Impact of electronic portal image quality on Elekta AQUA® collimator isocenter

PURPOSE

The collimator radiation isocenter position determined in AQUA^® v3.0 [Elekta AB, Stockholm, Sweden] software test "MLC Leaf and Jaw Position" was independently validated using an in-house MATLAB [Natick, MA: The MathWorks Inc.] script.

METHODS

The AQUA test determines radiation isocenter using the mean field center of nine 4 cm × 4 cm electronic portal imager (EPID) exposures at equidistant collimator angles. Impact of EPID image quality on AQUA reported isocenter for thirteen Elekta linear accelerators with Agility MLC heads were evaluated.

RESULTS

Of the thirteen, three had visually and quantitatively identifiable artifacts. For the ten good EPID's there was a systematic 0.25 mm offset of the MATLAB calculated mean field center relative to AQUA in the X-axis and Y-axis. This corresponds to one image pixel and was found to be due to differences in software co-ordinate convention. After subtracting this offset there was no significant difference in AQUA and MATLAB calculated isocenter.

CONCLUSIONS

For the three machines with poor image quality there was a demonstrated variation in AQUA calculated field center and therefore radiation isocenter relative to MATLAB. Restricting the region of interest (ROI) in AQUA software to only the irradiated section of the EPID brought AQUA and MATLAB result for these three machines into agreement.

Machine Log File and Calibration Errors-based Patient-specific Quality Assurance (QA) for Volumetric Modulated Arc Therapy (VMAT)

INTRODUCTION

Dose reconstructed based on linear accelerator (linac) log-files is one of the widely used solutions to perform patient-specific quality assurance (QA). However, it has a drawback that the accuracy of log-file is highly dependent on the linac calibration. The objective of the current study is to represent a new practical approach for a patient-specific QA during Volumetric modulated arc therapy (VMAT) using both log-file and calibration errors of linac.

METHODS

A total of six cases, including two head and neck neoplasms, two lung cancers, and two rectal carcinomas, were selected. The VMAT-based delivery was optimized by the TPS of Pinnacle^3 subsequently, using Elekta Synergy VMAT linac (Elekta Oncology Systems, Crawley, UK), which was equipped with 80 Multi-leaf collimators (MLCs) and the energy of the ray selected at 6 MV. Clinical mode log-file of this linac was used in this study. A series of test fields validate the accuracy of log-file. Then, six plans of test cases were delivered and log-file of each was obtained. The log-file errors were added to the corresponding plans through the house script and the first reconstructed plan was obtained. Later, a series of tests were performed to evaluate the major calibration errors of the linac (dose-rate, gantry angle, MLC leaf position) and the errors were added to the first reconstruction plan to generate the second reconstruction plan. At last, all plans were imported to Pinnacle and recalculated dose distribution on patient CT and ArcCheck phantom (SUN Nuclear). For the former, both target and OAR dose differences between them were compared. For the latter, γ was evaluated by ArcCheck, and subsequently, the surface dose differences between them were performed.

RESULTS

Accuracy of log-file was validated. If error recordings in the log file were only considered, there were four arcs whose proportion of control points with gantry angle errors more than ± 1°larger than 35%. Errors of leaves within ± 0.5 mm were 95% for all arcs. The distinctness of a single control point MU was bigger, but the distinctness of cumulative MU was smaller. The maximum, minimum, and mean doses for all targets were distributed between -6.79E-02-0.42%, -0.38-0.4%, 2.69E-02-8.54E-02% respectively, whereas for all OAR, the maximum and mean dose were distributed between -1.16-2.51%, -1.21-3.12% respectively. For the second reconstructed dose: the maximum, minimum, and mean dose for all targets was distributed between 0.0995~5.7145%, 0.6892~4.4727%, 0.5829~1.8931% separately. Due to OAR, maximum and mean dose distribution was observed between -3.1462~6.8920%, -6.9899~1.9316%, respectively.

CONCLUSION

Patient-specific QA based on the log-file could reflect the accuracy of the linac execution plan, which usually has a small influence on dose delivery. When the linac calibration errors were considered, the reconstructed dose was closer to the actual delivery and the developed method was accurate and practical.

