Multileaf Collimator Quality Assurance Research Articles

An EPID‐based system for gantry‐resolved MLC quality assurance for VMAT

Multileaf collimator (MLC) positions should be precisely and independently measured as a function of gantry angle as part of a comprehensive quality assurance (QA) program for volumetric‐modulated arc therapy (VMAT). It is also ideal that such a QA program has the ability to relate MLC positional accuracy to patient‐specific dosimetry in order to determine the clinical significance of any detected MLC errors. In this work we propose a method to verify individual MLC trajectories during VMAT deliveries for use as a routine linear accelerator QA tool. We also extend this method to reconstruct the 3D patient dose in the treatment planning system based on the measured MLC trajectories and the original DICOM plan file. The method relies on extracting MLC positions from EPID images acquired at 8.41 fps during clinical VMAT deliveries. A gantry angle is automatically tagged to each image in order to obtain the MLC trajectories as a function of gantry angle. This analysis was performed for six clinical VMAT plans acquired at monthly intervals for three months. The measured trajectories for each delivery were compared to the MLC positions from the DICOM plan file. The maximum mean error detected was 0.07 mm and a maximum root‐mean‐square error was 0.8 mm for any leaf of any delivery. The sensitivity of this system was characterized by introducing random and systematic MLC errors into the test plans. It was demonstrated that the system is capable of detecting random and systematic errors on the range of 1–2 mm and single leaf calibration errors of 0.5 mm. The methodology developed in the work has potential to be used for efficient routine linear accelerator MLC QA and pretreatment patient‐specific QA and has the ability to relate measured MLC positional errors to 3D dosimetric errors within a patient volume.PACS number(s): 87.55.Qr

A clinically observed discrepancy between image-based and log-based MLC positions.

To present a clinical case in which real-time intratreatment imaging identified an multileaf collimator (MLC) leaf to be consistently deviating from its programmed and logged position by >1 mm. An EPID-based exit-fluence dosimetry system designed to prevent gross delivery errors was used to capture cine during treatment images. The author serendipitously visually identified a suspected MLC leaf displacement that was not otherwise detected. The leaf position as recorded on the EPID images was measured and log-files were analyzed for the treatment in question, the prior day's treatment, and for daily MLC test patterns acquired on those treatment days. Additional standard test patterns were used to quantify the leaf position. Whereas the log-file reported no difference between planned and recorded positions, image-based measurements showed the leaf to be 1.3 ± 0.1 mm medial from the planned position. This offset was confirmed with the test pattern irradiations. It has been clinically observed that log-file derived leaf positions can differ from their actual position by >1 mm, and therefore cannot be considered to be the actual leaf positions. This cautions the use of log-based methods for MLC or patient quality assurance without independent confirmation of log integrity. Frequent verification of MLC positions through independent means is a necessary precondition to trust log-file records. Intratreatment EPID imaging provides a method to capture departures from MLC planned positions.

Clinical significance of multi-leaf collimator calibration errors.

This planning study investigates the clinical impact of multi-leaf collimator (MLC) calibration errors on three common treatment sites; head and neck (H&N), prostate and stereotactic body radiotherapy (SBRT) for lung. All plans used using either volumetric modulated adaptive therapy or dynamic MLC techniques. Five patient plans were retrospectively selected from each treatment site, and MLC errors intentionally introduced. MLC errors of 0.7, 0.4 and 0.2 mm were sufficient to cause major violations in the PTV planning criteria for the H&N, prostate and SBRT lung plans. Mean PTV dose followed a linear trend with MLC error, increasing at rates of 3.2-5.9% per millimeter depending on treatment site. The results indicate that an MLC quality assurance program that provides sub-millimeter accuracy is an important component of intensity modulated radiotherapy delivery techniques.

