Multileaf Collimator Positional Accuracy Research Articles

Characterization of positional accuracy of MLC using an ionization chamber array

PurposeThe purpose of this study is to characterize the positional accuracy of the multileaf collimator (MLC) using an ionization chamber array and develop an MLC quality assurance (QA) method. MethodsHalf of the volume for each ionization chamber located at the lower layer of the device was occluded by a specific leaf using the half-beam block (HBB) method, and the corresponding ionization chamber response was used as a baseline for the subsequent MLC QA study. The sensitivity of the method was assessed by introducing known leaf positional errors, varying from 0.5 to 2.5 mm in 0.5 mm increments in the positive and negative directions, into the baseline. The relative outputs of the detectors were linearly corrected according to the leaf position error recorded. The relationship between detector response variation relative to the baseline and the introduced leaf error was determined by function. The usefulness of the MLC QA technique developed in this study was verified weekly over 5 months. ResultsDuring the development of the algorithm, the maximum leaf absolute random position error was 0.3 mm. A strong linear relationship was observed between the relative output of the detector and the MLC positional error introduced, and the slope and coefficient of determination were −0.05159 and 0.99928, respectively. The mean MLC positional accuracy across all leaves for the error field and HBB baseline field over the 5-month assessment was 0.0649 ± 0.1340 mm and 0.0002± 0.0835 mm, respectively. ConclusionsThe automated MLC QA procedure we proposed has proven to be effective and robust, and satisfies the American Association of Physicists in Medicine (AAPM) Task Group Report 198 guideline criteria of ± 1 mm.

Elekta Unity MR-linac commissioning: mechanical and dosimetry tests

Abstract We report the commissioning results of Elekta Unity for the dosimetric performance and mechanical quality assurance (QA), and propose additional commissioning procedures. Mechanical tests included multi-leaf collimator (MLC) positional accuracy, radiation isocenter diameter at the center and off-center position, and coincidence between the magnetic resonance (MR) image center and radiation isocenter. Comparisons between the measurements and calculations of the simple irradiated field, intensity modulated radiation therapy (IMRT) commissioning, MLC output factor ratio, validation of independent dose calculation software and end-to-end testing were performed to evaluate dosimetric performance. The average values of the MLC positional accuracy for film- and imaging device-based analysis were −0.1 and 0.3 mm, respectively. The measured radiation isocenter size was 0.41 mm, and the off-center results were within 1 mm. The coincidence was −0.21, −1.19 and 0.49 mm along the x-, y- and z-axes, respectively. The calculated percent depth doses (PDD) and profiles agreed with the measurements. The results of independent dose calculation were within the action level recommended by American Associations of Physicist in Medicine. The gamma passing rate (GPR) for IMRT commissioning was 98.6 ± 0.9%, and end-to-end testing of adapted plans showed agreement within 2% between the measurement and calculation. We reported the results of mechanical and dosimetric performances of Elekta Unity, and proposed novel commissioning procedures. Our results should provide knowledge to the physics community for enhancing the QA programs.

Commissioning of a 1.5T Elekta Unity MR-linac: A single institution experience.

MR image‐guided radiotherapy has the potential to improve patient care, but integration of an MRI scanner with a linear accelerator adds complexity to the commissioning process. This work describes a single institution experience of commissioning an Elekta Unity MR‐linac, including mechanical testing, MRI scanner commissioning, and dosimetric validation. Mechanical testing included multileaf collimator (MLC) positional accuracy, measurement of radiation isocenter diameter, and MR‐to‐MV coincidence. Key MRI tests included magnetic field homogeneity, geometric accuracy, image quality, and the accuracy of navigator‐triggered imaging for motion management. Dosimetric validation consisted of comparison between measured and calculated PDDs and profiles, IMRT measurements, and end‐to‐end testing. Multileaf collimator positional accuracy was within 1.0 mm, the measured radiation isocenter walkout was 0.20 mm, and the coincidence between MR and MV isocenter was 1.06 mm, which is accounted for in the treatment planning system (TPS). For a 350‐mm‐diameter spherical volume, the peak‐to‐peak deviation of the magnetic field homogeneity was 4.44 ppm and the geometric distortion was 0.8 mm. All image quality metrics were within ACR recommendations. Navigator‐triggered images showed a maximum deviation of 0.42, 0.75, and 3.0 mm in the target centroid location compared to the stationary target for a 20 mm motion at 10, 15, and 20 breaths per minute, respectively. TPS‐calculated PDDs and profiles showed excellent agreement with measurement. The gamma passing rate for IMRT plans was 98.4 ± 1.1% (3%/ 2 mm) and end‐to‐end testing of adapted plans showed agreement within 0.4% between ion‐chamber measurement and TPS calculation. All credentialing criteria were satisfied in an independent end‐to‐end test using an IROC MRgRT phantom.