[Impact of multi-leaf collimator positioning accuracy on quality control of volumetric modulation arc therapy plan for cervical cancer treated with Elekta linear accelerator]

OBJECTIVE

To investigate the influence of positioning accuracy of the multi-leaf collimators (MLC) on the passing rate of the plan dose verification for volumetric modulation arc therapy (VMAT) of cervical cancer using an Elekta linear accelerator.

METHODS

The dose distributions were measured using Sun Nuclear's Mapcheck and Arccheck semiconductors matrix before and after MLC calibration in30 cervical cancer patients undergoing VMAT. Dosimetric comparisons were performed with 2D and 3D gamma passing rates of 3%, 3 mm and 2%, and 2 mm. The 3D gamma distribution was reconstructed with respect to the patient's anatomy using 3DVH software to evaluate the possible influence of MLC positioning accuracy.

RESULTS

Before and after MLC calibration, the gamma passing rates of Mapcheck were (88.80±1.81)% and (99.25 ± 0.53)% under 3% and 3 mm standard, respectively, with an average increase of 10.45%. The corresponding gamma passing rates of Arccheck were (87.61±1.98)% and (98.13±0.99)%, respectively, with an average increase of 10.52%. The gamma passing rates of 3DVH were (89.87±2.28)% and (98.3±1.15)%, respectively, with an average increase of 8.43%.

CONCLUSION

The MLC positioning accuracy is one of the main factors influencing dosimetric accuracy of VMAT for cervical cancer. The application of Autocal software facilitates MLC calibration and improves the accuracy and safety of VMAT delivery for cervical cancer.

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.

Evaluation of QA software system analysis for the static picket fence test

Intensity modulation treatments are widely used in radiotherapy because of many known advantages. In this context, the picket fence test (PF) is a relevant test to check the Multileaf Collimator (MLC) performances. So this work compares and evaluates three analysis platforms for the PF used routinely by three different institutions. This study covers two linear accelerators (Linac) with two MLC types, a Millenium 120 MLC and Millenium 120 High Definition MLC respectively on a Varian Truebeam and Truebeam STx. Both linacs include an As 1200 portal imager (EPID). From a reference PF plan, MLC errors have been introduced to modify the slits in position or width (shifts from 0.1 to 0.5 mm on one or both banks). Then errors have been defined on the EPID to investigate detection system deviations (signal sensitivity and position variations). Finally, 110 DICOM‐RT images have been generated and analyzed by each software system. All software systems have shown good performances to quantify the position errors, even though the leaf pair identifications can be wrong in some cases regarding the analysis method considered. The slit width measurement (not calculated by all software systems) has shown good sensitivity, but some quantification difficulties have been highlighted regardless of the analysis method used. Linked to the expected accuracy of the PF test, the imager variations have demonstrated considerable influence in the results. Differences in the results and the analysis methods have been pointed out for each software system. The results can be helpful to optimize the settings of each analysis software system depending on expectations and treatment modalities of each institution.

AAPM Task Group 198 Report: An implementation guide for TG 142 quality assurance of medical accelerators

The charges on this task group (TG) were as follows: (a) provide specific procedural guidelines for performing the tests recommended in TG 142; (b) provide estimate of the range of time, appropriate personnel, and qualifications necessary to complete the tests in TG 142; and (c) provide sample daily, weekly, monthly, or annual quality assurance (QA) forms. Many of the guidelines in this report are drawn from the literature and are included in the references. When literature was not available, specific test methods reflect the experiences of the TG members (e.g., a test method for door interlock is self-evident with no literature necessary). In other cases, the technology is so new that no literature for test methods was available. Given broad clinical adaptation of volumetric modulated arc therapy (VMAT), which is not a specific topic of TG 142, several tests and criteria specific to VMAT were added.

Efficient quality assurance method with automated data acquisition of a single phantom setup to determine radiation and imaging isocenter congruence