Evaluation of AutoCAL for electronic portal imaging device-based multi-leaf collimator quality assurance

Modern radiotherapy treatment techniques commonly include multi-leaf collimators (MLCs) to shape the treatment fields and to conform the radiation dose to the target volume. MLCs require accurate and frequent quality assurance (QA) to ensure spatial and temporal accuracy of the leaves in order to allow optimal dose delivery to the patient. In this study, the accuracy and efficacy of AutoCAL, a commercial software for MLC QA, were evaluated. The software was found to be reproducible to within 0.2mm and to correspond with conventional QA methods (within 1.2 and 0.9mm of film and water tank measurements, respectively.) Thus, AutoCAL was found to be an accurate and efficient tool for routine MLC QA and calibration.

SU‐E‐T‐646: Quality Assurance of Truebeam Multi‐Leaf Collimator Using a MLC QA Phantom

Purpose:To perform a routine quality assurance procedure for Truebeam multi‐leaf collimator (MLC) using MLC QA phantom, verify the stability and reliability of MLC during the treatment.Methods:MLC QA phantom is a specialized phantom for MLC quality assurance (QA), and contains five radio‐opaque spheres that are embedded in an “L” shape. The phantom was placed isocentrically on the Truebeam treatment couch for the tests. A quality assurance plan was setted up in the Eclipse v10.0, the fields that need to be delivered in order to acquire the necessary images, the MLC shapes can then be obtained by the images. The images acquired by the electronic portal imaging device (EPID), and imported into the PIPSpro software for the analysis. The tests were delivered twelve weeks (once a week) to verify consistency of the delivery, and the images are acquired in the same manner each time.Results:For the Leaf position test, the average position error was 0.23mm±0.02mm (range: 0.18mm∼0.25mm). The Leaf width was measured at the isocenter, the average error was 0.06mm±0.02mm (range: 0.02mm∼0.08mm) for the Leaf width test. Multi‐Port test showed the dynamic leaf shift error, the average error was 0.28mm±0.03mm (range: 0.2mm∼0.35mm). For the leaf transmission test, the average inter‐leaf leakage value was 1.0%±0.17% (range: 0.8%∼1.3%) and the average inter‐bank leakage value was 32.6%±2.1% (range: 30.2%∼36.1%).Conclusion:By the test of 12 weeks, the MLC system of the Truebeam is running in a good condition and the MLC system can be steadily and reliably carried out during the treatment. The MLC QA phantom is a useful test tool for the MLC QA.

SU-E-T-195: Gantry Angle Dependency of MLC Leaf Position Error

Purpose: The aim of this study was to investigate the gantry angle dependency of the multileaf collimator (MLC) leaf position error. Methods: An automatic MLC quality assurance system (AutoMLCQA) was developed to evaluate the gantry angle dependency of the MLC leaf position error using an electronic portal imaging device (EPID). To eliminate the EPID position error due to gantry rotation, we designed a reference maker (RM) that could be inserted into the wedge mount. After setting up the EPID, a reference image was taken of the RM using an open field. Next, an EPID-based picket-fence test (PFT) was performed without the RM. These procedures were repeated at every 45° intervals of the gantry angle. A total of eight reference images and PFT image sets were analyzed using in-house software. The average MLC leaf position error was calculated at five pickets (-10, -5, 0, 5, and 10 cm) in accordance with general PFT guidelines using in-house software. This test was carried out for four linear accelerators. Results: The average MLC leaf position errors were within the set criterion of <1 mm (actual errors ranged from -0.7 to 0.8 mm) for all gantry angles, but significant gantry angle dependency was observed inmore » all machines. The error was smaller at a gantry angle of 0° but increased toward the positive direction with gantry angle increments in the clockwise direction. The error reached a maximum value at a gantry angle of 90° and then gradually decreased until 180°. In the counter-clockwise rotation of the gantry, the same pattern of error was observed but the error increased in the negative direction. Conclusion: The AutoMLCQA system was useful to evaluate the MLC leaf position error for various gantry angles without the EPID position error. The Gantry angle dependency should be considered during MLC leaf position error analysis.« less

SU-E-T-134: Automated Analysis of Leaf Position Accuracy for Multi-Leaf Collimator Quality Assurance

