Fast and accurate leaf verification for dynamic multileaf collimation using an electronic portal imaging device

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.

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.

Using a TRAPS upstream transmission detector to verify multileaf collimator positions during dynamic radiotherapy delivery

With the advancement of high-precision radiotherapy and the increasing use of higher intensity beams, the risk to the patient increases should the radiotherapy machine malfunction. Hence more accurate treatment verification is required. In this paper we provide a solution for real-time monitoring of X-ray beams from radiotherapy linear accelerators using monolithic active pixel sensors. We show that leaf errors can be detected with high precision in static fields and IMRT step and shoot, and accurate leaf tracking is possible in Volumetric Modulated Arc Therapy. The prototype MAPS detector meets the criteria of 1% attenuation acceptable for clinical use.

Evaluation of the correlation between prostatic displacement and rectal deformation using the Dice similarity coefficient of the rectum

To estimate the relationship between the three-dimensional (3D) displacement error of the prostate and rectal deformation for reduction of deviation between the planned and treatment dose, using multiple acquisition planning CT (MPCT) and the Dice similarity coefficient (DSC) for rectal deformation for treatment of patients with prostate cancer. The 3D displacement error between the pelvic bone and a matching fiducial marker was calculated using MPCT in 24 patients who underwent prostate volumetric-modulated arc therapy for prostate cancer. We calculated the 3D displacement error between the pelvic bone and a matching fiducial marker on MPCT. The correlation of the 3D displacement error with the DSC of the rectum, calculated from MPCT images, was evaluated based on deformable image registration. The 3D displacement error of the prostate showed a slight correlation between MPCT and cone-beam computed tomography (adjusted r² = 0.241). The 3D displacement error, based on the pelvic bone and a fiducial marker on MPCT images, showed a moderate correlation with the DSC of the rectum (adjusted r² = 0.645) and was improved by a mean of 3.94 mm, based on MPCT, during the treatment period. The 3D displacement error on MPCT correlates with the 3D displacement error of daily cone-beam computed tomography; optimal selection of MPCT can potentially facilitate on-board setup of prostate patients to enable more accurate radiotherapy. The advance information of the 3D displacement error and rectal deformation is useful for optimal planning CT that can minimize the deviation between the planned dose and the treatment dose in patients receiving treatment for prostate cancer.

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).

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

Multileaf collimator (MLC) positions should be precisely and independently mea-sured 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 sys-tem based on the measured MLC trajectories and the original DICOM plan file. The method relies on extracting MLC positions from EPID images acquired at 8.41fps 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 1-2mm 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.

Analysis of direct clinical consequences of MLC positional errors in volumetric-modulated arc therapy using 3D dosimetry system

In advanced, intensity-modulated external radiotherapy facility, the multileaf collimator has a decisive role in the beam modulation by creating multiple segments or dynamically varying field shapes to deliver a uniform dose distribution to the target with maximum sparing of normal tissues. The position of each MLC leaf has become more critical for intensity-modulated delivery (step-and-shoot IMRT, dynamic IMRT, and VMAT) compared to 3D CRT, where it defines only field boundaries. We analyzed the impact of the MLC positional errors on the dose distribution for volumetric-modulated arc therapy, using a 3D dosimetry system. A total of 15 VMAT cases, five each for brain, head and neck, and prostate cases, were retrospectively selected for the study. All the plans were generated in Monaco 3.0.0v TPS (Elekta Corporation, Atlanta, GA) and delivered using Elekta Synergy linear accelerator. Systematic errors of +1, +0.5, +0.3, 0, -1, -0.5, -0.3 mm were introduced in the MLC bank of the linear accelerator and the impact on the dose distribution of VMAT delivery was measured using the COMPASS 3D dosim-etry system. All the plans were created using single modulated arcs and the dose calculation was performed using a Monte Carlo algorithm in a grid size of 3 mm. The clinical endpoints D95%, D50%, D2%, and Dmax,D20%, D50% were taken for the evaluation of the target and critical organs doses, respectively. A significant dosimetric effect was found for many cases even with 0.5 mm of MLC positional errors. The average change of dose D 95% to PTV for ± 1 mm, ± 0.5 mm, and ±0.3mm was 5.15%, 2.58%, and 0.96% for brain cases; 7.19%, 3.67%, and 1.56% for head and neck cases; and 8.39%, 4.5%, and 1.86% for prostate cases, respectively. The average deviation of dose Dmax was 5.4%, 2.8%, and 0.83% for brainstem in brain cases; 8.2%, 4.4%, and 1.9% for spinal cord in H&N; and 10.8%, 6.2%, and 2.1% for rectum in prostate cases, respectively. The average changes in dose followed a linear relationship with the amount of MLC positional error, as can be expected. MLC positional errors beyond ± 0.3 mm showed a significant influence on the intensity-modulated dose distributions. It is, therefore, recommended to have a cautious MLC calibration procedure to sufficiently meet the accuracy in dose delivery.

