Collimator Quality Research Articles

What patient positioning in chest X-ray is still acceptable? – An empirical study on thresholding quality metrics

AbstractBackground and AimIssues in patient positioning during chest X-ray (CXR) acquisition impair diagnostic quality and potentially increase radiation dose. Automated quality assessment was proposed to address this. Our objective is to determine thresholds on some quality control metrics following international guidelines, that represent expert knowledge and can be applied in a comprehensible and explainable AI approach for such an automatic quality assessment.Materials and MethodsAn AI-method estimating collimation distance to the ribcage, balancing between both clavicle heads, and number of ribs above the diaphragm as metrics for collimation, rotation, and inhalation quality was applied on 64,315 posteroanterior CXR images from a public dataset (ChestX-ray8). From this set 920 CXR images were sampled and manually annotated to gain additional trusted reference metrics. Seven readers from different institutions then classified the acquisition quality of these images independently into okay, inadequate, or unacceptable following the criteria of international guidelines. Optimal thresholds on the metrics were determined to reproduce these classes using the metrics only.ResultsA fair to moderate agreement between the experts was found. When disregarding all inadequate rates a classification on the metrics was able to separate okay rated cases from unacceptable cases for collimation (AUC 0.97), rotation (AUC = 0.93) and inhalation (AUC = 0.97).ConclusionSuitable thresholds were determined to reproduce expert opinions in the assessment of the most important quality criteria in CXR acquisition. These thresholds were finally applied on the AI-method's estimates to automatically classify image acquisition quality comprehensibly and according to the guidelines.

A novel EPID-based MLC QA method with log files achieving submillimeter accuracy.

The purpose of this study is to develop an electronic portal imaging device-based multi-leaf collimator calibration procedure using log files. Picket fence fields with 2-14mm nominal strip widths were performed and normalized by open field. Normalized pixel intensity profiles along the direction of leaf motion for each leaf pair were taken. Three independent algorithms and an integration method derived from them were developed according to the valley value, valley area, full-width half-maximum (FWHM) of the profile, and the abutment width of the leaf pairs obtained from the log files. Three data processing schemes (Scheme A, Scheme B, and Scheme C) were performed based on different data processing methods. To test the usefulness and robustness of the algorithm, the known leaf position errors along the direction of perpendicular leaf motion via the treatment planning system were introduced in the picket fence field with nominal 5, 8, and 11mm. Algorithm tests were performed every 2weeks over 4 months. According to the log files, about 17.628% and 1.060% of the leaves had position errors beyond ±0.1 and ±0.2mm, respectively. The absolute position errors of the algorithm tests for different data schemes were 0.062±0.067 (Scheme A), 0.041±0.045 (Scheme B), and 0.037±0.043 (Scheme C). The absolute position errors of the algorithms developed by Scheme C were 0.054±0.063 (valley depth method), 0.040±0.038 (valley area method), 0.031±0.031 (FWHM method), and 0.021±0.024 (integrated method). For the efficiency and robustness test of the algorithm, the absolute position errors of the integration method of Scheme C were 0.020±0.024 (5mm), 0.024±0.026 (8mm), and 0.018±0.024 (11mm). Different data processing schemes could affect the accuracy of the developed algorithms. The integration method could integrate the benefits of each algorithm, which improved the level of robustness and accuracy of the algorithm. The integration method can perform multi-leaf collimator (MLC) quality assurance with an accuracy of 0.1mm. This method is simple, effective, robust, quantitative, and can detect a wide range of MLC leaf position errors.

How good are collimated Gaussian beams produced with engineered diffusers?