We developed a quality assurance (QA) method to determine the isocenter congruence of Optical Surface Monitoring System (OSMS, Varian, CA, USA), kilovoltage (kV), and megavoltage (MV) imaging, and the radiation isocenter using a single setup of the OSMS phantom for frameless Stereotactic Radiosurgery (SRS) treatment. After aligning the phantom to the OSMS isocenter, a cone‐beam computed tomography (CBCT) of the phantom was acquired and registered to a computed tomography (CT) scan of the phantom to determine the CBCT isocenter. Without moving the phantom, MV and kV images were simultaneously acquired at four gantry angles to localize MV and kV isocenters. Then, Winston‐Lutz (W‐L) test images of the central BB in the phantom were acquired to analyze the radiation isocenter. The gantry and couch were automatically controlled using the TrueBeam Developer Mode during MV, kV, and W‐L image acquisition. All the images were acquired weekly for 17 weeks to track the congruence of all the imaging modalities' isocenter in six‐dimensional (6D) translations and rotations, and the radiation isocenter in three‐dimensional (3D) translations. The shifts of isocenters of all imaging modalities and the radiation isocenter from the OSMS isocenter were within 0.2 mm and 0.2° on average over 17 weeks. The maximum discrepancy between OSMS and other imaging modalities or radiation isocenters was 0.8 mm and 0.3°. However, systematic shifts of radiation isocenter anteriorly and laterally relative to the OSMS isocenter were observed. The measured discrepancies were consistent from week‐to‐week except for two weeks when the isocenter discrepancies of 0.8 mm were noted due to drifts of the OSMS isocenter. Once recalibration was performed on OSMS, the discrepancy was reduced to 0.3 mm and 0.2°.By performing the proposed QA on a weekly basis, the isocenter congruencies of multiple imaging systems and radiation isocenter were validated for a linear accelerator.

Quality assurance of linear accelerator: a comprehensive system using electronic portal imaging device

Aim

The Electronic Portal Imaging Device (EPID), primarily used for patient setup during radiotherapy sessions is also used for dosimetric measurements. In the present study, the feasibility of EPID in both machine and patient-specific quality assurance (QA) are investigated. We have developed a comprehensive software tool for effective utilisation of EPID in our institutional QA protocol.

Materials and methods

Portal Vision aS1000, amorphous silicon portal detector attached to Clinac iX—Linear Accelerator (LINAC) was used to measure daily profile and output constancy, various Multi-Leaf Collimator (MLC) checks and patient plan verification. Different QA plans were generated with the help of Eclipse Treatment Planning System (TPS) and MLC shaper software. The indigenously developed MATLAB programs were used for image analysis. Flatness, symmetry, output constancy, Field Width at Half Maximum (FWHM) and fluence comparison were studied from images obtained from TPS and EPID dosimetry.

Results

The 3 years institutional data of profile constancy and patient-specific QA measured using EPID were found within the acceptable limits. The daily output of photon beam correlated with the output obtained through solid phantom measurements. The Pearson correlation coefficients are 0.941 (p = 0.0001), 0.888 (p = 0.0188) and 0.917 (p = 0.0007) for the years of 2014, 2015 and 2016, respectively. The accuracy of MLC for shaping complex treatment fields was studied in terms of FWHM at different portions of various fields, showed good agreement between TPS-generated and EPID-measured MLC positions. The comparison of selected patient plans in EPID with an independent 2D array detector system showed statistically significant correlation between these two systems. Percentage difference between TPS computed and EPID measured fluence maps calculated for number of patients using MATLAB code also exhibited the validity of those plans for treatment.

Physical and Dosimetric Aspect of Euromechanics Add-on Multileaf Collimator on Varian Clinac 2100 C/D

BACKGROUND

Before treatment planning and dose delivery, quality assurance of multi-leaf collimator (MLC) has an important role in intensity-modulated radiation therapy (IMRT) due to the creation of multiple segments from optimization process.

OBJECTIVE

The purpose of this study is to assess the quality control of MLC leaves using EBT3 Gafchromic films.

MATERIAL AND METHODS

Leaf Position accuracy and leaf gap reproducibility were checked with Garden fence test. The garden fence test consists of 5 thin bands A) 0.2 Cm width spaced at 2 Cm intervals and B) 1 Cm width spaced at 1 Cm intervals. Each leaf accuracy was analyzed with measuring the full-width half-maximum (FWHM). Maximum and average leaf transmission were measured with gafchromic EBT3 films from Ashland for both 6 MV and 18 MV beams.

RESULTS

Leaf positions were found to be in a range between 1.78 - 2.53 mm, instead of nominal 2 mm for the test A and between 9.09 - 10.36 mm, instead of nominal 10 mm for the test B. The Average radiation transmission of the MLC was noted 1.79% and 1.98% of the open 10x10 Cm² field at isocenter for 6 MV and 18 MV beams, respectively. Maximum radiation transmission was noted 4.1% and 4.4% for 6 MV and 18 MV beams, respectively.