Purpose: The traditional QA method for checking the multi‐leaf collimator (MLC) positional accuracy is to expose a picket‐fence (PF) pattern on a piece of film. Evaluation of the film is often cumbersome or qualitative. We have developed a method that is simpler, more accurate, and capable of analyzing images from both static gantry and rotational gantry (i.e., VMAT) PF tests. Methods: PF tests with static gantry or VMAT delivery were performed on a Varian 21EX linac. The resulting EPID images were analyzed with an in‐house Matlab program. First, the PF lines were detected with a Hough transform algorithm. Then the leaf pairs were identified by fitting the image intensity profile of interleaf leakage with the ideal profile. For each leaf pair, the positions and widths of the PF gaps were computed by fitting the individual gaps with a Lorentzian function. Finally, the VMAT PF was overlaid and compared with the static gantry PF. Results: In the static gantry PF images, the distances between adjacent MLC PFs were <0.1 mm from the expected values. The distances between the MLC PFs and the jaw PF were <0.3 mm from expectation. The maximum angular difference between all PFs was 0.03 degrees. In one occasion the software detected a 0.34 mm gap position error. The corresponding gap width was 0.40 mm smaller than the neighboring gap widths. The misaligned leaf was identified automatically. For the VMAT PF tests, the distances between the VMAT PFs and static gantry PFs were <0.5 mm, while the angular differences were <0.2 degrees. Conclusion: Software was developed to analyze the static gantry and VMAT PF images. The software is sensitive and is able to detect leaf position errors of <0.4 mm. The overall leaf position accuracy is excellent in both static gantry and VMAT delivery.

SU-E-T-390: Monitorization of the Dynamic Multi-Leaf Collimator Performance Using Log Files - a Quality Assurance Tool for Intensity Modulated Radiotherapy.

Commission baseline values of the multi-leaf collimator (MLC) using Varian's dynalog file viewer software (DFV) and establish tolerance levels to use as a periodic quality assurance tool in intensity modulated radiotherapy (IMRT). A sliding window test was created to have all leafs moving at a constant high speed of 2.5cm/s. This test was performed for 100 days at gantry angles of 0°, 90° and 270° during the morning quality checks in an accelerator performing RapidArc treatments with a Millennium 120 MLC. The DFV was used to collect the percentage of counts in bins 1 (0.05mm) to 8 (deviations in steps of 0.5mm) and the root mean square error (RMS). These were analyzed statistically and a baseline value and tolerance levels were established. Results show that the greater percentage of counts is found at bin 4 with an average of 97%. Bins 1 to 3 have less than 1 % and bin 5 presents an average of 1.7%. As for bin 6, the average was below 0.1%. Bins 7 and 8 presented no counts. When comparing the results of the different gantry angles, significant differences are only found in bin 4 and 5, with a better performance for the 0° gantry. As for the RMS error, no difference was found between different carriages or gantry angles and the average value was 0.129. A tolerance level was established for bins that correspond to greater deviations - bins 5 to 8, with an upper limit equivalent to twice the standard deviation of each bin. With the introduction of IMRT, the MLC quality assurance became more extensive and thorough. However a quantitative analysis is often difficult to interpret and time consuming. The purposed method allows the monitorization of the stability and performance of the MLC in a quantitative and easy way.

Investigation of the feasibility of a simple method for verifying the motion of a binary multileaf collimator synchronized with the rotation of the gantry for helical tomotherapy

In this paper, we suggest a new method for verifying the motion of a binary multileaf collimator (MLC) in helical tomotherapy. For this we used a combination of a cylindrical scintillator and a general‐purpose camcorder. The camcorder records the light from the scintillator following photon irradiation, which we use to track the motion of the binary MLC. The purpose of this study is to demonstrate the feasibility of this method as a binary MLC quality assurance (QA) tool. First, the verification was performed using a simple binary MLC pattern with a constant leaf open time; secondly, verification using the binary MLC pattern used in a clinical setting was also performed. Sinograms of simple binary MLC patterns, in which leaves that were open were detected as “open” from the measured light, define the sensitivity which, in this case, was 1.000. On the other hand, the specificity, which gives the fraction of closed leaves detected as “closed”, was 0.919. The leaf open error identified by our method was −1.3±7.5%. The 68.6% of observed leaves were performed within ± 3% relative error. The leaf open error was expressed by the relative errors calculated on the sinogram. In the clinical binary MLC pattern, the sensitivity and specificity were 0.994 and 0.997, respectively. The measurement could be performed with −3.4±8.0% leaf open error. The 77.5% of observed leaves were performed within ± 3% relative error. With this method, we can easily verify the motion of the binary MLC, and the measurement unit developed was found to be an effective QA tool.PACS numbers: 87.56.Fc, 87.56.nk