Collimating a Gaussian beam from an uncollimated laser source has been achieved via the deployment of engineered diffusers (EDs)—also referred to as light shaping diffusers. When compared to conventional pinhole-based optical collimation systems, this method of beam collimation ensures high optical transmission efficiency at the expense of the introduction of additional speckle and a resulting reduction in spatial coherence. Despite a lower collimation quality, these ED-produced collimated beams are attractive and promising in terms of their deployment in various benchtop or tabletop systems that involve shorter beam propagation distances of up to a few meters while requiring a high optical power throughput. This paper aims to further the understanding of collimation quality and propagation properties of ED-produced Gaussian collimated beams via carefully designed experiments and accompanying analysis. We measure and document the beam divergence, Rayleigh distance, and M2 factor, as well as evolution of the wavefront radius of curvature (RoC), of these ED-generated beams over a few meters of propagation—a propagation distance which encapsulates a vast majority of optical systems. We further investigate the changes in the beam profile with the addition of a laser speckle reducer (SR) and compare the ED-produced beams with a near-ideal collimated beam produced with spatial filtering systems.

Commissioning of the First MRlinac in Latin America.

To show the workflow for the commissioning of a MRlinac, and some proposed tests; off-axis targets, output factors for small fields, dose in inhomogeneities, and multileaf collimator quality assurance (MLC QA). The tests were performed based on TG-142, TG-119, ICRU 97, TRS-398, and TRS-483 recommendations as well as national regulations for radiation protection and safety. The imaging tests are in agreement with the protocols. The radiation isocenter was 0.34 mm, and for off-axis targets location was up to 0.88 mm. The dose profiles measured and calculated in treatment planning system (TPS) passed in all cases the gamma analysis of 2%/2 mm (global dose differences). The output factors of fields larger than 2 cm × 2 cm are in agreement with the model of the MRlinac in the TPS. However, for smaller fields, their differences are higher than 10%. Picket fence test for different gantry angles showed a maximum leaf deviation up to 0.2 mm. Displacements observed in treatment couch adding weight (50 kg) are lower than 1 mm. Cryostat, bridge, and couch attenuation was up to 1.2%, 10%, and 24%, respectively. The implemented tests confirm that the studied MRlinac agrees with the standards reported in the literature and that the strict tolerances established as a baseline should allow a smoother implementation of stereotactic treatments in this machine.

MR-linac MLC positioning QA by digitally stitching dual double-exposed films.

The picket fence (PF) test is highly recommended for multi-leaf collimator (MLC) quality assurance. However, since the electronic portal imaging device (EPID) on the Elekta Unity only covers a small area, it is not feasible to perform the PF test for the entire MLC. Here, we propose a technique for the PF test by stitching two double-exposed films. Two EBT3 films were used to encompass the entire MLC, with each one covering one half of the area. Two fields were employed to apply double exposure: a PF pattern consisting of 11 2 mm wide pickets and a 2.84cm x 22cm open field. The edges of the open field defined by the diaphragms were used to correct film rotation as well as align them horizontally. The PF pattern was also measured with the EPID where the pickets were used to align the films vertically. Individual leaf positions were detected on the merged film for quantitative analysis. Various MLC positioning errors were introduced to evaluate the technique's sensitivity. The merged films covered 72 leaf pairs properly (four leaf pairs on both sides were outside the treatment couch). With the EPID, the leaf positioning accuracy was -0.02±0.07mm (maximum: 0.29mm) and the picket width variation was 0.00±0.03mm (maximum: 0.11mm); with the films, the position accuracy and width variation were -0.03±0.13mm (maximum: 0.80mm) and 0.00±0.13mm (maximum: 0.74mm), respectively. The EPID was able to detect errors of 0.5mm or above with submillimeter accuracy; the films were only able to detect errors>1.0mm. We developed a quantitative technique for the PF test on the Elekta Unity. The merged films covered nearly the entire MLC leaf banks. The technique exhibited clinically acceptable accuracy and sensitivity to MLC positioning errors.

Is a weekly qualitative picket fence test sufficient? A proposed alternate EPID-based weekly MLC QA program.

