradual and accurate ramping up and down of the couch, and at the middle of the arc remained the same as noticed for the system defined couch speed (RMSE: median 0.09⁰, range 0.065-0.12⁰). <h3>Conclusion</h3> This feasibility study serves as an end-to-end test and is probably the first of its kind that thoroughly validates various ways to control the couch speed and its integration with patient motion during the dose delivery. Treatments utilizing dynamic couch-based dose delivery must include patient immobilization. Our study takes patient motion into account and provides gradual start and stop motions of the couch, which helps minimize the patient motion for treatments utilizing dynamic couch motion.

Performance of a linear accelerator couch positioning quality control task using an electronic portal imaging device.

Short and semi-automated quality assurance (QA) programs are becoming one of the most popular and highly demanding tasks in radiotherapy. The current research investigates the accuracy of a four degrees of freedom (4DoF) medical linear accelerator couch positioning with a fast and accurate method based on images acquired using an electronic portal imaging device (EPID). An accurate EPID QA phantom and a proper in-house code were used. A Siemens medical linear accelerator equipped with an a-Si EPID was used to acquire portal images. For verifying the mechanical performance of the EPID positioning, EPID sensitivity, and accuracy of the code response from the image processing point of view were investigated. To characterize the results, three deviations in the phantom positioning were deliberately created. The translational and rotational displacements of the treatment couch were then evaluated. The loading effect on the treatment couch was then investigated. The results of prerequisite tests, including the mechanical performance of the EPID, and the sensitivity and accuracy of the recognition codes, were assessed. The results were found to be within the tolerance range reported at AAPM TG-142. The mean deviations of the tests between expected and measured displacements by 4DoF treatment couch were found to be 0.13° ± 0.11°, 0.12 ± 0.17mm, 0.17 ± 0.13mm, and 0.04 ± 0.09mm for rotational, longitudinal, lateral, and vertical shifts, respectively. The results showed that the proposed method is a reliable and fast approach to find the uncertainties occurring intreatment couch positioning.

Treatment couch positioning uncertainties using an EPID-based method

Introduction: The accuracy of patient positioning plays an important role for radiotherapy tasks. Short and automated Quality Assurance (QA) programs are becoming one of the challenging tasks in Radiotherapy. The current study, investigates the accuracy of treatment couch positioning with four degrees of freedom (4DoF) using a fast and accurate method based on an image acquired using an electronic portal imaging device (EPID). Materials and Methods: A Siemens medical linear accelerator equipped with an a-Si EPID was used to capture portal images. An accurate EPID QA phantom (PTW EPID QC Phantom®) and a proper in-house algorithm were used. The phantom has different sections to assess EPID image quality. Preliminary tests including Mechanical performance of the EPID positioning, EPID sensitivity and the code response from image processing point of view were investigated through several deviations deliberately created to evaluate translational and rotational displacements of treatment couch. Loading effect on treatment couch was also investigated. Results: The results of EPID preliminary tests and in-house code reliability were within tolerances expected. The mean deviations of tests for (4DoF) treatment couch were 0.25°±0.10˚, 0.428±0.112 mm, 0.428±0.112 mm and 0.286±0.112 mm for rotational, longitudinal, lateral and vertical shifts, respectively. The results of incline caused by a 130kg loading on the couch showed that the proposed method is a reliable and fast to find the uncertainties occurred for treatment couch positioning. Conclusion: As a part of linac QA program, treatment couch tests including translational and rotational motions under unloaded/loaded conditions were performed. The corresponding errors for table positioning including minimum detectable rotational and translational displacements, and couch sagging effect under loaded conditions were evaluated using an EPID. Creating one major test for the whole assessments for couch checks, fast and accurate and no labour intensive procedure, and electronic portal image instead of film and other imagers can be addressed as advantages of the method proposed.

Stereoscopic X-ray imaging, cone beam CT, and couch positioning in stereotactic radiotherapy of intracranial tumors: preliminary results from a cross-modality pilot installation.