SU-E-T-477: Multileaf Collimator Quality Assurance for Volumetric Modulated Arc Therapy

Purpose: To develop a quality assurance procedure for the multileaf collimator(MLC) in volumetric modulated arc therapy (VMAT) beam delivery. Method and Materials: A general pattern for MLC QA is proposed in this study to test the interaction between gantry speed and the MLC traveling speed on different linacs for VMAT dose delivery. An independently mounted film was used to catch the entire arc movement. To utilize similar concept for picket‐fence test, we divided the gantry 360degrees into two counterclockwise fields, 180 to 0 and 0 to 180degrees, and dynamic abutted segments were delivered on every 15 degree gantry rotation. The MLC traveled 3 cm distance in each segment dynamically, from starting opening at 0 and ending at 3cm. Starting point for each 15 degree segment was from 0th, 3rd, 6th, 9th…to 36th cm with MLC bank B moving. But each segment was delivered twice, first in counterclockwise and second time in clockwise directions with MLC travelling in opposite directions with starting point 3rd, 6th, 9th, 12th …to 39th cm with MLC bank A moving. These two fields had the same MLC segments. The accumulated dose on film was analyzed. Results: With perfect speed combination between gantry and MLC, the total dose profile along the leaf‐ motion direction is uniform. All leaves in each bank move together that will be reflected in the width of the strip and conjunction lines, while one leaf is out of range by 1 mm, the profile will show non‐uniform profile only at that leaf position. The conjunction lines of each segment also represent the reproducibility of the combination of gantry and MLC positions. Conclusion: We have investigated a quality assurance procedure for the MLC used in VMAT treatment. It can detect the mismatch between gantry speed and MLC motion to quantify the accuracy of VMAT dose delivery.

An MLC calibration method using a detector array

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

SU‐FF‐T‐307: A Multileaf Collimator Quality Assurance Tool for Modulated Electron Radiation Therapy

Purpose: Modulated electron radiation therapy (MERT) is an efficient treatment modality for shallow tumors. MERT provides superior target dose uniformity and conformity and spares distal critical structures. We have acquired a portable, motorized eMLC for MERT delivery at our institution. This work aims to investigate a quality assurance (QA) tool to check the leaf position accuracy for the eMLC using aSi‐EPID images. Materials and methods: The motorized eMLC consists of 27 pairs of 6mm width, 2cm thick tungsten leaves. Standard eMLC patterns were designed to test actual leaf positions using EPID images (pixel size:0.392mm) taken with 6MV photon beams. Compared to electron beams, photon beams provide sharper EPID images and therefore more precise leaf location determination. The leaf‐end positions were detected using an edge function, calibrated using film data to derive the optimal window‐level and contrast. A computer program was developed to check the leaf‐end position automatically using the EPID images. The position tolerance was set to 1mm. Results: Optimal image parameters have been obtained to detect eMLC leaf‐end position accurately. Extensive analysis of standard (reference) eMLC fields showed that EPID detected leaf‐end positions agreed their actual positions to ±2 pixels. After the eMLC leaves were moved back and forward 50 times the maximum leaf position error was 3 pixels. Online MERT eMLC segmented fields showed higher quality; leaf positions were within 0.5mm of their actual positions. We are using this eMLC QA tool to study the eMLC long‐term accuracy. Conclusion: An eMLC QA tool has been developed to check the leaf‐end position accuracy for MERT beam delivery using EPID images. This can be used as a routine QA procedure to ensure the accuracy of the eMLC so as to use a Monte Carlo based dose verification tool with the MUs and leaf sequences from the record and verify system.

