Linac Parameters Research Articles

Assessing focal spot alignment in clinical linear accelerators: a comprehensive evaluation with triplet phantoms.

A fundamental parameter to evaluate the beam delivery precision and stability on a clinical linear accelerator (linac) is the focal spot position (FSP) measured relative to the collimator axis of the radiation head. The aims of this work were to evaluate comprehensive data on FSP acquired on linacs in clinical use and to establish the ability of alternative phantoms to detect effects on patient plan delivery related to FSP. FSP measurements were conducted using a rigid phantom holding two ball-bearings at two different distances from the radiation source. Images of these ball-bearings were acquired using the electronic portal imaging device (EPID) integrated with each linac. Machine QA was assessed using a radiation head-mounted PTW STARCHECK phantom. Patient plan QA was investigated using the SNC ArcCHECK phantom positioned on the treatment couch, irradiated with VMAT plans across a complete 360° gantry rotation and three X-ray energies. This study covered eight Elekta linacs, including those with 6 MV, 18 MV, and 6 MV flattening-filter-free (FFF) beams. The largest range in the FSP was found for 6 MV FFF. The FSP of one linac, retrofitted with 6 MV FFF, displayed substantial differences in FSP compared to 6 MV FFF beams on other linacs, which all had FSP ranges less than 0.50mm and 0.25mm in the lateral and longitudinal directions, respectively. The PTW STARCHECK phantom proved effective in characterising the FSP, while the SNC ArcCHECK measurements could not discern FSP-related features. Minor variations in FSP may be attributed to adjustments in linac parameters, component replacements necessary for beam delivery, and the wear and tear of various linac components, including the magnetron and gun filament. Consideration should be given to the ability of any particular phantom to detect a subsequent impact on the accuracy of patient plan delivery.

Using KMeans Clustering to Evaluate and Alert for Deviations of Linac Photon Beam Parameters.

To analyse the daily measured Dosimetric Quality Assurance (QA) parameters of linear accelerator (linac) using Unsupervised Machine Learning (ML) Algorithm thereby evaluating the current machine status and to highlight the probable cause of the 'out-of-range' measured parameter. Five Parameters measured using PTW QuickCheckwebline device in a linac is subjected to KMeans clustering technique. The measured parameters comprise of Central Axis Dose (CAX), Beam Flatness, SymmetryLR, SymmetryGT and Beam Quality (BQF). Data from Varian with 55- and 107-day's measurements and from Elekta with 75 days measurements from 2 beam matched linacs were used in this clustering technique. This evaluation is used to review the current linac status and obtain 1) Upper and lower limits of each parameter (CAX, Flatness, Symmetry, Beam Quality), 2) Frequency of the days when the linac parameters are closer to the target value and when they deviate from the target value. 3) The date when these parameters deviate from the estimated limits. 4) The probable reason for the deviation and 5) Finally if the machine requires maintenance. This methodology ensures that the machine is always closest to the target value, thus providing quality radiation treatment for the cancer patients. Moreover, the performance of the linac is studied meticulously and the need for maintenance is alerted before the linac beam shows marked deviation from the base value. KMeans clustering is a very simple and easy to use ML tool. With quick computation time and with lesser data it can arrive at the actual limits of the linac parameters and help to determine if the linac needs maintenance well in advance.

Development of a new MC Fast Dose Engine for small compact radiation fields generated by the CyberKnifeⓇ Iris medical linac