CONCLUSION

In this study, application of gafchromic EBT3 films for the quality assurance of Euromechanics multileaf collimator was studied. Our results showed that the average leaf leakage and positional accuracy of this type of MLC were in the acceptance level based on the Protocols.

MLC positioning verification for small fields: a new investigation into automatic EPID-based verification methods

Multileaf-collimator (MLC) defined small fields in radiotherapy are used in high dose, ultra-conformal techniques such as stereotactic radiotherapy and stereotactic radiosurgery. Proximity to critical structures and irreversible damage arising from inaccurate delivery mean that correct positioning of the MLC system is of the utmost importance. Some of the existing techniques for MLC positioning quality assurance make use of electronic portal imaging device (EPID) images. However, conventional collimation verification algorithms based on the full width at half maximum (FWHM) fail when applied to small field images acquired by an EPID due to overlapping aperture penumbrae, lateral electron disequilibrium and radiation source occlusion. The objective of this study was to investigate sub-pixel edge detection and other techniques with the aim of developing an automatic and autonomous EPID-based method suitable for MLC positional verification of small static fields with arbitrary shapes. Methods investigated included derivative interpolation, Laplacian of Gaussian (LoG) and an algorithm based on the partial area effect hypothesis. None of these methods were found to be suitable for MLC positioning verification in small field conditions. A method is proposed which uses a manufacturer-specific empirically modified FWHM algorithm which shows improvement over the conventional techniques in the small field size range. With a measured mean absolute difference from planned position for Varian linacs of 0.01 ± 0.26 mm, compared with the erroneous FWHM value of 0.70 ± 0.51 mm. For Elekta linacs the proposed algorithm returned 0.26 ± 0.25 mm, in contrast to the FWHM result of 1.79 ± 1.07 mm.

Normalize the response of EPID in pursuit of linear accelerator dosimetry standardization

Normalize the response of electronic portal imaging device (EPID) is the first step toward an EPID‐based standardization of Linear Accelerator (linac) dosimetry quality assurance. In this study, we described an approach to generate two‐dimensional (2D) pixel sensitivity maps (PSM) for EPIDs response normalization utilizing an alternative beam and dark‐field (ABDF) image acquisition technique and large overlapping field irradiations. The automated image acquisition was performed by XML‐controlled machine operation and the PSM was generated based on a recursive calculation algorithm for Varian linacs equipped with aS1000 and aS1200 imager panels. Cross‐comparisons of normalized beam profiles and 1.5%/1.5 mm 1D Gamma analysis was adopted to quantify the improvement of beam profile matching before and after PSM corrections. PSMs were derived for both photon (6, 10, 15 MV) and electron (6, 20 MeV) beams via proposed method. The PSM‐corrected images reproduced a horn‐shaped profile for photon beams and a relative uniform profiles for electrons. For dosimetrically matched linacs equipped with aS1000 panels, PSM‐corrected images showed increased 1D‐Gamma passing rates for all energies, with an average 10.5% improvement for crossline and 37% for inline beam profiles. Similar improvements in the phantom study were observed with a maximum improvement of 32% for 15 MV and 22% for 20 MeV. The PSM value showed no significant change for all energies over a 3‐month period. In conclusion, the proposed approach correct EPID response for both aS1000 and aS1200 panels. This strategy enables the possibility to standardize linac dosimetry QA and to benchmark linac performance utilizing EPID as the common detector.

Rapid acceptance testing of modern linac using on-board MV and kV imaging systems

PURPOSE

The purpose of this study was to develop a novel process for using on-board MV and kV Electronic Portal Imaging Devices (EPIDs) to perform linac acceptance testing (AT) for two reasons: (a) to standardize the assessment of new equipment performance, and (b) to reduce the time to clinical use while reducing physicist workload.

METHODS AND MATERIALS

In this study, Varian TrueBeam linacs equipped with amorphous silicon-based EPID (aS1000) were used. The conventional set of AT tests and tolerances were used as a baseline guide. A novel methodology was developed or adopted from published literature to perform as many tests as possible using the MV and kV EPIDs. The developer mode on Varian TrueBeam linacs was used to automate the process. In the EPID-based approach, most of mechanical tests were conducted by acquiring images through a custom phantom and software tools were developed for quantitative analysis to extract different performance parameters. The embedded steel-spheres in a custom phantom provided both visual and radiographic guidance for beam geometry testing. For photon beams, open field EPID images were used to extract inline/crossline profiles to verify the beam energy, flatness and symmetry. EPID images through a double wedge phantom were used for evaluating electron beam properties via diagonal profile. Testing was augmented with a commercial automated application (Machine Performance Check) which was used to perform several geometric accuracy tests such as gantry, collimator rotations, and couch rotations/translations.