BackgroundTo assess the accuracy and precision of a fully integrated pilot installation of stereoscopic X-ray imaging and kV-CBCT for automatic couch positioning in stereotactic radiotherapy of intracranial tumors. Positioning errors as detected by stereoscopic X-ray imaging are compared to those by kV-CBCT (i.e. the accuracy of the new method is verified by the established method), and repeated X-ray images are compared (i.e. the precision of new method is determined intra-modally).MethodsPreliminary results are reported from a study with 32 patients with intracranial tumors. Patients were treated with stereotactic radiotherapy guided by stereoscopic X-ray imaging and kV-CBCT. Patient positioning was automatically corrected by a robotic couch. Cross-modal discrepancies in position detection were measured (N = 42). Intra-modal improvements after correction and re-verification by stereoscopic X-ray imaging were measured (N = 70). The accuracy and precision of stereoscopic X-ray imaging and the accuracy and precision of CBCT were confirmed in phantom measurements (N = 12 shifts of a ball bearing phantom, N = 24 shifts of a head phantom).ResultsAfter correction based on stereoscopic X-ray imaging 95% of residual mean errors were below 0.4, 0.4, 0.5, and 0.7 mm (lateral, longitudinal, vertical, radial, respectively). Stereoscopic X-ray imaging and CBCT were in close agreement with an average discrepancy of 0.1, 0.5, 0.3 and 0.8 mm, respectively. 95% of discrepancies were below 0.8, 1.2, 1.0, and 1.4 mm, respectively. After correction and re-verification by stereoscopic X-ray imaging, the remaining intra-modal residual error was consistent with zero (p = 0.31, p = 0.48, p = 0.81 in lateral, longitudinal, and vertical direction; p-values from two-tailed t-test). The inherent technical accuracy and precision of stereoscopic X-ray imaging and the accuracy and precision of CBCT were found to be of the order of 0.1 mm in controlled phantom settings.ConclusionsIn a routine clinical setting, both stereoscopic X-ray imaging and CBCT were able to reduce positioning errors by an order of magnitude. The end-to-end precision of the system, measured from the discrepancy (mean) between ExacTrac and CBCT, in a clinical setting seems to be about 0.8 mm radially, including couch positioning. The precision (measured from repeatability of ExacTrac, intra-modal) was found to be about 0.7 mm radially in a clinical setting.

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.

MO‐F‐CAMPUS‐J‐03: Evaluation and Use of Cubic Phantoms for Daily IGRT QA Process with TrueBeam 6DoF Couch

Purpose:To improve efficiency and efficacy of Daily IGRT QA process and to prepare for Varian TrueBeam 6DoF couch implementationMethods:MIMI phantom from Standard Imaging was compared to PentaGuide phantom from Modus Medical in order to replace and update our daily IGRT QA process. The following metrics were used as our evaluation criteria: contrast structures, predefined shift mark, TG‐142 compliance, support for 6DoF and ease of use. To improve process compliance, daily IGRT QA set up and process, an acrylic in‐house baseplates with known offsets were designed. New Daily IGRT QA protocol for 3DoF and 6DoF systems was developed and implemented into our practice. The selected phantom was used for the commissioning of Varian TrueBeam 6DoF couches and test the reproducibility and accuracy of 2D/2D and 3D/3D matching and localization processes. Baseline and tolerances for Daily IGRT QA for 6DoF couch were determined.Results:Based on our evaluation criteria, the MIMI phantom fit best our QA processes and matching goals. Comprehensive daily IGRT QA which includes CBCT 3D automatic registration, couch positioning, coincidence of kV/MV isocenters, image quality, and laser alignment were accurately and efficiently validated using the MIMI phantom and the setup baseplate with Varian TrueBeam and EX systems with 3DoF and 6DoF couches. Also, the use of MIMI phantom and setup baseplate was found to be a useful and effective tool for the commissioning of the Varian TrueBeam 6DoF couches.Conclusion:Implementation of MIMI phantom improved our Daily IGRT QA process by eliminating the use multiple phantoms which resulted in 40% Daily IGRT QA time reduction. Feedback from therapists has been positive and 100% compliance to new protocol has been achieved with minimal training. MIMI phantom was used to test the stability of the couch and to set baseline for Daily QA target and tolerance values.

Reducing patient posture variability using the predicted couch position

A method is presented in which the couch position is predicted before the treatment instead of obtaining a reference position at the first treatment fraction. This prevents systematic differences in patient posture between preparation and treatment. In literature, only limited data are available on couch positioning. We position our patients at the planned couch position, allowing a small difference between skin marks and lasers, followed by online imaging. For a 3-month period, our standard deviations (mm) in couch position in the vertical, longitudinal, and lateral directions were head and neck—1.6, 2.8, and 2.5; thorax—2.9, 5.5, and 4.5; breast—3.0, 4.1, and 4.0; and pelvis—3.5, 4.0, and 4.7, respectively. We have improved the reproducibility of patient posture in our institute by using the predicted couch position. Our data may serve as a reference for other institutes because the couch position variation is less than that published in literature.