Assessment of Multi-leaf Collimator Positional Accuracy Using Radiochromic EBT3 Film and an Electronic Portal Imaging Device

This study aimed to evaluate multi-leaf collimator (MLC) positional accuracy by using an electronic portal imaging device (EPID) and radiochromic EBT3 film. Furthermore, the MLC’s positional accuracy at different gantry and collimator angles of a Siemens ONCOR linear accelerator (linac) was evaluated. A picket fence test was performed to evaluate the MLC’s positional accuracy at various gantry and collimator angles of the linac. The EPID and the EBT3 films were sequentially irradiated seven times at 2-cm intervals by making a rectangular field (0.3 × 19 cm2). The full-width at half-maximum (FWHM) of each band (field) was calculated for all leaves by using inhouse software. Then, variations between the actual and the planned leaf locations were obtained by using the EPID and the EBT3 film at various gantry and collimator angles. The mean FWHM, acquired using the EPID and the EBT3 film ranged from 2.331 - 3.647 mm and 2.059 - 4.001 mm, respectively. Variations between the actual and the planned leaf locations were found to be affected by changes in the collimator and the gantry angles. Moreover, a -0.060 - 1.588 mm difference we seen between the result obtained from the EBT3 film and that obtained from the EPID. The mean FWHM, at most angles, for the EBT3 film was larger than that for the EPID. The findings showed average deviations for the EPID (0.001 - 0.669 mm) and EBT3 film (0.007 - 1.001 mm); these values agreed within the tolerance level (±1 mm). Furthermore, good agreement was found between the results obtained from the EPID and the EBT3 film; these two dosimetric methods can be used interchangeably, but each must be chosen considering its advantages and disadvantages.

Characterization of positional accuracy of a double-focused and double-stack multileaf collimator on an MR-guided radiotherapy (MRgRT) Linac using an IC-profiler array.

With advance magnetic resonance (MR)-guided online adaptive radiotherapy (MRgoART) relying on calculation-based intensity-modulated radiation therapy (IMRT) quality assurance (QA), accurate and sensitive QA of the multileaf collimator (MLC) becomes an increasingly essential component for routine machine QA. As such, it is important to assure compliance with the AAPM TG142 guidelines to supplement calculation-based QA methods for an online adaptive radiotherapy program. We have developed and implemented an efficient and highly sensitive QA procedure using an ionization chamber profiler (ICP) array to enable real-time characterization of the positional accuracy of a double-focused and double-stacked MLC on a clinical MR-guided radiotherapy (MRgRT) system and to supplement calculation-based QA for an MRgoART program. An in-house MR-compatible jig was used to position the ICP (detector resolution 5mm on X/Y axis) at an extended SDD of 108.4cm to enable each MLC leaf (8.3mm leaf width at isocenter) to be uniquely determined by two neighboring ion chambers. The MRgRT linac system utilizes a novel jawless, double-focused, and double-stacked MLC design such that the upper bank (MLC1) and lower bank (MLC2) are offset by half a leaf width. Positional accuracy was characterized by three methods: single bank half-beam block (HBB) at central axis, forward slash diagonal (FSD), and backslash diagonal (BSD) at off-axis. Measurements were performed for each bank in which each leaf occludes half of a detector. A corresponding reference field with the MLC retracted from occlusion was measured. The sensitivities of HBB, FSD, and BSD were evaluated by introducing 0.5-2.5mm of known errors in 0.5mm increments, in both positive and negative directions. The relationship between detector response and MLC error was established. Over a 6-month longitudinal assessment, we have evaluated MLC performance with weekly QA of HBB among cardinal angles, and monthly QA of FSD and BSD. A strong correlation was found between detector response of percentage dose difference and MLC positional error introduced (N=350 introduced errors) for both HBB and FSD/BSD with coefficient of determination of 0.999 and 0.977, respectively. The relationship between detector response to MLC positional change was found to be 20.65%/mm for HBB and 11.14%/mm for FSD and BSD. At baseline, the mean MLC positional accuracy averaged across all leaves was 0.06±0.27mm (HBB), 0.04±0.52mm (FSD), -0.06±0.51mm (BSD). The mean MLC positional accuracy relative to baseline over the 6-month assessment was found to be highly reproducible at 0.00±0.12mm (HBB; N=28weeks), -0.02±0.19mm (FSD; N=6months), -0.03±0.19mm (BSD; N=6months). Positional accuracy of a novel jawless, double-focused, double-stacked MLC has been characterized and monitored over 6 months with an efficient, highly sensitive, and robust method using an ICP array. This routine QA method supplements calculation-based IMRT QA for an online adaptive radiotherapy program. Longitudinal assessment demonstrated no-drift in the MLC calibration. A highly reproducible jig setup allowed the validation of MLC positional accuracy to be within TG142 criteria of ±1mm for 99% of measurements (i.e., 100% HBB, 95% BSD, 95% FSD) over the 6-month assessment.