On the sensitivity of patient‐specific IMRT QA to MLC positioning errors

Accurate multileaf collimator (MLC) leaf positioning plays an essential role in the effective implementation of intensity modulated radiation therapy (IMRT). This work evaluates the sensitivity of current patient‐specific IMRT quality assurance (QA) procedures to minor MLC leaf positioning errors. Random errors of up to 2 mm and systematic errors of ±1mm and ±2mm in MLC leaf positions were introduced into 8 clinical IMRT patient plans (totaling 53 fields). Planar dose distributions calculated with modified plans were compared to dose distributions measured with both radiochromic films and a diode matrix. The agreement between calculation and measurement was evaluated using both absolute distance‐to‐agreement (DTA) analysis and γ index with 2%/2mm and 3%/3mm criteria. It was found that both the radiochromic film and the diode matrix could only detect systematic errors on the order of 2 mm or above. The diode array had larger sensitivity than film due to its excellent detector response (such as small variation, linear response, etc.). No difference was found between DTA analysis and γ index in terms of the sensitivity to MLC positioning errors. Higher sensitivity was observed with 2%/2mm than with 3%/3mm in general. When using the diode array and 2%/2mm criterion, the IMRT QA procedure showed strongest sensitivity to MLC position errors and, at the same time, achieved clinically acceptable passing rates. More accurate dose calculation and measurement would further enhance the sensitivity of patient‐specific IMRT QA to MLC positioning errors. However, considering the significant dosimetric effect such MLC errors could cause, patient‐specific IMRT QA should be combined with a periodic MLC QA program in order to guarantee the accuracy of IMRT delivery.PACS numbers: 87.50.Gi, 87.52.Df, 87.52.Px, 87.53.Dq, 87.53.Tf, 87.53.Kn, 87.56.Fc

TU‐A‐AUD A‐01: LINAC and MLC QA for IMRT

Intensity‐modulated radiotherapy (IMRT) is now common in many therapy centers. Quality assurance (QA) procedures for a linear accelerator (linac) and multi‐leaf collimator (MLC) designed for conventional static fields do not address the unique dosimetric issues pertinent to IMRT planning and delivery. IMRT portals are composed of many irregular, small, off‐center, and abutting field segments throughout the target volume, each delivering only a few MU. In these regards, emphasis should be placed on beam stability for small MU and leaf position accuracy with gantry rotation. At the same time, the target volume, shielded by the MLC for much of the treatment, sees a larger transmitted dose, especially for MLC with rounded leaf ends, The impact of delivery technique, segmented (step and shoot) or dynamic (sliding window), on the required leaf position tolerance is not fully appreciated and will be addressed. While the mechanics and software of the various linacs and MLC distinguish them, there are many aspects of QA in common. This course will discuss: (1) the specific QA related to the MLC design; (2) additional QA for linear accelerators pertinent to the small MU and small field sizes used in IMRT; (3) tools and procedures often used to perform these QA tasks; (4) specific QA issues for different IMRT delivery methods.Educational Objectives:1. Understand different IMRT delivery methods and their specific QA issues.2. Understand effect of QA on the IMRT delivery accuracy.3. Understand the importance of proper commissioning of the planning system to avoid confusion with dose delivery issues.