PurposeWell‐designed routine multileaf collimator (MLC) quality assurance (QA) is important to assure external‐beam radiation treatment delivery accuracy. This study evaluates the clinical necessity of a comprehensive weekly (C‐Weekly) MLC QA program compared to the American Association of Physics in Medicinerecommended weekly picket fence test (PF‐Weekly), based on our seven‐year experience with weekly MLC QA.MethodsThe C‐Weekly MLC QA program used in this study includes 5 tests to analyze: (1) absolute MLC leaf position; (2) interdigitation MLC leaf position; (3) picket fence MLC leaf positions at static gantry angle; (4) minimum leaf‐gap setting; and (5) volumetric‐modulated arc therapy delivery. A total of 20,226 QA images from 16,855 tests (3,371 tests × 5) for 11 linacs at 5 photon clinical sites from May 2014 to June 2021 were analyzed. Failure mode and effects analysis was performed with 5 failure modes related to the 5 tests. For each failure mode, a risk probability number (RPN) was calculated for a C‐Weekly and a PF‐Weekly MLC QA program. The probability of occurrence was evaluated from statistical analyses of the C‐Weekly MLC QA.ResultsThe total number of failures for these 16,855 tests was 143 (0.9%): 39 (27.3%) for absolute MLC leaf position, 13 (9.1%) for interdigitation position, 9 (6.3%) for static gantry picket fence, 2 (1.4%) for minimum leaf‐gap setting, and 80 (55.9%) for VMAT delivery. RPN scores for PF‐Weekly MLC QA ranged from 60 to 192 and from 48 to 96 for C‐Weekly MLC QA.ConclusionRPNs for the 5 failure modes of MLC QA tests were quantitatively determined and analyzed. A comprehensive weekly MLC QA is imperative to lower the RPNs of the 5 failure modes to the desired level (<125); those from the PF‐Weekly MLC QA program were found to be higher (>125). This supports the clinical necessity for comprehensive weekly MLC QA.

Variation in Elekta iView electronic portal imager pixel scale factor with gantry angle, and impact on multi-leaf collimator quality assurance.

For Elekta Agility linear accelerators, the iViewGT electronic portal imaging device (EPID) is positioned at a nominal X‐Ray source‐to‐panel distance of 1600 mm. For display, image registration, and data processing purposes, the image pixels are scaled to spatial units at the treatment isocenter plane. This is achieved by applying a pixel scaling factor (PSF). During this investigation, the dependence of the PSF at cardinal gantry angles was determined along with the resulting effects on the multi‐leaf collimator (MLC) quality assurance (QA) results for three linear accelerators (linacs). The PSF was found to vary by 0.0018–0.0022 mm/pixel during gantry rotation, which resulted in variations in the mean MLC reported error of up to 0.8 mm at 100 mm off‐axis with the gantry rotated to 180°. Measurement and application of a gantry angle–specific PSF is a simple process that can be implemented to improve the accuracy of EPID‐based MLC QA at cardinal gantry angles.