Picket fence test of Agility MLCs using linac log data

The multileaf collimator (MLC) performance is critical during intensity modulated radiotherapy (IMRT) delivery. Picket fence test is typically performed by acquiring images of MLC gaps using film or electronic portal imaging device (EPID) to assess the MLC positional accuracy as part of a comprehensive weekly quality assurance (QA) programme. This study aims to investigate the accuracy of MLC position of Agility MLCs (Elekta, Stockholm, Sweden) using linac log data. The positions of MLCs were tracked during picket fence test using Elekta’s propriety optical tracking system on the linac head. The data were recorded at four cardinal gantry angles over six weeks. The fluence from the picket fence was constructed using the tracked MLC positions and compared with the fluence obtained from EPID. The fluence of the picket fence pattern constructed from the tracking data of MLC position showed that the measured range of gap size error within -0.3 mm to 0.8 mm. The average MLC positional error tracked was 0.07 mm ± 0.14 mm (mean ± 1 SD). Gap size measured from tracking data was within the range of the results obtained from EPID. Tracking the MLC position using the optical tracking system provide a more accurate test method for routine QA of the MLC performance. Over the period of investigation, it was found that the MLC performance was within the tolerance of ± 1 mm recommended by AAPM TG142.

Position accuracy evaluation of multileaf collimator (MLC) static in linear accelerator

This study was aimed to evaluate the accuracy of static MLC positions using gafchromic BT3 film. EBT3 gafchromic film was scanned using Vidar dosimetry Pro Advantage and was analyzed using Matlab algorithm. Matlab algorithm which determined leaf edge of MLC was using Edge Detection Function with Laplacian of Gaussian Method. The precision of MLC leafs on Linac 1 and Linac 2 was measured using the Matlab application code and being compared to early data in TPS setting. The measurement showed that average value of the best precision was obtained at line 6 and the lowest at line 2 both in Linac 1 and 2. MLC leaf measurement error using Matlab was still within tolerance limit AAPM TG 142 which is ± 1 mm. An Error value of the MLC was increased if the leaf movement over the normal position. This effect was caused by an influence of vertical line scattering (VL) and inaccuracy positioning of gafchromic film EBT3 when scanned. The result of this study showed that MLC position accuracy in LINA C 1 and 2 was still within tolerance limit based on AAPM PG 142. It could be an alternative measurement of MLC position accuracy more easily.

Evaluation of application of EPID for rapid QC testing of linear accelerator.

Evaluation of application of EPID for rapid QC testing of linear accelerator. Quality control of a linear accelerator device is a time and energy intensive process. In this study, attempts have been made to perform the linear accelerator quality control using electronic portal imaging device (EPID), which is mounted on most accelerators. First, quality control and dosimetry parameters of the device were determined and measured based on standard protocols to ensure full calibration of the accelerator. Then, various features of EPID including spatial resolution and contrast resolution, the effect of buildup region, dose response and image uniformity were evaluated. In the next step, consistent with the parameters of linear accelerator quality control including field size, field flatness and symmetry, the light field coincidence with X-ray field, mechanical stability and multileaf collimator position accuracy test, the output images of device were obtained.After feeding images to the MATLAB software, their pixel content was analyzed. All measurements of the three photon beams were repeated three times. The EPID image had a desirable resolution, contrast and uniformity and displayed high sensitivity to dose changes with linear dose response. Seven qualitative parameters of the linear accelerator were then controlled by EPID. The results of the linear accelerator quality control using the EPID were consistent with practice. Quality control using the EPID was more convenient and faster than conventional methods.