MC-based computerized TPS is an essential tool for improving treatment quality in external radiation therapy chains. However, incorporating these systems into clinical TPS is challenging due to their long computation time. To address this problem, Phase Spaces (PhSp) can be utilized, which reduce the computation time of such simulations while maintaining accuracy levels comparable to full simulations. Since PhSps can only generate a finite number of particles, recycling the same PhSp several times during a simulation will increase statistical uncertainty. To carry out this disadvantage, the present study employs a numerical reconstruction model to develop a novel generation of MC Fast Dose Engine (MC-FDE) that relies on the Cyberknife linac parameters publicly available in the IAEA database. The MC-FDE model’s performance is evaluated based on the precision of the reconstructed parameters and the accuracy of dose calculation in the 3D BEAMSCAN water phantom and patient CT datasets. In contrast to the recently published Wasserstein Generative Adversarial Networks (WGAN), the MC-FDE reconstruction model produces significant agreement with measured data, IAEA PhSp files and TPS Accuray, as determined by marginal distributions, correlation metrics, 2-3D gamma indexes and the Dose Volume Histogram (DVH) test. Regarding calculation speed, MC-FDE has a very fast process and can generate an infinite number of particles, unlike the limited number of particles recorded in IAEA PhSp files. According to the findings of this study, the proposed MC-FDE model is considered suitable for use as part of clinical TPS.

A formalism and methodology for measurement and control of LINAC isocenter.

Despite the acknowledged need for a stable reference point for LINAC isocenter quality assurance (QA), no standard for such a reference point has been established. This paper introduces a practical and robust technique for measuring and tuning LINAC isocenter within a stable reference frame based on the collimator axes ofrotation. We develop a framework based on physical isocenter, a refinement of the approach by Skworcow etal. The physical isocenter provides a relatively stable, first principles spatial point from which other LINAC parameters can be referenced. An optical tracking system was used to measure the collimator axes with high precision and an isocenter cost function was implemented to ensure a unique isocenter location. The same optical tracking system was used to (a) align the couch axis to the physical isocenter, (b) align the radiation beam to the collimator axes, and (c) position a marker precisely at the physical isocenter to demonstrate the effectiveness of theapproach. The framework was successfully demonstrated on an Elekta LINAC. The physical isocenter was shown to be repeatable with a standard deviation of 0.03 mm for the position and 0.03 mm for the radius. The couch axis was aligned to physical isocenter within 0.07 mm. The average collimator to beam axis distance before beam alignment was 0.19 and 0.10 mm after. All these steps were performed within 3 h, showing that the method is efficient when applied to isocenter optimization. The time required to measure physical isocenter and guide a marker to it for day-to-day isocenter QA was under 10min. We have presented a modular, practical framework for isocenter characterization and optimization based on physical isocenter, which is a stable and fixed referencepoint.

Verification of electron beam parameters in an intraoperative linear accelerator using dosimetric and radiobiological response methods.

BackgroundThe availability of linear accelerators (linac) for research purposes is often limited and therefore alternative radiation sources are needed to conduct radiobiological research. The National Centre for Radiation Research in Poland recently developed an intraoperative mobile linac that enables electron irradiation at energies ranging from 4 to 12 MeV and dose rates of 5 or 10 Gy/min. The present study was conducted to evaluate the electron beam parameters of this intraoperative linac and to verify the set-up to evaluate out-of-field doses in a water phantom, which were determined through dosimetric and biological response measurements.Materials and methodsThe distribution of radiation doses along and across the radiation beam were measured in a water phantom using a semiconductor detector and absolute doses using an ionisation chamber. Two luminal breast cancer cell lines (T-47D and HER2 positive SK-BR-3) were placed in the phantom to study radiation response at doses ranging from 2 to 10 Gy. Cell response was measured by clonogenic assays.Results and ConclusionThe electron beam properties, including depth doses and profiles, were within expected range for the stated energies. These results confirm the viability of this device and set-up as a source of megavoltage electrons to evaluate the radiobiological response of tumour cells.