Collimation of an anastigmat objective with three off-axis aspherical mirrors

Subject of study. Objectives for a wide variety of wavelengths from the visible to the mid-infrared are currently under development for use in small spacecraft. Because of advantages such as the ability to operate over a broad spectral range, the use of low-weight mirror fabrication technologies, and their smaller size, reflective objectives are preferred for space-based applications. This paper discusses the collimation of reflective objectives with off-axis aspheric mirrors (objectives with an off-axis field of view). Collimation of reflective objectives has traditionally been performed using interferometers operating in autocollimation mode, with collimation being verified using compensatory designs. In the case of compact multimirror objectives, it is not always possible to find space for the collimation equipment (either an interferometer with an auxiliary mirror or a holographic compensator). Method. An alternative technique was developed for sequential element-by-element collimation of a prototype multimirror objective, based on monitoring focal-spot shapes and comparing them against a calculated focal-spot shape. We discuss collimation of a four-mirror anastigmat objective, in which the first mirror is an off-axis concave ellipsoid, the second mirror is an off-axis convex hyperboloid, the third mirror is a flat, and the fourth mirror is an off-axis concave ellipsoid. The objective was collimated using a He-Ne laser (wavelength 0.63 µm), with the distribution of light in the focal spot being recorded with a television camera; collimation was verified using a continuously tunable CO laser (emission wavelength of 5.415 µm) and an HF laser (emission wavelength of 2.8–3.2 µm), and the distribution of light in the focal spots was measured with an InSb photodetector array. The focal spots were inspected using a microscope whose object plane coincided with the focal plane of the element being collimated. The entrance aperture of the objective was illuminated by a collimated laser beam parallel to the collimation axis. The quality of collimation was determined by comparing the shapes of the observed focal spots against those theoretically calculated for an ideally collimated objective. Main results. Our research revealed that a test collimation technique based on the shape of the focal spots is insufficiently sensitive to miscollimation of individual mirrors and scattering within a multiple-mirror system. In addition, the technique fails to provide the required collimation accuracy. Practical significance. The proposed technique will be used to develop specialized optoelectronic instrumentation for continuous, step-by-step verification of all optical elements in a four-mirror anastigmat to sufficient accuracy for collimation.

Investigation of 2D radiophotoluminescence films in radiotherapy: Multileaf collimator Quality Assurance and small field dosimetry

In radiotherapy, accurate dose determination and precise dose delivery to the tumor are directly associated with better treatment outcomes, such as better tumor control and lower post treatment complications. Challenges in Quality Assurance (QA) are a.o. (1) the varying amounts of primary and secondary scattered photons, the varying gantry angles and the energy heterogeneity and (2) the small field sizes used in Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT).As alternative to the commonly used GafChromic™ EBT films, radiophotoluminescence (RPL) films gained attention for high resolution 2D dosimetry in radiotherapy thanks to their insensitivity to ambient light, reusability, absence of fading and the few image corrections needed.In this work we characterize the response of improved Al2O3:C,Mg RPL films in a clinical 6 MV beam. Film response was evaluated in terms of uniformity, dose response, minimum detectable dose (MDD), small field dosimetry and its performance was evaluated for multileaf collimator (MLC) QA tests.The new films showed linear dose response up to 8 Gy, followed by supralinear response for higher doses up to 24 % at 20 Gy. QA checks for the MLC, including the chair and the alternating leaves, were compared with calculations from the treatment planning system (TPS) and showed an agreement within 5% for alternating leaves and a deviation in Full Width Half Maximum (FWHM) below 1.5 % for the chair test. The maximum FWHM discrepancy between RPL film results and reference data (diode) for small field sizes (30 × 30, 20 × 20, 10 × 10, 8 × 8, 6 × 6 mm2) was 1.9 %. Notwithstanding the variations in dose response, the studied films exhibit overall good agreement with reference data and demonstrate potential for QA in IMRT and small field radiotherapy

Quality of Hand Radiograph Collimation Determined by Artificial Intelligence Algorithm Correlates with Radiograph Quality Scores Assigned by Radiologists

Providing direct feedback to technologists has become challenging for radiologists due to geographic separation and other reasons. As such, there is a need for automated solutions to solve quality issues in radiography. We evaluated the feasibility of using a computer vision artificial intelligence (AI) algorithm to classify hand radiographs into quality categories in order to automate quality assurance processes in radiology. A bounding box was placed over the hand on 300 hand radiographs. These inputs were employed to train the computational neural network (CNN) to automatically detect hand boundaries. The trained CNN detector was used to place bounding boxes over the hands on an additional 100 radiographs, independently of the training or validation sets. A computer algorithm processed each output image to calculate unused air spaces. The same 100 images were classified by two musculoskeletal radiologists into four quality categories. The correlation between the AI-calculated unused space metric and radiologist-assigned quality scores was determined using the Spearman correlation coefficient. The kappa statistic was used to calculate the inter-reader agreement. The best negative correlation between the AI-assigned metric and the radiologists’ assigned quality scores was achieved using the calculation of the unused space at the top of the image. The Spearman correlation coefficients were −0.7 and −0.6 for the two radiologists. The kappa correlation coefficient for interobserver agreement between the two radiologists was 0.6. Automatic calculation of the percentage of unused space or indirect collimation at the top of hand radiographs correlates moderately well with radiographic collimation quality.