SU‐E‐T‐434: Evaluation of the Machine Performance Check (MPC) Program for Truebeam 2.0

Purpose:Machine Performance Check (MPC) is a software application used to verify that the TrueBeam machine is operating within major specifications prior to treatment. Used in combination with a phantom named Isocal, it verifies beam output, beam uniformity, treatment isocenter size, coincidence of treatment and imaging isocenters, positioning accuracy of kV and MV imaging systems, accuracy of collimator and gantry rotation angle, positioning accuracy of jaws and MLC leafs, and couch positioning. The tests can be performed semi‐automatically and requires approximately 10 minutes of machine time. It is the purpose of this study to report the performance of this program.Methods:A pre‐release version of the MPC tool was installed on a Truebeam linac with 6D couch at our center. Baseline beam output measurements were taken for 5 photon beams (6–15 MV, 6 FFF, 10 FFF) and 5 electron beams (6–20 MeV). Deviations from the baseline output were subsequently recorded for several days and compared against independent measurements from a PTW farmer chamber and our daily QA device (Fluke Biomedical Tracker) as part of an ongoing evaluation.Results:The beam output deviations between the MPC and the PTW chamber measurements were within ±0.7% for photons beams and ±1.0% for electrons beams. This was similar to the tracker performance. There were some isolated incidents where the MPC measurements had unexplained spikes (&gt;3%) that disappeared on a repeat measurement. MPC was also able to detect maximum positioning errors in the jaws (1.12 mm), MLCs (1.14 mm), and couch roll (0.11°).Conclusion:Overall, the ability of the MPC to monitor linac output stability was comparable to that of ionization chamber‐based measurements. MPC also provided fast daily mechanical tests not currently available in the clinic. How best to utilize this previously unavailable data is still under investigation.

SU‐E‐J‐30: Intuitive Display Coordinates for Six‐Degree‐Of‐Freedom Treatment Couches

Purpose: To define couch display coordinates of six‐degree‐of‐freedom treatment couches in a manner that is more intuitively linked to the position of isocenter in the patient, permits easy identification of irregularities in couch positioning, and permits the tightest possible override tolerances for all couch motions. The display coordinates should be calculated from the IEC table‐top coordinate system with no patient‐specific information. Methods: Couch display coordinates are proposed as the position of radiation isocenter in IEC table‐top coordinates. Coordinate systems are compared across multiple radiation therapy machines and then compared to the proposed display coordinates. Examples are provided showing how the proposed definition can be applied across a variety of different couch designs. Results: Consistent with IEC standards, most machines with six‐degree‐of‐freedom couches, and particularly those with robotic couches, measure rotations about a point fixed with respect to the couch and independent of all translational motions. This convention has the undesirable consequence of eliminating the one‐to‐one relationship between the translational coordinates displayed and the position of the radiation isocenter in the patient. By defining the display coordinates as the position of radiation isocenter in IEC table‐top coordinates, the one‐to‐one relationship is reestablished. Examples provided show that the proposed definition insures that a one‐to‐one relationship between the position of isocenter in the patient and the three translational display coordinates regardless of the values of the three angular degrees of motion. Conclusion: The examples illustrate how the proposed couch display coordinates lead to easier identification of irregularities in couch positioning such as identifying laterality of isocenter or shifts in multi‐isocenter treatments. The proposed display also allows tighter tolerances for couch overrides. These advantages suggest a revised IEC display standard following these conventions has the potential to reduce setup errors at the treatment machine and enhance the utility of the couch display coordinates.

SU-E-T-543: The Use of a Proportional-Integral-Derivative Design for Optimized Real-Time Head Motion Correction in Frameless SRS

Purpose: While less invasive, the immobilization devices used in frameless SRS techniques generally allow for more head movement than a frame, leading to a larger PTV. To maintain traditional SRS accuracies, we have developed a robotic 3D head motion stage to correct for these sub‐ millimeter head deviations occurring during treatment. To achieve optimal head motion management we report on our use of a proportional‐integral‐ derivative (PID) feedback loop to monitor and compensate for real‐time patient motion during frameless SRS. Methods: A PID controller consists of three corrective parameters: a proportional term to account for the error difference between the desired patient position and the actual position; an integral term that accelerates the position correction toward the desired setpoint value; and a derivative term that accounts for the instantaneous rate of change of the patient position error. These parameters were tuned according to manual and heuristic Zielger‐Nichols methods. The PID algorithm was integrated into existing Labview software designed to control patient motion via a computer‐controlled 3D stepper motor stage. For realtime head position feedback either a Polaris external marker tracker or VisionRT system was used. The PID correction was tested both on phantom and healthy volunteers. Results: With volunteers in a relaxed supine position, the PID method maintained head motion to under 0.25 mm as based on the optical monitoring system in all three directions for greater than 90% of the time. Artificial introduction of extreme 1–5 mm head shifts by manual couch moving were corrected 2–3 times faster by PID when compared to our prior constant motor velocity approach Conclusions: In the event of patient head deviations, a properly tuned PID controller can offer significant time sparing advantages. This would allow for quicker initial setup after coarse couch positioning and a higher duty cycle during treatment.