Enhanced seeded free electron laser performance with a “cold” electron beam

FERMI is a seeded free electron laser (FEL) based on the high gain harmonic generation (HGHG) scheme and has been generating intense and fully coherent extreme ultra-violet and soft x-ray pulses for several years. The high-degree performance of the FEL leans on a high brightness electron beam, with small transverse emittance ($\ensuremath{\sim}1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$) and a peak current of about 700-800 A. The main constraint on lasing at high harmonics of the seed is low electron time-slice energy spread. The optimization of the photoinjector and of some linac parameters has allowed a reduction of the relative slice energy spread to the level of few times of ${10}^{\ensuremath{-}5}$. With these new conditions, the FEL can be operated without the need of a laser heater to suppress micro bunching instabilities and this ``cold'' beam has allowed generation of extreme UV pulses with pulse energy exceeding a mJ, and with peak power of about 10 GW. We describe the electron beam characterization and the FEL performance improvement, including the extension of the range of harmonics of the seed which can be amplified, up to the twenty-fifth harmonic, i.e., 10 nm.

Choice of Linac parameters to minimize the space-charge effects

In high-intensity linear accelerators, the tune spreads induced by the space-charge forces in the radial and longitudinal planes are key parameters for halo formation and beam losses. For matched beams, they are the parameters governing the number of resonances (including coupling resonances), which affect the beam and determine the respective sizes of the stable and halo areas in phase space. The number and strength of the resonances excited in mismatched beams leading to even higher amplitude halos are also directly linked to the tune spreads. In this paper, the equations making the link between the basic linac parameters (rf frequency, zero-current phase advances, beam intensity and emittances) and the tune spreads are given. The analysis of the way these linac parameters can be chosen to minimize the tune spreads is presented. The ESS linac parameters are used as an example for this study.

RF Measurements and Tuning of the 325 MHz Ladder-RFQ

Based on the positive results of the unmodulated 325 MHz Ladder-RFQ prototype from 2013 to 2016, we developed and designed a modulated 3.4 m Ladder-RFQ*. The unmodulated Ladder-RFQ features a very constant voltage along the axis. It accepted 3 times the operating power of which is needed in operation**. That level corresponds to a Kilpatrick factor of 3.1 with a pulse length of 200 µs. The 325 MHz RFQ is designed to accelerate protons from 95 keV to 3.0 MeV according to the design parameters of the proton linac within the FAIR project. This particular high frequency for a 4-ROD-RFQ creates difficulties, which triggered the development of a Ladder-RFQ with its high symmetry. The results of the unmodulated prototype have shown, that the Ladder-RFQ is very well suited for that frequency. The duty cycle is up to 5% for the applied cooling concept. Manufacturing has been completed in September 2018. We will show the finalization of assembly after manufacturing as well as low level RF measurements. The final machining step for both flatness and frequency tuning has been finished in April 2019.

8 ASN-IRSN Survey on in vivo dosimetry in external radiotherapy: what solutions and strategies to push the frontier of ”technically measurable”?

Introduction In Vivo Dosimetry (IVD) is mandatory in France for every ”technically measurable” beam, since 2008 (criterion No. 15 of National Cancer Institute, for the practice of external radiotherapy). IVD was historically based on punctual measurement techniques adapted to conventional radiotherapy. The applicability of this method has greatly decreased with the dissemination of modulated radiotherapy techniques (IMRT, VMAT). The majority of French radiotherapy centers practice these advanced techniques (nearly 70% of them in 2013 [1] ), for sometimes 100% of the treatments. So, the proportion of ”non-technically measurable” beams grows very rapidly and new solutions must be considered to ensure a satisfactory level of treatments control. From this statement, resulting from the practices observed during inspections, Nuclear Safety Authority (ASN) wished to know what IVD solutions - especially those called ”transit” based on the use of the portal imaging device (EPID) – are available or in development, the technical and organizational constraints of the ”transit” IVD and its relations with other types of controls. Methods IRSN met providers of IVD solutions, or comparable control systems, and visited radiotherapy departments experienced in the field of transit IVD. The Medical Physicists representatives (SFPM) point of view has been collected, in continuation with the SFPM work of 2014 [2] . Results This study focused on main ”transit” IVD solutions currently proposed and nearly expected, as well as alternative or complementary solutions such as the analysis and monitoring of the LINAC parameters. Interviews with medical physicists revealed that IVD is part of an overall process of dose verification and cannot replace the controls performed without the patient. The definition of the intervention criteria or validation of DIV results, the identification of dose gap origin of and the interpretation of the results are the main mentioned difficulties, as well as the ergonomics of the devices, the tools of monitoring and analysis, the accessibility of data and the need for consistent human resources. The teams’ expectations with respect to IVD, and therefore the requirements and resources allocated, can be on one hand a simple wish of regulatory compliance or, on the other hand, to use IVD as an adaptive radiotherapy tool . . Conclusions The offer of IVD solutions in recent radiotherapy techniques is progressing. The EPID-based IVD is the most widely used but, routinely, is often confined to conventional techniques. The exploitation and interpretability of the results is the main difficulty encountered by the teams, leading them to develop a treatment verification strategy in which the IVD is a one part of dose control and not the ultimate and global solution.