Challenges of Irradiation on Extrathoracic and Non-thoracic Organs in Portable Neonatal Chest Radiography: Do We Need Mandatory Protective Rules?

Background: Chest X-ray (CXR) is known as the most common radiography used for adult and pediatric patients worldwide. Improper X-ray field collimation can result in excessive radiation dose on non-thoracic organs in chest radiographs. Objectives: This study was to investigate X-ray field collimation quality in neonatal chest radiography. Methods: A total of 213 chest radiographs of neonates from three hospitals were analyzed for collimation quality assessment in a retrospective study. Accordingly, ideal imaging field (IIF) and current imaging field (CIF) were initially defined. The margins of the IIF included acromioclavicular (AC) level to lower costal margin (i.e. top to bottom) and one centimeter beyond the broadest area of the chest on each side (that is, right to left). The CIF size was also defined as the square borders of collimators. Results: The findings revealed that the area of the CIF was 1.65 ± 0.39 times to the ideal imaging firlddd (IIF) for three hospitals, suggesting that collimation quality in neonatal chest radiographs was not accurate and it had defects. According to the results, acceptable collimation percentage (36.6%) in Hospital A was more than that in two other centers, and the given center also provided the lowest radiation due to the exposure of non-thoracic structures to primary beams. Conclusions: It was concluded that training radiographers and using patient immobilization devices and stabilizers were important points that could reduce radiation exposure to non-thoracic organs in pediatric CXR.

Evaluation of MLC errors of LINAC based on log file

This study aimed to investigate the Multi Leaf Collimator (MLC) quality assurance (QA) and fluence monitor unit (MU) reconstruction based on log files of Varian Linear Accelerator. The graphical user interface (GUI) was developed using MATLAB to analyze the Log files. The MLC QA was analyzed using the error root mean square (RMS) of leaf parameters. Meanwhile, the fluence MU were reconstructed based on the information of leaf position, jaw position, and fractional MU for each 50 ms. The gamma index evaluation between actual and expected fluence MU was calculated to analyze the segment potential error caused by the MLC position. The average of mean error RMS leaves for Bank A and Bank B are 0.213 ± 0.098 mm and 0.207 ± 0.073 mm, whereas the average of maximum error RMS leaves for Bank A and Bank B are 0.491 ± 0.056 mm and 0.48 ± 0.061 mm, respectively. The mean gamma index results of 2%, 2mm and 3%, 3mm criteria between actual and expected fluence MU for the composite beam for all case are 98.2 ± 1.0% and 99.4 ± 0.3%, respectively. The error of MLC based on the log file analysis is within the tolerance level. The results showed that parameters irradiation such as MLC position, jaw position and fractional MU proper with planning.