RESULTS

The developed process demonstrated that the tests, which required customer demonstration, were efficiently performed using EPIDs. The AT tests that were performed using EPIDs were fully automated using the developer mode on the Varian TrueBeam system, while some tests, such as the light field versus radiation field congruence, and collision interlock checks required user interaction.

CONCLUSIONS

On-board imagers are quite suitable for both geometric and dosimetric testing of linac system involved in AT. Electronic format of the acquired data lends itself to benchmarking, transparency, as well as longitudinal use of AT data. While the tests were performed on a specific model of a linear accelerator, the proposed approach can be extended to other linacs.

A quantitative method to the analysis of MLC leaf position and speed based on EPID and EBT3 film for dynamic IMRT treatment with different types of MLC

A quantitative method based on the electronic portal imaging system (EPID) and film was developed for MLC position and speed testing; this method was used for three MLC types (Millennium, MLCi, and Agility MLC). To determine the leaf position, a picket fence designed by the dynamic (DMLC) model was used. The full‐width half‐maximum (FWHM) values of each gap measured by EPID and EBT3 were converted to the gap width using the FWHM versus nominal gap width relationship. The algorithm developed for the picket fence analysis was able to quantify the gap width, the distance between gaps, and each individual leaf position. To determine the leaf speed, a 0.5 × 20 cm²MLC‐defined sliding gap was applied across a 14 × 20 cm² symmetry field. The linacs ran at a fixed‐dose rate. The use of different monitor units (MUs) for this test led to different leaf speeds. The effect of leaf transmission was considered in a speed accuracy analysis. The difference between the EPID and film results for the MLC position is less than 0.1 mm. For the three MLC types, twice the standard deviation (2 SD) is provided; 0.2, 0.4, and 0.4 mm for gap widths of three MLC types, and 0.1, 0.2, and 0.2 mm for distances between gaps. The individual leaf positions deviate from the preset positions within 0.1 mm. The variations in the speed profiles for the EPID and EBT3 results are consistent, but the EPID results are slightly better than the film results. Different speeds were measured for each MLC type. For all three MLC types, speed errors increase with increasing speed. The analysis speeds deviate from the preset speeds within approximately 0.01 cm s⁻¹. This quantitative analysis of MLC position and speed provides an intuitive evaluation for MLC quality assurance (QA).

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

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

PACS number(s): 87.53.Ly, 87.56.Fc

Optimization of the rounded leaf offset table in modeling the multileaf collimator leaf edge in a commercial treatment planning system

An editable rounded leaf offset (RLO) table is provided in the Pinnacle³ treatment planning software. Default tables are provided for major linear accelerator manufacturers, but it is not clear how the default table values should be adjusted by the user to optimize agreement between the calculated leaf tip value and the actual measured value. Since we wish for the calculated MLC‐defined field edge to closely match the actual delivered field edge, optimal RLO table values are crucial. This is especially true for IMRT fields containing a large number of segments, since any errors would add together. A method based on the calculated MLC‐defined field edge was developed for optimizing and modifying the default RLO table values. Modified RLO tables were developed and evaluated for both dosimetric and light field‐based MLC leaf calibrations. It was shown, using a Picket Fence type test, that the optimized RLO table better modeled the calculated leaf tip than the Pinnacle³ default table. This was demonstrated for both an Elekta Synergy 80‐leaf and a Varian 120‐leaf MLC.