Progress of the modulated 325 MHz Ladder RFQ

Based on the positive results of the unmodulated 325 MHz Ladder-RFQ prototype from 2013 to 2016 [1], we developed and designed a modulated 3.3 m Ladder-RFQ. The idea of the Ladder type RFQ firstly came up in the late eighties [2, 3] and was realized successfully for the CERN Linac3 operating at 101 MHz [4] and for the CERN antiproton decelerator ASACUSA at 202 MHz [5].The unmodulated Ladder-RFQ features a very constant voltage along the axis. The RFQ was high power tested at the GSI test stand [6]. It accepted 3 times the RF power level needed in operation [7]. The highest level corresponds to a Kilpatrick factor of 3.1 with a pulse length of 200 μs. The 325 MHz RFQ is designed to accelerate protons from 95keV to 3.0 MeV according to the design parameters of the proton linac within the FAIR project. This particular high frequency creates difficulties for a 4-ROD type RFQ, which triggered the development of a Ladder RFQ with its higher symmetry. The results of the unmodulated prototype have shown, that the Ladder-RFQ is a suitable candidate for that frequency. For the present design duty cycles are feasible up to 5%. The basic design and tendering of the RFQ has been successfully completed in 2016 [8]. Manufacturing will be completed until May 2018. In this paper we present the latest results of manufacturing and beam dynamics simulations for the matching between LEBT and RFQ.

P217] Comprehensive procedure for IMRT QA testing

Purpose To design a versatile procedure for testing the repeatability and accuracy of geometric setup i.e. leaves motion, gantry and collimator position and isocenter stability, as well as dosimetric properties of beam parameters in conditions representative for intensity modulated radiation therapy (IMRT) techniques, with respect to ease for performance and automated analysis. Methods Tests were design for the cubic water-equivalent phantom with the insertion of MatriXX Evolution dosimetric device, consisted of 1024 air ionization chambers arranged in 2D array for dose distribution acquiring. Measurements were analyzed using OmniPro I’mRT software. All tests were checked for phantom setup reproducibility contribution in the final uncertainty of each result. The capability to detect in these tests the gross errors in gantry, collimator and treatment couch setup was checked. The influence of dosimetric device placement with respect to the beam central axis on reconstructed dose values and localization was also investigated, taking into account the limited resolution related to ionization chambers spacing. Results Stability of isocenter position performed for full-arc rotating gantry and 1×1 cm2 field size together with cone beam CT, analyzed against the convergence of MV to kV point of rotation showed accuracy of 0.2 mm whereas 0.5 mm is declared by the linac manufacturer. The test involving the re-loading of the field size for non-zero gantry position showed 0.5% accuracy of leaves positioning. Dose analysis for strip test was appeared to be sensitive to small couch angles often difficult to perceive, resulting in about 2% dose discrepancy for 2–5 deg. angles mismatch. Conclusions It is possible to verify the parameters of linac operation which are crucial for accurate and precise IMRT realization in a simple tests involving the equipment commonly used for dose distribution verification. The advantage of using 2D array of dosimeters is the possibility to compare two subsequent measurements in terms of gamma index, what enables to maintain the single linac operation stability aa well as twin parameters across the set of linacs. For IMRT QA purpose 3% and 1 mm criteria are sensitive enough to detect the mismatch in geometric or dosimetric parameters of linac of 0.2 mm and 1%, respectively.