X-ray source with quasi-monochromatic parallel beam

In inertial confined fusion experiments, an excellent-performance and high-efficiency X-ray source plays an important role in X-ray radiography schemes. Indeed, it can be used in a variety of X-ray experimental techniques. The mono-chromaticity, flux intensity, degree of collimation (the radiation can be transported long distances without loss), and spot size of the X-ray source affect the quality of imaging. Ray-tracing simulations, which are validated by experimental results, demonstrate that high-intensity collimated X-ray beams can be produced from an isotropic X-ray source. Therefore, a method of improving the performance of an X-ray source from a laser-produced plasma is presented. A spherically bent crystal is used to collimate mono-chromatic X-rays emitted from a laser-produced plasma. Here we design a spherically bent crystal spectrometer system for collimating the laser-produced X-rays. The system performance is experimentally tested at the Shenguang Ⅱ (SGⅡ) laser facility located in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. The beam divergence is measured by using a metal grid placed downstream from the crystal, the metal grid that possesses wires with 60 μm in diameter and 127 μm in period. An imaging plate (IP) is placed at various distances downstream from grid. The quality of the generated beam is monitored by measuring the dimensions of the grid image formed by the beam on IP. While the narrow range of wavelength is measured with a spherically bent crystal spectrometer. Experimental results show that the spherically bent crystal spectrometer system can produce quasi-monochromatic (10&lt;sup&gt;-3&lt;/sup&gt; &lt; △ &lt;i&gt;λ/λ&lt;/i&gt; &lt;10&lt;sup&gt;-2&lt;/sup&gt;) X-ray beams with a high degree of collimation (less than 2 mrad divergence), uniform spot size (~500 μm), and a relative tenability in the wide spectral range. The influences of various experimental parameters on the quality of beam collimation are evaluated in two ways. They can be investigated in test experiments by representing the beam divergence distribution as a function of Bragg angle. In another study of the effect of the aberrations, when the incident beam on the spherically bent crystal is not normal, the beam is less collimated in the tangential plane, and out of collimation in the sagittal plane. Following the ray-tracing method, we analyze the diffracted beam divergence produced by the astigmatic aberration. The qualitative conclusion is that the good agreement with the experimental results is obtained. By fully utilizing limited Bragg angle range, the spherically bent crystal spectrometer system can realize collimated diffracted X-ray beams with divergence of less than 1 mrad by using a laser-produced plasma X-ray source under the appropriately experimental parameters.

Alignment error of mirror modules of advanced telescope for high-energy astrophysics due to wavefront aberrations

One of the approaches that is being tested for the integration of the mirror modules of the advanced telescope for high-energy astrophysics x-ray mission of the European Space Agency consists in aligning each module on an optical bench operated at an ultraviolet wavelength. The mirror module is illuminated by a plane wave and, in order to overcome diffraction effects, the centroid of the image produced by the module is used as a reference to assess the accuracy of the optical alignment of the mirror module itself. Among other sources of uncertainty, the wave-front error of the plane wave also introduces an error in the position of the centroid, thus affecting the quality of the mirror module alignment. The power spectral density of the position of the point spread function centroid is here derived from the power spectral density of the wave-front error of the plane wave in the framework of the scalar theory of Fourier diffraction. This allows the defining of a specification on the collimator quality used for generating the plane wave starting from the contribution to the error budget allocated for the uncertainty of the centroid position. The theory generally applies whenever Fourier diffraction is a valid approximation, in which case the obtained result is identical to that derived by geometrical optics considerations.

06. - Digitization of multi leaf collimator quality assurance

06. - Digitization of multi leaf collimator quality assurance

Beam Collimation during Lumbar Spine Radiography: A Retrospective Study.

Collimating the primary beam to the area of diagnostic interest (ADI) has been strongly recommended as an effective method to reduce patient's radiation dose and to improve image quality during radiology practice. Lack or inadequate collimation results in excessive radiation dose to patients and deterioration image quality. To assess the quality of beam collimation during lumbar spine radiography at two general hospitals in Ahvaz, Iran. We retrospectively reviewed 830 digital antero-posterior (AP) lumbar spine radiographs in term of beam collimation. For each radiograph, the distance between current and optimal collimation was calculated (in cm). The area of ADI and total field size for each radiograph were also calculated (in cm2). The total mean ADI and irradiated region outside ADI for each radiograph were estimated 360 and 454 cm2, respectively. The total irradiated region outside ADI was 1.26 times more than ADI. In contrast to cranial regions outside ADI, caudal regions were more commonly included inside the primary beam (12% vs. 24.4%; P-value <0.005). At least in 62% of radiographs evaluated, ovaries were included in the primary beam. Radiographers should make considerable effort to limit the primary beam to the ADI to reduce patient's exposure and to increase image quality.