Feasibility study of performing IGRT system daily QA using a commercial QA device

The purpose of this study was to investigate the feasibility of using a single QA device for comprehensive, efficient daily QA of a linear accelerator (Linac) and three image‐guided stereotactic positioning systems (IGSPSs). The Sun Nuclear Daily QA 3 (DQA3) device was used to perform daily dosimetry and mechanical accuracy tests for an Elekta Linac, as well as daily image geometric and isocenter coincidence accuracy tests for three IGSPSs: the AlignRT surface imaging system; the frameless SonArray optical tracking System (FSA) and the Elekta kV CBCT. The DQA3 can also be used for couch positioning, repositioning, and rotational tests during the monthly QA. Based on phantom imaging, the Linac coordinate system determined using AlignRT was within 0.7 mm/0.6° of that of the CBCT system. The difference is attributable to the different calibration methods that are utilized for these two systems. The laser alignment was within 0.5 mm of the isocenter location determined with the three IGSPSs. The ODI constancy was ± 0.5 mm. For gantry and table angles of 0°, the mean isocenter displacement vectors determined using the three systems were within 0.7 mm and 0.6° of one another. Isocenter rotational offsets measured with the systems were all ≤ 0.5°. For photon and electron beams tested over a period of eight months, the output was verified to remain within 2%, energy variations were within 2%, and the symmetry and flatness were within 1%. The field size and light‐radiation coincidence were within 1mm ± 1 mm. For dosimetry reproducibility, the standard deviation was within 0.2% for all tests and all energies, except for photon energy variation which was 0.6%. The total measurement time for all tasks took less than 15 minutes per QA session compared to 40 minutes with our previous procedure, which utilized an individual QA device for each IGSPS. The DQA3 can be used for accurate and efficient Linac and IGSPS daily QA. It shortens QA device setup time, eliminates errors introduced by changing phantoms to perform different tests, and streamlines the task of performing dosimetric checks.PACS number: 87.56.Fc

高解像度ディジタルカメラを用いた高精度放射線治療用６軸寝台の位置精度測定法の検討

In this study, we proposed and evaluated a positional accuracy assessment method with two high-resolution digital cameras for add-on six-degrees-of-freedom radiotherapy (6D) couches. Two high resolution digital cameras (D5000, Nikon Co.) were used in this accuracy assessment method. These cameras were placed on two orthogonal axes of a linear accelerator (LINAC) coordinate system and focused on the isocenter of the LINAC. Pictures of a needle that was fixed on the 6D couch were taken by the cameras during couch motions of translation and rotation of each axis. The coordinates of the needle in the pictures were obtained using manual measurement, and the coordinate error of the needle was calculated. The accuracy of a HexaPOD evo (Elekta AB, Sweden) was evaluated using this method. All of the mean values of the X, Y, and Z coordinate errors in the translation tests were within ±0.1 mm. However, the standard deviation of the Z coordinate errors in the Z translation test was 0.24 mm, which is higher than the others. In the X rotation test, we found that the X coordinate of the rotational origin of the 6D couch was shifted. We proposed an accuracy assessment method for a 6D couch. The method was able to evaluate the accuracy of the motion of only the 6D couch and revealed the deviation of the origin of the couch rotation. This accuracy assessment method is effective for evaluating add-on 6D couch positioning.