OA011] DoseTracker: In-house developed software program for real-time reconstruction of motion-induced dose errors during radiotherapy

Purpose A problem of current plan-specific quality assurance (QA) in radiotherapy is that it ignores organ motion, although motion may cause dose discrepancies much graver than the treatment delivery errors assessed by the QA procedure. Furthermore, the QA is not feasible for real-time adaptive treatments like MLC tracking, where the treatment machine behavior is determined on-the-fly as a response to tumor motion. To overcome these limitations, we have developed the software program DoseTracker that performs real-time motion-including dose reconstruction during treatment delivery. Here, we validate DoseTracker in experiments and simulated treatments. Methods DoseTracker performs real-time motion-including dose reconstruction, based on streamed linac parameters and target positions, using a pencil-beam algorithm. An arbitrary set of points can be chosen for the dose calculations and the calculations points can move independently as function of time. DoseTracker has been validated in phantom experiments and simulated patient treatments. The phantom experiments used a biplanar diode array on a programmable motion stage. DoseTracker performed online real-time reconstruction of the diode doses at 15 Hz and calculated the 3%/3 mm gamma failure rate (comparing motion doses with static doses) at 1 Hz during five VMAT liver SBRT treatments with and without MLC tracking. The gamma errors were retrospectively compared with diode measurements. In the treatment simulations, DoseTracker made (offline) real-time tumor dose reconstructions at 2–5 Hz in patient anatomy for four liver SBRT patients previously treated with motion monitoring by electromagnetic transponders. Simulations were performed with and without respiratory gating. The reduction in CTV D95 relative to the planned intent ( Δ D95) was retrospectively compared between DoseTracker and motion-including dose reconstructions performed in the treatment planning system (TPS) by isocenter shifts. DoseTracker currently assumes water densities so another set of TPS calculations were performed on water densities. Results Experiments yielded 2.0%-point gamma failure rate root-mean-square difference between DoseTracker and measurements. Simulated treatments yielded 1.2%-points (CT-densities) and 0.6%-point (water-densities) differences in Δ D95. Conclusions A program to reconstruct motion-induced dose errors was developed and tested in phantom studies and patients, yielding high accuracies, and allowing supervision of treatment correctness and action based on dose discrepancies. Planned improvements include CT-densities, rotations, etc.

First online real-time evaluation of motion-induced 4D dose errors during radiotherapy delivery.

In radiotherapy, dose deficits caused by tumor motion often far outweigh the discrepancies typically allowed in plan-specific quality assurance (QA). Yet, tumor motion is not usually included in present QA. We here present a novel method for online treatment verification by real-time motion-including four-dimensional (4D) dose reconstruction and dose evaluation and demonstrate its use during stereotactic body radiotherapy (SBRT) delivery with and without MLC tracking. Five volumetric-modulated arc therapy (VMAT) plans were delivered with and without MLC tracking to a motion stage carrying a Delta4 dosimeter. The VMAT plans have previously been used for (nontracking) liver SBRT with intratreatment tumor motion recorded by kilovoltage intrafraction monitoring (KIM). The motion stage reproduced the KIM-measured tumor motions in three dimensions (3D) while optical monitoring guided the MLC tracking. Linac parameters and the target position were streamed to an in-house developed software program (DoseTracker) that performed real-time 4D dose reconstructions and 3%/3mm γ-evaluations of the reconstructed cumulative dose using a concurrently reconstructed planned dose without target motion as reference. Offline, the real-time reconstructed doses and γ-evaluations were validated against 4D dosimeter measurements performed during the experiments. In total, 181,120 dose reconstructions and 5,237 γ-evaluations were performed online and in real time with median computation times of 30ms and 1.2s, respectively. The mean (standard deviation) difference between reconstructed and measured doses was -1.2% (4.9%) for transient doses and -1.5% (3.9%) for cumulative doses. The root-mean-square deviation between reconstructed and measured motion-induced γ-fail rates was 2.0%-point. The mean (standard deviation) sensitivity and specificity of DoseTracker to predict γ-fail rates above a given threshold was 96.8% (3.5%) and 99.2% (0.4%), respectively, for clinically relevant thresholds between 1% and 30% γ-fail rate. Real-time delivery-specific QA during radiotherapy of moving targets was demonstrated for the first time. It allows supervision of treatment accuracy and action on treatment discrepancy within 2s with high sensitivity and specificity.