MLC quality assurance using EPID: A fitting technique with subpixel precision

Amorphous silicon based electronic portal imaging devices (EPIDs) have been shown to be a good alternative to radiographic film for routine quality assurance (QA) of multileaf collimator (MLC) positioning accuracy. In this work, we present a method of acquiring an EPID image of a traditional strip-test image using analytical fits of the interleaf and leaf abutment image signatures. After exposure, the EPID image pixel values are divided by an open field image to remove EPID response and radiation field variations. Profiles acquired in the direction orthogonal to the leaf motion exhibit small peaks caused by interleaf leakage. Gaussian profiles are fitted to the interleaf leakage peaks, the results of which are, using multiobjective optimization, used to calculate the image rotational angle with respect to the collimator axis of rotation. The relative angle is used to rotate the image to align the MLC leaf travel to the image pixel axes. The leaf abutments also present peaks that are fitted by heuristic functions, in this case modified Lorentzian functions. The parameters of the Lorentzian functions are used to parameterize the leaf gap width and positions. By imaging a set of MLC fields with varying gaps forming symmetric and asymmetric abutments, calibration curves with regard to relative peak height (RPH) versus nominal gap width are obtained. Based on this calibration data, the individual leaf positions are calculated to compare with the nominal programmed positions. The results demonstrate that the collimator rotation angle can be determined as accurate as 0.01 degrees. A change in MLC gap width of 0.2 mm leads to a change in RPH of about 10%. For asymmetrically produced gaps, a 0.2 mm MLC leaf gap width change causes 0.2 pixel peak position change. Subpixel resolution is obtained by using a parameterized fit of the relatively large abutment peaks. By contrast, for symmetrical gap changes, the peak position remains unchanged with a standard deviation of 0.05 pixels, or 0.026 mm. A trial run of 36 test images, each with gap widths varying from 0.4 to 1.4 mm, were used to analyze 8640 abutments. The leaf position variations were detected with a precision of 0.1 mm at a 95% confidence level, with a mean of 0.04 mm and a standard deviation of 0.03 mm. The proposed method is robust and minimizes the effect of image noise and pixel size and may help physicists to establish reliable and reasonable action levels in routine MLC QA.

Siemens Multileaf Collimator Characterization and Quality Assurance Approaches for Intensity-Modulated Radiotherapy

Application of the multileaf collimator (MLC) has evolved from replacing blocks to create treatment fields to creating photon fluence modulation for intensity-modulated radiotherapy (IMRT). Multileaf collimator system performance requirements are far more stringent for such applications and will require increased performance for future applications, such as motion tracking. This article reviews Siemens MLC systems, including a technical description and dosimetric characteristics of 56-, 82-, and 160-leaf designs. Routine quality assurance of MLC for IMRT necessitates frequent and critical assessment of MLC leaf position calibration errors that can present in many different ways (e.g., accuracy, reproducibility, longevity, hysteresis, and collimator/gantry angle dependencies). Several techniques for measuring these errors are presented, along with qualitative and quantitative techniques for analyzing results. In particular, increased accuracy of leaf position measurement at variable gantry angles is enabled by spatial transformations to electronic portal imaging device position quantified by calibration protocols introduced with megavoltage cone beam. Measured values of X-ray transmission (intra-leaf, inter-leaf, and through abutting leaf pairs) and penumbra (leaf end, leaf tongue, leaf groove) are presented with an evaluation of their characterization by a treatment-planning system. The dosimetric impact of planning system model inadequacies is demonstrated for collimator scatter, dose profile values within 30 mm of the field edge, and the resultant effect demonstrated on clinical cases. Finally, a description of automated quality assurance delivery, analysis, and calibration protocols applicable for the specific vendor's system is provided.

Multileaf Collimator Characteristics and Reliability Requirements for IMRT Elekta System

Understanding the characteristics of a multileaf collimator (MLC) system, modeling MLC in a treatment planning system, and maintaining the mechanical accuracy of the linear accelerator gantry head system are important factors in the safe implementation of an intensity-modulated radiotherapy program. We review the characteristics of an Elekta MLC system, discuss the necessary MLC modeling parameters for a treatment planning system, and provide a novel method to establish an MLC leaf position quality assurance program. To perform quality assurance on 40 pairs of individual MLC leaves is a time-consuming and difficult task. In this report, an effective routine MLC quality assurance method based on the field edge of a backup jaw as referenced in conjunction with a diode array as a radiation detector system is discussed. The sensitivity of this test for determining the relative leaf positions was observed to be better than 0.1 mm. The Elekta MLC leaf position accuracy measured with this system has been better than 0.3 mm.