Development of a novel methodology for QA of respiratory-gated and VMAT beam delivery using Octavius 4D phantom

The objective of this study was to develop and evaluate a series of quality assurance (QA) techniques based on Octavius 4D phantom for testing of respiratory-gated treatment delivery, integrity of dose rate vs gantry speed in volumetric-modulated arc therapy (VMAT) commissioning, and multileaf collimator (MLC) positioning accuracy of a linear accelerator. An Octavius 4D phantom capable of rotating with the gantry and recording the detector signal with a sampling rate of 10 Hz was isocentrally set up and an inclinometer was also installed to measure the gantry angle simultaneously. A simple arc test was created and delivered with gating function activated to measure the timing accuracy of the gating window. A tailor-made dose rate vs gantry speed plan was also designed to test the accuracy of measured dose rate, gantry speed, and actual control points. All experiments were conducted while machine log files were collected for comparison. The variations of beam flatness, symmetry, and field size were analyzed as a function of gantry angle to evaluate the influence from the modulation of dose rate and gantry speed. MLC position accuracy was evaluated based on specific garden fence plans. The time of gating window was measured to be less than 10-millisecond deviation from the log data. Gantry backlash was observed and quantified to be 1.72° with an extra stabilization time of 1.16 seconds for a gating arc with gantry speed of 6°/s. In the dose rate vs gantry speed test, the mean deviation between measured gantry angle and log data was less than 0.2° after a time delay of 0.25 second was corrected. The measured dose rate agreed with the log data very well with a mean deviation of 0.05%, and even the transit of modulation was tracked successfully. There was a statistically significant difference on the variation of beam parameters between a VMAT plan and a simple arc plan. The induced MLC position errors were detected with an accuracy of 0.05 mm. The leaf position reproducibility was found to be better than 0.02 mm, whereas the routine MLC position accuracy was better than 0.1 mm. A time-resolved method using Octavius 4D phantom has been developed and proven to be convenient for respiratory gating QA, dose rate vs gantry speed test, and MLC QA. Gating time, dose rate, and gantry speed-induced leave position error could be directly measured with high accuracy after comparison with the machine log data. This study also highlights the capability of the phantom in quantifying the variation of flatness, symmetry, and field size during gantry rotation.

Multileaf Collimator Position Accuracy of Respiratory Gated VMAT

Respiratory gated VMAT (volumetric modulated arc therapy) repeats rapid stop and go operations of a MLC (multileaf collimator) by turning the beam on and off by respiratory gating. The rapid stop and go operations of the MLC during respiratory gated irradiation may induce position error of the MLC and may affect output error and dose distribution. The purpose of this study was to clarify the relationship between the MLC position accuracy of the respiratory gated VMAT and the VMAT parameters. In the method, 1 arc, 2 arcs, and 4 arcs plan were created for the virtual target and irradiation was performed without the gated respiration and with the gated respiration. The respiratory gated system used a RPM (real-time position management system). The MLC position error, gap size error, and the MLC leaf speed were calculated from a log-file. In the histogram of the gap size error, the frequency of falling within the error range up to 0.2 mm was about 12 percentage points higher for the gated respiration. The MLC position error increased with increasing the MLC leaf speed. The correlation coefficient between the MLC leaf speed and the MLC position error exceeded 0.96, showing a strong correlation. Dose rate of VMAT parameters decreased with increasing arc number with the gated respiration and without the gated respiration. Gated irradiation was temporarily stopped, and it decreased by about 27% with respect to the dose rate without the gated respiration. The gantry rotation speed repeated the stop and re-rotation operations when gated irradiation was performed. For all arcs, the rotation speed decreased by about 30% compared with the rotation speed without the gated respiration. The pass rate of gamma analysis for each arc plan was about 95%. No effect on gated irradiation dose distribution was observed. Respiratory gated irradiation reduced dose rate change and gantry rotation speed of the VMAT. Reduction of the MLC leaf speed occurred, and the MLC position error and gap size error decreased. The MLC positional accuracy was secured, and it was confirmed that there was no effect on dose distribution by the respiratory gated VMAT.