Automated ultrafast kilovoltage–megavoltage cone-beam CT for image guided radiotherapy of lung cancer: System description and real-time results

PurposeTo establish a fully automated kV-MV CBCT imaging method on a clinical linear accelerator that allows image acquisition of thoracic targets for patient positioning within one breath-hold (∼15s) under realistic clinical conditions. Methods and materialsOur previously developed FPGA-based hardware unit which allows synchronized kV-MV CBCT projection acquisition is connected to a clinical linear accelerator system via a multi-pin switch; i.e. either kV-MV imaging or conventional clinical mode can be selected. An application program was developed to control the relevant linac parameters automatically and to manage the MV detector readout as well as the gantry angle capture for each MV projection. The kV projections are acquired with the conventional CBCT system. GPU-accelerated filtered backprojection is performed separately for both data sets. After appropriate grayscale normalization both modalities are combined and the final kV-MV volume is re-imported in the CBCT system to enable image matching. To demonstrate adequate geometrical accuracy of the novel imaging system the Penta-Guide phantom QA procedure is performed. Furthermore, a human plastinate and different tumor shapes in a thorax phantom are scanned. Diameters of the known tumor shapes are measured in the kV-MV reconstruction. ResultsAn automated kV-MV CBCT workflow was successfully established in a clinical environment. The overall procedure, from starting the data acquisition until the reconstructed volume is available for registration, requires ∼90s including 17s acquisition time for 100° rotation. It is very simple and allows target positioning in the same way as for conventional CBCT. Registration accuracy of the QA phantom is within ±1mm. The average deviation from the known tumor dimensions measured in the thorax phantom was 0.7mm which corresponds to an improvement of 36% compared to our previous kV-MV imaging system. ConclusionsDue to automation the kV-MV CBCT workflow is speeded up by a factor of >10 compared to the manual approach. Thus, the system allows a simple, fast and reliable imaging procedure and fulfills all requirements to be successfully introduced into the clinical workflow now, enabling single-breath-hold volume imaging.

Feasibility of using the linac real-time log data for VMAT treatment verification

This study investigates the feasibility of using the real-time log data from a linac to verify Volumetric Modulated Arc Therapy (VMAT) treatment. The treatment log data for an Elekta Synergy linac can be recorded at a sampling rate of 4 Hz using the service graphing tool on the linac control computer. A treatment plan that simulates a VMAT treatment was delivered from the linac and all the dynamic treatment parameters including monitor unit (MU), Multileaf Collimator (MLC) position, jaw position, gantry angle and collimator angle were recorded in real-time using the service graphing tool. The recorded raw data were extracted and analysed using algorithms written in Matlab (MathWorks, Natick, MA). The actual treatment parameters logged using the service graphing tool was compared to the prescription and the deviations were analysed. The MLC position errors travelling at the speed range from −3.25 to 5.92 cm/s were between −1.7 mm to 2.5 mm, well within the 3.5 mm tolerance value (AAPM TG-142). The discrepancies of other delivery parameters were also within the tolerance. The real-time linac parameters logged using the service graphing tool can be used as a supplementary data for patient specific VMAT pre-treatment quality assurance.

Possible parameters of proton acceleration using backward traveling wave harmonic

Analysis shows that, when accelerating protons of intermediate energy range using the field of backward harmonic of the traveling wave, a range of practically accessible parameters of accelerating structure exists, where it is possible to provide simultaneously the stability of longitudinal and transverse particle motion and high rates of acceleration. The focusing effect is provided by the field of slow fundamental harmonic. The calculated characteristics of accelerating structure and the assessment of parameters of the proton linac are obtained in a range of 15−230 MeV.