PACS numbers: 87.55.D‐, 87.55.de, 87.55.Qr

Detector system dose verification comparisons for arc therapy: couch vs. gantry mount

The aim of this study was to assess the performance of a gantry‐mounted detector system and a couch set detector system using a systematic multileaf collimator positional error manually introduced for volumetric‐modulated arc therapy. Four head and neck and esophagus VMAT plans were evaluated by measurement using an electronic portal imaging device and an ion chamber array. Each plan was copied and duplicated with a 1 mm systematic MLC positional error in the left leaf bank. Direct comparison of measurements for plans with and without the error permitted observational characteristics for quality assurance performance between detectors. A total of 48 different plans were evaluated for this testing. The mean percentage planar dose differences required to satisfy a 95% match between plans with and without the MLCPE were 5.2% ± 0.5% for the chamber array with gantry motion, 8.12% ± 1.04% for the chamber array with a static gantry at 0°, and 10.9% ± 1.4% for the EPID with gantry motion. It was observed that the EPID was less accurate due to overresponse of the MLCPE in the left leaf bank. The EPID always images bank‐A on the ipsilateral side of the detector, whereas for a chamber array or for a patient, that bank changes as it crosses the ‐90° or +90° position. A couch set detector system can reproduce the TPS calculated values most consistently. We recommend it as the most reliable patient specific QA system for MLC position error testing. This research is highlighted by the finding of up to 12.7% dose variation for H/N and esophagus cases for VMAT delivery, where the mere source of error was the stated clinically acceptability of 1 mm MLC position deviation of TG‐142.

PACS numbers: 87.56.‐v, 87.55.‐x, 07.57.KP, 29.40.‐n, 85.25.Pb

An independent system for real-time dynamic multileaf collimation trajectory verification using EPID

A new tool has been developed to verify the trajectory of dynamic multileaf collimators (MLCs) used in advanced radiotherapy techniques using only the information provided by the electronic portal imaging devices (EPID) measured image frames. The prescribed leaf positions are resampled to a higher resolution in a pre-processing stage to improve the verification precision. Measured MLC positions are extracted from the EPID frames using a template matching method. A cosine similarity metric is then applied to synchronise measured and planned leaf positions for comparison. Three additional comparison functions were incorporated to ensure robust synchronisation. The MLC leaf trajectory error detection was simulated for both intensity modulated radiation therapy (IMRT) (prostate) and volumetric modulated arc therapy (VMAT) (head-and-neck) deliveries with anthropomorphic phantoms in the beam. The overall accuracy for MLC positions automatically extracted from EPID image frames was approximately 0.5 mm. The MLC leaf trajectory verification system can detect leaf position errors during IMRT and VMAT with a tolerance of 3.5 mm within 1 s.

Evaluation of relative transmitted dose for a step and shoot head and neck intensity modulated radiation therapy using a scanning liquid ionization chamber electronic portal imaging device

The dose delivery verification for a head and neck static intensity modulated radiation therapy (IMRT) case using a scanning liquid ionization chamber electronic portal imaging device (SLIC-EPID) was investigated. Acquired electronic portal images were firstly converted into transmitted dose maps using an in-house developed method. The dose distributions were then compared with those calculated in a virtual EPID using the Pinnacle³ treatment planning system (TPS). Using gamma evaluation with the ΔD_max and DTA criteria of 3%/2.54 mm, an excellent agreement was observed between transmitted dose measured using SLIC-EPID and that calculated by TPS (gamma score approximately 95%) for large MLC fields. In contrast, for several small subfields, due to SLIC-EPID image blurring, significant disagreement was found in the gamma results. Differences between EPID and TPS dose maps were also observed for several parts of the radiation subfields, when the radiation beam passed through air on the outside of tissue. The transmitted dose distributions measured using portal imagers such as SLIC-EPID can be used to verify the dose delivery to a patient. However, several aspects such as accurate calibration procedure and imager response under different conditions should be taken into the consideration. In addition, SLIC-EPID image blurring is another important issue, which should be considered if the SLIC-EPID is used for clinical dosimetry verification.

Quality assurance of dynamic parameters in volumetric modulated arc therapy

OBJECTIVES

The purpose of this study was to demonstrate quality assurance checks for accuracy of gantry speed and position, dose rate and multileaf collimator (MLC) speed and position for a volumetric modulated arc treatment (VMAT) modality (Synergy S; Elekta, Stockholm, Sweden), and to check that all the necessary variables and parameters were synchronous.

METHODS

Three tests (for gantry position-dose delivery synchronisation, gantry speed-dose delivery synchronisation and MLC leaf speed and positions) were performed.