Effects of Low MU in Respiratory Gated IMRT on MLC Position Accuracy and Dose Distribution

The purpose of this research is to clarify the effects of low monitor unit (MU) on multileaf collimator (MLC) position accuracy and dose distribution in intensity modulated radiotherapy (IMRT) using respiratory gated. In the phantom experiment, irradiation without respiratory gated and respiratory gated with low MU (3, 5, and 7 MU) were performed, and positional accuracy and dose distribution of MLC were analyzed. MLC positional accuracy was calculated from the log-files and the MLC position error, gap size error, MLC leaf speed were calculated and compared with the planned value. Gamma analysis of the dose distribution obtained from the irradiated films and the dose distribution of the treatment plans were carried out. Without respiratory gated and respiratory gated, the frequency of gap size error that did not exceed 0.2 mm were more than 93% under all conditions. MLC position error increased with increasing MLC leaf speed. The determination coefficient of respiratory gated irradiation was lower by about 20% compared with that without respiratory gated, and variation from the approximate straight line occurs. The output difference due to low MU irradiation during respiratory gated was within 1% of the planned value. Although, the pass rate of gamma analysis differed in tumor size, the dose distribution well conformity at 96% or more for both without respiratory gated and respiratory gated. However, in the comparison of the profile in the MLC movement direction, respiratory gated irradiation at 3 MU showed a difference of about 9% at the edge of the irradiated field and about 6% at the point where the dose rapidly changed. It was shown that MLC position accuracy due to stop and go of MLC leaf can be secured even with low MU irradiation of about 3 MU. However, attention should be paid to the dose of risk organs adjacent to the tumor margin.

Commissioning and quality assurance for VMAT delivery systems: An efficient time-resolved system using real-time EPID imaging.

An ideal commissioning and quality assurance (QA) program for Volumetric Modulated Arc Therapy (VMAT) delivery systems should assess the performance of each individual dynamic component as a function of gantry angle. Procedures within such a program should also be time-efficient, independent of the delivery system and be sensitive to all types of errors. The purpose of this work is to develop a system for automated time-resolved commissioning and QA of VMAT control systems which meets these criteria. The procedures developed within this work rely solely on images obtained, using an electronic portal imaging device (EPID) without the presence of a phantom. During the delivery of specially designed VMAT test plans, EPID frames were acquired at 9.5Hz, using a frame grabber. The set of test plans was developed to individually assess the performance of the dose delivery and multileaf collimator (MLC) control systems under varying levels of delivery complexities. An in-house software tool was developed to automatically extract features from the EPID images and evaluate the following characteristics as a function of gantry angle: dose delivery accuracy, dose rate constancy, beam profile constancy, gantry speed constancy, dynamic MLC positioning accuracy, MLC speed and acceleration constancy, and synchronization between gantry angle, MLC positioning and dose rate. Machine log files were also acquired during each delivery and subsequently compared to information extracted from EPID image frames. The largest difference between measured and planned dose at any gantry angle was 0.8% which correlated with rapid changes in dose rate and gantry speed. For all other test plans, the dose delivered was within 0.25% of the planned dose for all gantry angles. Profile constancy was not found to vary with gantry angle for tests where gantry speed and dose rate were constant, however, for tests with varying dose rate and gantry speed, segments with lower dose rate and higher gantry speed exhibited less profile stability. MLC positional accuracy was not observed to be dependent on the degree of interdigitation. MLC speed was measured for each individual leaf and slower leaf speeds were shown to be compensated for by lower dose rates. The test procedures were found to be sensitive to 1mm systematic MLC errors, 1mm random MLC errors, 0.4mm MLC gap errors and synchronization errors between the MLC, dose rate and gantry angle controls systems of 1°. In general, parameters measured by both EPID and log files agreed with the plan, however, a greater average departure from the plan was evidenced by the EPID measurements. QA test plans and analysis methods have been developed to assess the performance of each dynamic component of VMAT deliveries individually and as a function of gantry angle. This methodology relies solely on time-resolved EPID imaging without the presence of a phantom and has been shown to be sensitive to a range of delivery errors. The procedures developed in this work are both comprehensive and time-efficient and can be used for streamlined commissioning and QA of VMAT delivery systems.