A system for intensity modulated dose plan verification based on an experimental pencil beam kernel obtained by deconvolution

The number of intensity modulated radiation therapy (IMRT) procedures is continuously growing worldwide and it is necessary to develop tools for patient specific quality assurance (QA) that avoid using machine time that could be employed in treating additional patients. One way of achieving this goal is to perform a multileaf collimator quality assurance periodically in the linear accelerator and check the treatment planning system (TPS) calculation by employing an independent calculation system. Within the work frame of the pencil beam kernel approach, a new system was developed for obtaining an experimental kernel. This new technique is based on a deconvolution procedure using the Hankel transform. The resulting kernel is obtained in a way completely independent of those employed in commercial treatment planning systems, usually calculated by Monte Carlo simulations. Also provided are comparisons between calculated and measured doses with radiographic film, linear array of diodes, and ionization chamber. Measurements taken in polystyrene and water for clinical IMRT plans demonstrate that this method can calculate IMRT dose distributions, as well as treatment times, with great accuracy. Apart from other applications, it can be used as a double-check algorithm for IMRT QA.

Quality assurance of the multileaf collimator with helical tomotherapy: Design and implementation

Quality assurance (QA) of the multileaf collimator (MLC) is a critical step for the delivery of intensity modulated radiation therapy treatment plan. While QA procedures for motor-driven MLC have been published extensively, those for binary MLCs such as the one used for helical tomotherapy have not been presented in the literature, as this is still a fairly new technology. In this study, seven test patterns for the MLC QA of a helical tomotherapy unit have been designed and implemented. The seven test patterns check the MLC alignment, MLC leakage, MLC timing and MLC leaf position error in detail. Those patterns can be easily implemented in any center with a helical tomotherapy unit as part of the routine QA. The QA procedures can be performed using existing QA resources such as solid water phantom and EDR2 film. A software toolkit called "Tomo MLC QA" has been developed to assist in generating the QA procedures and analyzing the results. Our results showed that the helical tomotherapy MLC is very robust, exhibiting interleaf leakage of 0.53% +/-0.09%. Several issues with the MLC have been found and discussed. The QA results also illustrate the utilization and usefulness of the proposed QA procedures.

Evaluation of MLC leaf positioning using a scanning liquid ionization chamber EPID

A method was developed to determine the accuracy of multileaf collimator (MLC) positioning using transmitted dose maps measured by a scanning liquid ionization chamber electronic portal imaging device (SLIC-EPID). Several MLC fields were designed, using the Varian C-series standard MLC-80, as reference fields for open fields. The MLC leaves were then shifted from the reference positions along the direction of MLC leaf movement towards the central axis from 0.1 to 1.6 mm. The electronic portal images (EPIs), acquired for each case, were converted to two-dimensional dose maps using an appropriate calibration method and the relative dose difference maps were then calculated. The experiment was then performed at non-zero gantry angles in the presence of an anthropomorphic phantom for typical prostate and head and neck fields. Several standard edge detection algorithms were also used in order to find the shifted MLC leaf position. In addition, the short-term reproducibility of MLC leaf positioning was evaluated using the above-mentioned methods. It was found that the relationship between the relative dose difference and MLC leaf spatial displacement is linear. A variation of 0.2 mm in leaf position leads to approximately 4% change in the relative dose values for open fields. The variation of the relative dose difference for phantom studies depends on the phantom positioning and the EPI normalization. From the standard edge detection algorithms, used in the current study, the ‘Canny’ algorithm was found to be the optimum method to identify the minimum detectable MLC leaf displacements with a precision of approximately 0.1 mm for all cases. However, the result of edge detection algorithms generally is binary and there is no additional information compared to the relative dose maps. The reproducibility of MLC positions was found to be within 0.3 mm. In conclusion, a SLIC-EPID can be used for regular quality assurance (QA) of MLC leaf positioning. Despite significant difference in the pixel size of the acquired SLIC-EPIs, it can be concluded that the SLIC-EPID can be used for MLC quality assurance protocols with similar accuracy compared to amorphous silicon (a-Si) EPID results.