RESULTS

The average error in gantry position was 0.5° and the average difference was 3 MU for a linear and a parabolic relationship between gantry position and delivered dose. In the third part of this test (sawtooth variation), the maximum difference was 9.3 MU, with a gantry position difference of 1.2°. In the sweeping field method test, a linear relationship was observed between recorded doses and distance from the central axis, as expected. In the open field method, errors were encountered at the beginning and at the end of the delivery arc, termed the "beginning" and "end" errors. For MLC position verification, the maximum error was -2.46 mm and the mean error was 0.0153 ±0.4668 mm, and 3.4% of leaves analysed showed errors of >±1 mm.

CONCLUSION

This experiment demonstrates that the variables and parameters of the Synergy S are synchronous and that the system is suitable for delivering VMAT using a dynamic MLC.

Quality assurance of MLC leaf position accuracy and relative dose effect at the MLC abutment region using an electronic portal imaging device

We investigated an electronic portal image device (EPID)-based method to see whether it provides effective and accurate relative dose measurement at abutment leaves in terms of positional errors of the multi-leaf collimator (MLC) leaf position. A Siemens ONCOR machine was used. For the garden fence test, a rectangular field (0.2 20 cm) was sequentially irradiated 11 times at 2-cm intervals. Deviations from planned leaf positions were calculated. For the nongap test, relative doses at the MLC abutment region were evaluated by sequential irradiation of a rectangular field (2 20 cm) 10 times with a MLC separation of 2 cm without a leaf gap. The integral signal in a region of interest was set to position A (between leaves) and B (neighbor of A). A pixel value at position B was used as background and the pixel ratio (A/B 100) was calculated. Both tests were performed at four gantry angles (0, 90, 180 and 270°) four times over 1 month. For the nongap test the difference in pixel ratio between the first and last period was calculated. Regarding results, average deviations from planned positions with the garden fence test were within 0.5 mm at all gantry angles, and at gantry angles of 90 and 270° tended to decrease gradually over the month. For the nongap test, pixel ratio tended to increase gradually in all leaves, leading to a decrease in relative doses at abutment regions. This phenomenon was affected by both gravity arising from the gantry angle, and the hardware-associated contraction of field size with this type of machine.

QA of dynamic MLC based on EPID portal dosimetry

PURPOSE

Dynamic delivery of intensity modulated beams (dIMRT) requires not only accurate verification of leaf positioning but also a control on the speed of motion. The latter is a parameter that has a major impact on the dose delivered to the patient. Time consumed in quality assurance (QA) procedures is an issue of relevance in any radiotherapy department. Electronic portal imaging dosimetry (EPID) can be very efficient for routine tests. The purpose of this work is to investigate the ability of our EPID for detecting small errors in leaf positioning, and to present our daily QA procedures for dIMRT based on EPID.

METHODS AND MATERIALS

A Varian 2100 CD Clinac equipped with an 80 leaf Millennium MLC and with amorphous silicon based EPID (aS500, Varian) is used. The daily QA program consists in performing: Stability check of the EPID signal, Garden fence test, Sweeping slit test, and Leaf speed test.

RESULTS AND DISCUSSION

The EPID system exhibits good long term reproducibility. The mean portal dose at the centre of a 10 × 10 cm(2) static field was 1.002 ± 0.004 (range 1.013-0.995) for the period evaluated of 47 weeks. Garden fence test shows that leaf position errors of up to 0.2 mm can be detected. With the Sweeping slit test we are able to detect small deviations on the gap width and errors of individual leaves of 0.5 and 0.2 mm. With the Leaf speed test problems due to motor fatigue or friction between leaves can be detected.

CONCLUSIONS

This set of tests takes no longer than 5 min in the linac treatment room. With EPID dosimetry, a consistent daily QA program can be applied, giving complete information about positioning/speed MLC.

Dosimetric impact of systematic MLC positional errors on step and shoot IMRT for prostate cancer: a planning study