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

Commissioning of Axesse accelerator for volumetric modulated arc therapy

Objective To test the accuracy and reliability of Axesse accelerator for volumetric modulated arc therapy (VMAT). Methods The accuracy and reliability of Axesse accelerator for VMAT were tested in a stepwise manner, from the simple to the complex and from the part to the whole. For the parts of the system, the stability of dosimetric output and the position accuracy of multi-leaf collimator (MLC) were tested. For the process of the system, the variable VMAT dose rates and gantry speed modulation, the MLC speed and dose rate modulation, and dosimetric verification in patients were tested. Results Compared with fixed gantry irradiation, the variation in dosimetric output was below 1.0% for rotary irradiation including the slide rotary irradiation of the dynamic MLC. The MLC position error of 0.5 mm was visible using the electronic portal imaging system of Axesse, iViewGT 3.40. The MLC position accuracy was within 1 mm for fixed gantry irradiation and rotary irradiation. In the range of the dose rate applied in clinical practice, the testing results of variable VMAT dose rates and gantry speeds as well as variable dose rates and MLC speeds showed that the variation between different strip-field beam intensities was below 2.0%. Using a gamma criterion of 3 mm/3%, the pass rates in dosimetric verification of patients with cervical cancer, prostate cancer, and breast cancer were 96.52%, 95.72%, and 98.83%, respectively. Conclusions The Axesse system can precisely control MLC motion, variable dose rates, and gantry speeds in VMAT. The Axesse system is accurate and reliable for VMAT. Key words: Linear accelerator; Volumetric modulated arc therapy; Commissioning

Verification and calibration of leaf position of a multi-leaf collimator by ionization chamber arrays

Objective To verify and adjust the leaf position accuracy of a multi-leaf collimator (MLC) in a linear accelerator. Methods Ionization chamber arrays (StarTrack, IBA) were used to measure the leaf position accuracy of a MLC in a linear accelerator (Precise, ELEKTA) and adjust the leaves out of tolerance. Results After the adjustment of leaf position of the MLC, the analysis of the verification films taken at the offset position showed that the leaf position of the MLC was accurate. Conclusions The method developed in this study is convenient and simple for measurement and calibration of leaf position of the MLC in the linear accelerator, which meets the MLC position accuracy requirement for the linear accelerator. Key words: Multi-leaf collimators; Accuracy; Adjustment; Ionization chamber

The study of electronic portal imaging devices for the position accuracy of multi-leaf collimator instead of the film

Objective To explore the application of the electronic portal imaging device (EPID) to test on MLC leaves position accuracy instead of EBT3 film, and the feasibility analysis under any gantry angle. Methods RIT113 software was used to analyze the image data of radiochromic film EBT3 and EPID of Siemens ARTISTE accelerator. The replacement process from EBT3 film to EPID for the test MLC leaves positioning was completed by defining the positions of 50% isodose curve of the field edge of EBT3 after radiation as the actual positions of MLC leaves, therefore finding out the percentage isodose value of the same position in the EPID portal imaging from the corresponding radiation field. Results When the accelerator gantry angle was 0, the mean percentage isodose value was 44% at the MLC leaves positions determined by EBT3, and the maximum position error of MLC leaves is 0.12 mm. When the accelerator gantry was any other angles, the results were compared with the zero angle situation through DTA, all of the points passed when the DTA radium was 0.5 mm. Conclusions Using EPID instead of EBT3 to do the test MLC leaves positioning is feasible, furthermore, it is suitable under any gantry angle with excellent accuracy for the clinical practive, which deserves to be widely spread. Key words: Electronic portal imaging device; Autoradiography film; Multi-leaf collimator; Quality control; Phantom

Monitoring daily MLC positional errors using trajectory log files and EPID measurements for IMRT and VMAT deliveries