22. CyberKnife M6 InCise-2 multileaf collimator setup, modelization and quality insurance program

Introduction InCise-2 MLC approximately reduces treatment time by 30% by decreasing the total amount of beams and MUs. Beams library measurements, modelization validation and MLC quality insurance program are presented. Methods Commissioning: TMR, profiles and OF are measured for 11 beam sizes from 7.6 × 7.7 mm 2 to 115 × 100.1 mm 2 using a PTW diode in an IBA BluePhantom2 water phantom.OF are also measured with IBA CC13 and PTW PinPoint ionization chambers and Gafchromics EBT3 films placed in a solid water equivalent phantom. Absence of light field, the robot’s degrees of freedom and the leaves’ inclinaison (inducing asymmetrical penumbra) makes Water Phantom and MLC axes alignment, thereby profiles measurement, tricky. Measurements are compared to the composite data given by the manufacturer. Validation of the model: Treatment planning with the MLC can be achieved two different methods: Isocentric conformal or conformal avoidance, which is based on three different fluence modulation processes in the inverse planning system. Treatment plans based on each planning methods are calculated on the Standard Imaging SRS Dose Verification Phantom and delivered on the phantom where several EBT3 films have been inserted. 2D gamma index analysis (1 mm/5% and 3 mm/3%) is carried out using software made on site. Quality Control: TG-50 and Garden-Fence tests are carried out using Gafchromic EBT3 films to control leaves positioning accuracy.Transmission through MLC is also measured using films. Results Commissioning: Regarding OF, the different detectors match well for 23 × 23.1 mm 2 field size and above (maximum mismatch: 0.5%).Below this field size, mismatch between ionization chambers, diode and films is significant: e.g.diode versus film over-estimates OF by 5.3% for the field size 7.6 × 7.7 mm 2 . Moreover, TMR, profiles and OF measured match well with the composite data. Model validation: Gamma-index analysis results are better than 95% for each planning method. Quality control: TG-50 test is carried out every morning with visual validation.Garden-Fence test results are monthly quantitatively analyzed.Manufacturer’s criteria about leaves positioning accuracy are: –No drift above0.95 mm –No leaf takes more than one position with a drift over 0.5 mm –No more than 13 leaves belonging to the same side have drift over 0.5 mm In step with manufacturer’s criteria, leakage below 0.5% has been measured. Conclusions MLC has been clinically used since March 2016. As expected, treatment time is significantly reduced while dosimetry quality is maintained compared with Iris collimation. Though, the Monte-Carlo algorithm unavailability restrains the use of MLC. Manufacturer’s quality control recommendations are our guideline. Additional tests are being studied.

O19. Linear accelerator multileaf collimator quality control methodologies in radiotherapy: Analysis and recommendations

Introduction Multileaf collimator (MLC) system introduction into clinical Linear Accelerators (Linacs), facilitated computer-control and verification of complex treatment, and results in an increase in patient set up speed. A MLC system thus requires a re-evaluation of the quality assurance requirements for beam collimation. This study surveyed, developed, performed and evaluated quality assurance efforts for conventional MLCs with the aim to determine efficacy and reproducibility of the quality control procedures. Materials and methods The performance of MLCs for an Elekta (Livingstone Hospital, LH) and a Siemens (Charlotte Maxeke Johannesburg Academic Hospital, CMJAH) Linac were examined. The quality control procedures studied were leaf matching, leaf position accuracy, inter-leaf leakage and transmission through abutting leaves. Three planar portal imaging devices (radiographic film, radio chromic film and an Electronic Portal Imaging Device) and a LA48 Linear array were used as detectors. Record and verify data management systems were used to set up and execute the procedures. The calibration of all the portal imaging devices was also performed. Results The test procedures were successfully carried out and analysed using different detectors on two Linac systems at CMJAH and LH. The analysis of the data obtained showed consistency across devices and reproducibility in the results with time. A combined single executable procedure was developed and analysis indicated its viability and reproducibility on all platforms. Conclusion Further investigation is required in the event that leaf matching and leaf position accuracy test procedures exceed tolerance in order to identify and quantify the actual mechanical fault. The combined procedure provided the same qualitative information as the 4 individual tests and can therefore be used routinely to perform a basic constancy check of MLC performance.