Commissioning and implementation of Mobius, a commercial independent calculation system for IMRT

Introduction At NUH VMAT and Tomotherapy patient plans were checked using patient-specific QA (PSQA). Mobius was purchased to aid with the reduction of PSQA time (pre-treatment and on linac). Methods Tomotherapy plans were exported to Mobius and the results were compared with those acquired from our existing PSQA system (Delta4 and CheckTomo). The gamma pass rate and the percentage difference in the 95% coverage of the PTVs were used to determine acceptability. Mobius is currently being commissioned for IMRT from OMP and Monaco TPSs. Beam model data within Mobius were adjusted to reflect local equipment. MobiusFX monitors the communication between the linac and R&V system (Mosaiq). The accuracy of the delivery was assessed by checking linac parameters (MU, gantry, collimator, and MLC movements). The system was also tested by creating deliberate errors in DICOM plan files. Results The gamma pass rates for Mobius and CheckTomo were comparable, but not for the Delta4. The tolerances for the 95% coverage difference were set from the mean ± 2 S.D. Tolerances for the gamma pass rate were 96%, 94% and 95% for 5, 2.5 and 1 cm jaws respectively. The same metrics with Tomotherapy will be used for VMAT to assess plan acceptability and the tolerances will be selected as above. Preliminary results suggest that MobiusFX detects large errors in the beam delivery, and plan transfer between TPS and Mosaiq. Discussion and conclusion On Tomotherapy the PSQA workload was reduced, from 3 h/week to 1 h/2 weeks. The FX module workflow is viable and efficient.

Linac beam data measurement using a detector array

Introduction As new treatment advances are implemented in a radiotherapy department, the QA workload increases requiring more resources. Unplanned maintenance work in the middle of the daily linac schedule also demands quick methods to check beam parameters without sacrifices in the quality of measurements. New measurement modalities that can simplify procedures are thus of great value. Purpose To investigate the reliability of a detector array when beam adjustments are carried out by the linac engineer or QA work is being undertaken. Detector arrays provide an attractive alternative to water phantoms due to their compactness and ease of setup. Materials and methods A multi-axis ion chamber array (IC PROFILER™, SUN NUCLEAR) was used to collect reference data for clinical linac beams. Energies measured were photons of 6, 15 and 18 MV and electrons in the range 6 to 18 MeV. The data were compared to corresponding scans and point measurements obtained using the PTW MP3 (Freiburg, Germany) water phantom for the respective beams. Parameters checked were output constancy, dose rate dependence, energy constancy, beam profiles in cardinal and diagonal directions, beam flatness and symmetry and electron applicator output constancy. The array allowed also a quick check of beam startup characteristics and the verification of beam parameter stability at various gantry angles. Results All parameters (constancy, output factors, energy dependence, dose rate, beam symmetry and flatness), measured with the multi-axis chamber array and the water phantom for the various beams, were in accordance within 1% at most. The detector array provides comparable results and combines the reliability of a water phantom with the advantage of reduced linac time taken up by solid phantom setups. Conclusion The detector array has shown to have the reliability needed to substitute the water phantom for acquiring linac beam data. This is particularly useful for QA measurements and checking linac parameters during and after unplanned technical interventions. As new treatment advances are implemented in a radiotherapy department, the QA workload increases requiring more resources. Unplanned maintenance work in the middle of the daily linac schedule also demands quick methods to check beam parameters without sacrifices in the quality of measurements. New measurement modalities that can simplify procedures are thus of great value. To investigate the reliability of a detector array when beam adjustments are carried out by the linac engineer or QA work is being undertaken. Detector arrays provide an attractive alternative to water phantoms due to their compactness and ease of setup. A multi-axis ion chamber array (IC PROFILER™, SUN NUCLEAR) was used to collect reference data for clinical linac beams. Energies measured were photons of 6, 15 and 18 MV and electrons in the range 6 to 18 MeV. The data were compared to corresponding scans and point measurements obtained using the PTW MP3 (Freiburg, Germany) water phantom for the respective beams. Parameters checked were output constancy, dose rate dependence, energy constancy, beam profiles in cardinal and diagonal directions, beam flatness and symmetry and electron applicator output constancy. The array allowed also a quick check of beam startup characteristics and the verification of beam parameter stability at various gantry angles. All parameters (constancy, output factors, energy dependence, dose rate, beam symmetry and flatness), measured with the multi-axis chamber array and the water phantom for the various beams, were in accordance within 1% at most. The detector array provides comparable results and combines the reliability of a water phantom with the advantage of reduced linac time taken up by solid phantom setups. The detector array has shown to have the reliability needed to substitute the water phantom for acquiring linac beam data. This is particularly useful for QA measurements and checking linac parameters during and after unplanned technical interventions.