This work investigated the differences between multileaf collimator (MLC) positioning accuracy determined using either log files or electronic portal imaging devices (EPID) and then assessed the possibility of reducing patient specific quality control (QC) via phantom-less methodologies. In-house software was developed, and validated, to track MLC positional accuracy with the rotational and static gantry picket fence tests using an integrated electronic portal image. This software was used to monitor MLC daily performance over a 1 year period for two Varian TrueBeam linear accelerators, with the results directly compared with MLC positions determined using leaf trajectory log files. This software was validated by introducing known shifts and collimator errors. Skewness of the MLCs was found to be 0.03 ± 0.06° (mean ±1 standard deviation (SD)) and was dependent on whether the collimator was rotated manually or automatically. Trajectory log files, analysed using in-house software, showed average MLC positioning errors with a magnitude of 0.004 ± 0.003 mm (rotational) and 0.004 ± 0.011 mm (static) across two TrueBeam units over 1 year (mean ±1 SD). These ranges, as indicated by the SD, were lower than the related average MLC positioning errors of 0.000 ± 0.025 mm (rotational) and 0.000 ± 0.039 mm (static) that were obtained using the in-house EPID based software. The range of EPID measured MLC positional errors was larger due to the inherent uncertainties of the procedure. Over the duration of the study, multiple MLC positional errors were detected using the EPID based software but these same errors were not detected using the trajectory log files. This work shows the importance of increasing linac specific QC when phantom-less methodologies, such as the use of log files, are used to reduce patient specific QC. Tolerances of 0.25 mm have been created for the MLC positional errors using the EPID-based automated picket fence test. The software allows diagnosis of any specific leaf that needs repair and gives an indication as to the course of action that is required.

Impact of dose rates on the position accuracy of multi-leaf collimator

PurposeTo study the impact of dose rates on the position accuracy of a Varian multi-leaf collimator (MLC) in dynamic IMRT. MethodsFour nasopharyngeal carcinoma (NPC) patients who received dynamic IMRT were chosen. The dosimetric verification at different dose rates (400–600MU/min) was performed using a PTW 2D ion chamber Array in a Varian Clinac iX linear accelerator. Meanwhile, the actual MLC leaf position data was taken by the MLC controller. The passing ratios of dosimetric verification were compared at different dose rates, and the MLC leaf position deviations were evaluated through the parameters of Error Histogram and Error RMS. ResultsThe passing ratios of dosimetric verification both of the criteria of 3%/3mm and 2%/2mm did not show statistical differences between two dose rates (P>0.05). But using the criteria of 2%/1mm, significant differences were observed between two dose rates (P<0.05). Moreover, according to the analysis of the Error Histogram and Error RMS, the MLC leaf position deviations were higher with the increase of dose rate, and the differences between 400 and 600MU/min were statistically significant (P<0.05). ConclusionsThe higher dose rate will affect the accuracy of MLC leaf position in dynamic IMRT.

Exit fluence analysis using portal dosimetry in volumetric modulated arc therapy

AimIn measuring exit fluences, there are several sources of deviations which include the changes in the entrance fluence, changes in the detector response and patient orientation or geometry. The purpose of this work is to quantify these sources of errors. BackgroundThe use of the volumetric modulated arc therapy treatment with the help of image guidance in radiotherapy results in high accuracy of delivering complex dose distributions while sparing critical organs. The transit dosimetry has the potential of Verifying dose delivery by the linac, Multileaf collimator positional accuracy and the calculation of dose to a patient or phantom. Materials and methodsThe quantification of errors caused by a machine delivery is done by comparing static and arc picket fence test for 30 days. A RapidArc plan, created for the pelvis site was delivered without and with Rando phantom and exit portal images were acquired. The day to day dose variation were analysed by comparing the daily exit dose images during the course of treatment. The gamma criterion used for analysis is 3% dose difference and 3mm distance to agreement with a threshold of 10% of maximum dose. ResultsThe maximum standard deviation for the static and arc picket fence test fields were 0.19CU and 1.3CU, respectively. The delivery of the RapidArc plans without a phantom shows the maximum standard deviation of 1.85CU and the maximum gamma value of 0.59. The maximum gamma value for the RapidArc plan delivered with the phantom was found to be 1.2. The largest observed fluence deviation during the delivery to patient was 5.7% and the maximum standard deviation was 4.1CU. ConclusionIt is found from this study that the variation due to patient anatomy and interfraction organ motion is significant.