SU‐G‐TeP4‐03: A Multileaf Collimator Calibration and Quality Assurance Technique Using An Electronic Portal Imaging Device

Purpose:To develop an accurate and quick multileaf collimator (MLC) calibration and quality assurance technique using an electronic portal imaging device (EPID)Methods:The MLC models used include the MLCi and Agility (Elekta Ltd). This technique consists of two 22(L)x10(W) cm2 fields with 00 and 1800 collimator angles centered to an offset EPID. The MLC opening is estimated by calculating the profile at the image's center in the image's horizontal direction. Scans in the image's vertical direction were calculated every 20 pixels in the inner 70% of estimated MLC opening. The profiles’ edges were fitted with linear equations to determine the image's rotation angle. Then, crossline profiles were scanned at the center of each leaf taking into account the leaf's width at isocenter and the rotation angle. The profiles’ edges determine the location of the leaves’ edges and these were subtracted from the reference leaf's position in order to determine the relative leaf offsets. The edge location of all profiles was determined by using the parameterized gradient of the penumbra region. The technique was tested against an established diode array‐based method, and for different MLC systems, patterns, gantry angles, days, energies, beam modalities and MLC openings.Results:The differences between the proposed and established methods were 0.26±0.19mm. The leaf offsets’ deviation was &lt;0.3mm (5 months period). For pattern fields, the differences between predetermined and calculated offsets were 0.18±0.18mm. The leaf offset deviation of measurements with different energies and MLC openings were &lt;0.1mm and &lt;0.3mm, respectively. The differences between offsets of FF and FFF beams were 0.01±0.02mm (&lt;0.07mm). The differences between the offsets at different gantry angles were 0.08±0.15mm.Conclusion:The proposed method proved to be accurate and efficient in calculating the relative leaf offsets. Parameterized field edge is essential to obtain accurate result by eliminating the noise from EPID.

Collimating Montel mirror as part of a multi-crystal analyzer system for resonant inelastic X-ray scattering.

Advances in resonant inelastic X-ray scattering (RIXS) have come in lockstep with improvements in energy resolution. Currently, the best energy resolution at the Ir L3-edge stands at ∼25 meV, which is achieved using a diced Si(844) spherical crystal analyzer. However, spherical analyzers are limited by their intrinsic reflection width. A novel analyzer system using multiple flat crystals provides a promising way to overcome this limitation. For the present design, an energy resolution at or below 10 meV was selected. Recognizing that the angular acceptance of flat crystals is severely limited, a collimating element is essential to achieve the necessary solid-angle acceptance. For this purpose, a laterally graded, parabolic, multilayer Montel mirror was designed for use at the Ir L3-absorption edge. It provides an acceptance larger than 10 mrad, collimating the reflected X-ray beam to smaller than 100 µrad, in both vertical and horizontal directions. The performance of this mirror was studied at beamline 27-ID at the Advanced Photon Source. X-rays from a diamond (111) monochromator illuminated a scattering source of diameter 5 µm, generating an incident beam on the mirror with a well determined divergence of 40 mrad. A flat Si(111) crystal after the mirror served as the divergence analyzer. From X-ray measurements, ray-tracing simulations and optical metrology results, it was established that the Montel mirror satisfied the specifications of angular acceptance and collimation quality necessary for a high-resolution RIXS multi-crystal analyzer system.