SU‐GG‐T‐555: Dosimetric Effect of Couch Position Variation for Intensity Modulated Arc Therapy

Purpose: To investigate the dosimetric effect of variation in treatment couch positioning relative to isocenter for intensity modulated arc therapy. Method and Materials: 5 RapidArc (Varian Medical Systems, Palo Alto, CA) treatment plans (2 head and neck, 2 prostate, and a lung) were selected that were calculated including the treatment couch with the support rails in the lateral position. Calculations were done for the couch positioned ±3 cm relative to isocenter and for the support rails in the center position. Dose‐volume histograms were compared for the targets and critical structures. Results: For ±3 cm shift, the largest deviation of the minimum, maximum, and mean target doses was 0.2%. For the critical structures, the largest deviation of the minimum, maximum, and mean doses was 0.6%, 0.4%, and 0.4%, respectively. The largest deviations occurred for the head and neck plans. Excluding the head and neck plans, the largest deviation of the minimum, maximum, and mean critical structure doses was 0.2%. For the support rails in the center, largest deviation of the minimum, maximum, and mean target doses from the planned lateral rail position was 1.6%, 1.6%, and 1.1%, respectively. For the critical structures, the largest deviation of the minimum, maximum, and mean doses from the planned lateral rail position plan was 0.9%, 3.8%, and 1.2%, respectively. Conclusion: The effect of couch shifts of up to 3 cm relative to isocenter is small. For positioning systems indexed to the treatment couch, the daily variation of the couch position can be safely ignored. However, for positioning systems that are independent of the couch, care must be taken to limit the daily deviation. The support rail positions have a more significant effect than couch position relative to isocenter.Conflict of Interest: First author has a sponsored research agreement with Varian.

MO‐FF‐A2‐04: An Accurate Mechanical Quality Assurance Procedure for a New High Performance Linac

Purpose T o establish QA procedures for the mechanical systems of a new linac that has been developed to deliver radiation, using image‐guidance, to the target with improved spatial accuracy. The procedures are required to be able to detect mechanical errors of much less than 1 mm, be independent of the linac's own readouts and calibration procedures, and be fast enough for the physicist to perform on a monthly or more frequent schedule. Method and Materials In image guided delivery, mechanical properties that will affect the spatial accuracy with which dose is delivered include:• radiation isocenter ‐imaging origin displacement,• position errors of the jaws and MLC leaves,• accuracy with which the patient support couch can respond to a change in position request.To measure these quantities, we use the machine's kV, MV, and infra‐red imaging systems. We report on the techniques used, and the estimates of their accuracy. Results The preliminary estimate of the measurement uncertainty of the radiation isocenter — imaging origin offset is ± 0.3 mm. The observed offset is within the measurement error. The accuracy of the field size seen in the MV images is ± 0.2mm. Couch accuracy for shifts of up to 2 cm, the magnitudes expected using image guidance, was found to be within the measurement error. The ability to control the machine using scripts allows gantry and collimator positioning, couch positioning, beam delivery and imaging in all modes, to be sequenced and performed automatically. Thus the time required for a complete mechanical QA procedure is greatly shortened. Conclusion The imaging components of a new linac can be positioned with sufficient reproducibility and accuracy to allow their use in a mechanical QA program that can achieve the sub‐mm accuracy needed for this machine. Research supported by Varian Medical Systems

Virtual micro MLC commissioning

The resolution of multileaf collimators (MLCs) is limited by their finite leaf width. A commercial package (HD‐270™) uses 3D couch translation and leaf adjustments to emulate smaller leaf widths. In this paper, we report on the commissioning of this feature using software testing, dial gauge measurements, and film dosimetry. We also identify a variety of limitations: software bugs and truncation artifacts, MLC leaf positioning uncertainties (random variations, systematic gantry dependence and backlash), and uncertainties in couch positioning. These reduce the capabilities of this implementation below that achievable theoretically.PACS numbers: 87.53.Kn, 87.53.Uv, 87.53.Xd

Technical note: the implementation of patient position correction using a megavoltage imaging device on a linear accelerator.

The problem of using information from the analysis of megavoltage images to adjust patient set-up has been addressed. In the case of rotational corrections it has been assumed that the treatment head is to be adjusted, although for gantry angles of 0 degree and 180 degrees couch rotation may be used. In the case of translational shifts adjustment of the collimator jaws or of the couch have both been considered for arbitrary combinations of couch and gantry angle. For couch movement the case has been considered where it is desirable to minimize both the number of parameters to be adjusted and also the magnitude of the change in the patient's position. Values obtained for frequently used set-up parameters have been presented. Adjustment of the treatment couch positioning is the most desirable option, as this should bring the patient closer to the correct position for subsequent treatment fields. However, rotational errors are not correctable for all gantry angles and furthermore the collimator settings may be set more accurately than those of the treatment couch. Hence, in some cases, adjustment of the collimation system may be desirable or necessary. The formulae given in Equations (13) to (18) are currently being used in an intervention study to correct patient set-up during the course of a treatment fraction.