The positional accuracy of multileaf collimators (MLC) is crucial in ensuring precise delivery of intensity-modulated radiotherapy (IMRT). The aim of this planning study was to investigate the dosimetric impact of systematic MLC positional errors on step and shoot IMRT of prostate cancer. A total of 12 perturbations of MLC leaf banks were introduced to six prostate IMRT treatment plans to simulate MLC systematic positional errors. Dose volume histograms (DVHs) were generated for the extraction of dose endpoint parameters. Plans were evaluated in terms of changes to the defined endpoint dose parameters, conformity index (CI) and healthy tissue avoidance (HTA) to planning target volume (PTV), rectum and bladder. Negative perturbations of MLC had been found to produce greater changes to endpoint dose parameters than positive perturbations of MLC (p < 0.01). Negative and positive asynchronised MLC perturbations of -1 mm resulted in median changes in D(95) of -1.2 and 0.9% respectively. Negative and positive synchronised MLC perturbations of 1 mm in one direction resulted in median changes in D(95) of -2.3 and 1.8% respectively. Doses to rectum were generally more sensitive to systematic MLC errors compared to bladder (p < 0.01). Negative and positive synchronised MLC perturbations of 1 mm in one direction resulted in median changes in endpoint dose parameters of rectum and bladder from 1.0 to 2.5%. Maximum reduction of -4.4 and -7.3% were recorded for conformity index (CI) and healthy tissue avoidance (HTA) respectively due to synchronised MLC perturbation of 1 mm. MLC errors resulted in dosimetric changes in IMRT plans for prostate.

An MLC calibration method using a detector array

PURPOSE

The authors have developed a quantitative calibration method for a multileaf collimator (MLC) which measures individual leaf positions relative to the MLC backup jaw on an Elekta Synergy linear accelerator.

METHODS

The method utilizes a commercially available two-axis detector array (Profiler 2; Sun Nuclear Corporation, Melbourne, FL). To calibrate the MLC bank, its backup jaw is positioned at the central axis and the opposing jaw is retracted to create a half-beam configuration. The position of the backup jaws field edge is then measured with the array to obtain what is termed the radiation defined reference line. The positions of the individual leaf ends relative to this reference line are then inferred by the detector response in the leaf end penumbra. Iteratively adjusting and remeasuring the leaf end positions to within specifications completes the calibration. Using the backup jaw as a reference for the leaf end positions is based on three assumptions: (1) The leading edge of an MLC leaf bank is parallel to its backup jaw's leading edge, (2) the backup jaw position is reproducible, and (3) the measured radiation field edge created by each leaf end is representative of that leaf's position. Data from an electronic portal imaging device (EPID) were used in a similar analysis to check the results obtained with the array.

RESULTS

The relative leaf end positions measured with the array differed from those measured with the EPID by an average of 0.11+/-0.09 mm per leaf. The maximum leaf positional change measured with the Profiler 2 over a 3 month period was 0.51 mm. A leaf positional accuracy of +/-0.4 mm is easily attainable through the iterative calibration process. The method requires an average of 40 min to measure both leaf banks.

CONCLUSIONS

This work demonstrates that the Profiler 2 is an effective tool for efficient and quantitative MLC quality assurance and calibration.

Task Group 142 report: quality assurance of medical accelerators

The task group (TG) for quality assurance of medical accelerators was constituted by the American Association of Physicists in Medicine's Science Council under the direction of the Radiation Therapy Committee and the Quality Assurance and Outcome Improvement Subcommittee. The task group (TG-142) had two main charges. First to update, as needed, recommendations of Table II of the AAPM TG-40 report on quality assurance and second, to add recommendations for asymmetric jaws, multileaf collimation (MLC), and dynamic/virtual wedges. The TG accomplished the update to TG-40, specifying new test and tolerances, and has added recommendations for not only the new ancillary delivery technologies but also for imaging devices that are part of the linear accelerator. The imaging devices include x-ray imaging, photon portal imaging, and cone-beam CT. The TG report was designed to account for the types of treatments delivered with the particular machine. For example, machines that are used for radiosurgery treatments or intensity-modulated radiotherapy (IMRT) require different tests and/or tolerances. There are specific recommendations for MLC quality assurance for machines performing IMRT. The report also gives recommendations as to action levels for the physicists to implement particular actions, whether they are inspection, scheduled action, or immediate and corrective action. The report is geared to be flexible for the physicist to customize the QA program depending on clinical utility. There are specific tables according to daily, monthly, and annual reviews, along with unique tables for wedge systems, MLC, and imaging checks. The report also gives specific recommendations regarding setup of a QA program by the physicist in regards to building a QA team, establishing procedures, training of personnel, documentation, and end-to-end system checks. The tabulated items of this report have been considerably expanded as compared with the original TG-40 report and the recommended tolerances accommodate differences in the intended use of the machine functionality (non-IMRT, IMRT, and stereotactic delivery).