SU-F-P-43: Use of Freeware Business Intelligence Software to Trend TG-142 Compliant Linac QA Parameters

Purpose: To develop a software platform to track linac QA parameter trends using readily available freeware business intelligence (BI) software Methods: Spreadsheets were used to collect QA data by many, if not all, institutions; but the trending analysis with data from separate spreadsheets could be a cumbersome task. A freeware version of the Microsoft Power BI software was adapted to trend linac QA parameters to allow us to maintain nine dosimetrically equivalent linacs. A monthly QA spreadsheet (Microsoft Excel) has been used in our institution to collect QA data. We developed a C# computer program to mine QA data from spreadsheets in an automated way and store in a SQL database. The program runs every night automatically, crawls down a predetermined directory in a network hard-drive where all data spreadsheets are saved and searches for new data. If new data are found, they are written into the database. The implemented BI software reads QA data from the database and provides users with dynamic data trending interface as dashboards. The BI dashboard was configured to dynamically filter and drill down to specific parameters and conditions. This enables users to see the existence or missing data for specific tests and trends of linac parameters potentially alerting actions to be taken. Results: The developed software platform has been in use since 2015. More than 7680 data points corresponding to TG-142 compliant monthly QAs for nine linacs were automatically retrieved and stored in the database. The developed dashboard has provided access to data for physicists enabling them to dissect the data to observe trends in different parameters in just a few mouse clicks. Conclusion: The developed BI dashboard has been an invaluable tool providing a common data analysis platform for a quick and easy access to all linac QA parameters and their trends.

Linking log files with dosimetric accuracy – A multi-institutional study on quality assurance of volumetric modulated arc therapy

PurposeTo systematically evaluate machine specific quality assurance (QA) for volumetric modulated arc therapy (VMAT) based on log files by applying a dynamic benchmark plan. Methods and materialsA VMAT benchmark plan was created and tested on 18 Elekta linacs (13 MLCi or MLCi2, 5 Agility) at 4 different institutions. Linac log files were analyzed and a delivery robustness index was introduced. For dosimetric measurements an ionization chamber array was used. Relative dose deviations were assessed by mean gamma for each control point and compared to the log file evaluation. ResultsFourteen linacs delivered the VMAT benchmark plan, while 4 linacs failed by consistently terminating the delivery. The mean leaf error (±1SD) was 0.3±0.2mm for all linacs. Large MLC maximum errors up to 6.5mm were observed at reversal positions. Delivery robustness index accounting for MLC position correction (0.8–1.0) correlated with delivery time (80–128s) and depended on dose rate performance. Dosimetric evaluation indicated in general accurate plan reproducibility with γmean(±1SD)=0.4±0.2 for 1mm/1%. However single control point analysis revealed larger deviations and attributed well to log file analysis. ConclusionThe designed benchmark plan helped identify linac related malfunctions in dynamic mode for VMAT. Log files serve as an important additional QA measure to understand and visualize dynamic linac parameters.