Patient Specific Pre-Treatment QA Verification Using an EPID Approach

A software program [MU-EPID], has been developed to perform patient specific pre-treatment quality assurance (QA) verification for intensity modulated radiation therapy (IMRT) using fluence maps measured with an electronic portal imaging device (EPID). The software converts the EPID acquired images of each IMRT beam, to fluence maps that are equivalent to those calculated by the treatment planning system (TPS). The software has the capability to process Varian, Elekta and Siemens EPID DICOM images. In the present investigation, several IMRT plans for different treatment sites were used to validate the software using the Varian a-Si 1000 EPID with the Pinnacle TPS. A total of 20 IMRT plans of different treatment sites were analyzed. Isodose distributions, dose profiles, dose volume histograms (DVH's) and gamma analysis comparisons were performed to evaluate the accuracy of our method. A gamma index analysis of the isocenter coronal plane was done for each plan and showed an average of 97.44% of gamma passing rate using a 3% and 3 mm gamma criterion. Isodose, DVH and dose profile comparisons were conducted between the original calculated plan and the measured reconstructed plan from the EPID images processed through the MU-EPID software. The results suggest that MU-EPID can be used clinically for patient specific IMRT QA, providing a comprehensive 3D dosimetric evaluation through DVH comparison as well as an option for a 2D gamma analysis.

Accurate IMRT fluence verification for prostate cancer patients using 'in-vivo' measured EPID images and in-room acquired kilovoltage cone-beam CT scans

Background

To investigate for prostate cancer patients the comparison of ‘in-vivo’ measured portal dose images (PDIs) with predictions based on a kilovoltage cone-beam CT scan (CBCT), acquired during the same treatment fraction, as an alternative for pre-treatment verification. For evaluation purposes, predictions were also performed using the patients’ planning CTs (pCT).

Methods

To get reliable CBCT electron densities for PDI predictions, Hounsfield units from the pCT were mapped onto the CBCT, while accounting for non-rigidity in patient anatomy in an approximate way. PDI prediction accuracy was first validated for an anatomical phantom, using IMRT treatment plans of ten prostate cancer patients. Clinical performance was studied using data acquired for 50 prostate cancer patients. For each patient, 4–5 CBCTs were available, resulting in a total of 1413 evaluated images. Measured and predicted PDIs were compared using γ-analyses with 3% global dose difference and 3 mm distance to agreement as reference criteria. Moreover, the pass rate for automated PDI comparison was assessed. To quantify improvements in IMRT fluence verification accuracy results from multiple fractions were combined by generating a γ-image with values halfway the minimum and median γ values, pixel by pixel.

Results

For patients, CBCT-based PDI predictions showed a high agreement with measurements, with an average percentage of rejected pixels of 1.41% only. In spite of possible intra-fraction motion and anatomy changes, this was only slightly larger than for phantom measurements (0.86%). For pCT-based predictions, the agreement deteriorated (average percentage of rejected pixels 2.98%), due to an enhanced impact of anatomy variations. For predictions based on CBCT, combination of the first 2 fractions yielded gamma results in close agreement with pre-treatment analyses (average percentage of rejected pixels 0.63% versus 0.35%, percentage of rejected beams 0.6% versus 0%). For the pCT-based approach, only combination of the first 5 fractions resulted in acceptable agreement with pre-treatment results.

Conclusion

In-room acquired CBCT scans can be used for high accuracy IMRT fluence verification based on in-vivo measured EPID images. Combination of γ results for the first 2 fractions can largely compensate for small accuracy reductions, with respect to pre-treatment verification, related to intra-fraction motion and anatomy changes.

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.

Using an EPID for patient-specific VMAT quality assurance

PURPOSE

A patient-specific quality assurance (QA) method was developed to verify gantry-specific individual multileaf collimator (MLC) apertures (control points) in volumetric modulated arc therapy (VMAT) plans using an electronic portal imaging device (EPID).

METHODS

VMAT treatment plans were generated in an Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (aS1000) acquired in continuous acquisition mode were used for pretreatment QA. Each cine image file contains the grayscale image of the MLC aperture related to its specific control point and the corresponding gantry angle information. The TPS MLC file of this RapidArc plan contains the leaf positions for all 177 control points (gantry angles). In-house software was developed that interpolates the measured images based on the gantry angle and overlays them with the MLC pattern for all control points. The 38% isointensity line was used to define the edge of the MLC leaves on the portal images. The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement at each control point and the frequency in which each particular leaf mismatches for the entire arc.

RESULTS

Seven patients plans were analyzed using this method. The leaves with the highest mismatched rate were found to be treatment plan dependent.

CONCLUSIONS

This in-house software can be used to automatically verify the MLC leaf positions for all control points of VMAT plans using cine images acquired by an EPID.

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).

The use of EPID-measured leaf sequence files for IMRT dose reconstruction in adaptive radiation therapy

For intensity modulated radiation treatment (IMRT) dose reconstruction, multileaf collimator (MLC) log files have been shown applicable for deriving delivered fluence maps. However, MLC log files are dependent on the accuracy of leaf calibration and only available from one linear accelerator manufacturer. This paper presents a proof of feasibility and principles in (1) using an amorphous silicon electronic portal imaging device (aSi-EPID) to capture the MLC segments during an IMRT delivery and (2) reconstituting a leaf sequence (LS) file based on the leaf end positions calculated from the MLC segments and their associated fractional monitor units. These EPID-measured LS files are then used to derive delivered fluence maps for dose reconstruction. The developed approach was tested on a pelvic phantom treated with a typical prostate IMRT plan. The delivered fluence maps, which were derived from the EPID-measured LS files, showed slight differences in the intensity levels compared with the corresponding planned ones. The dose distribution calculated with the delivered fluence maps showed a discernible difference in the high dose region when compared to that calculated with the planned fluence maps. The maximum dose in the former distribution was also 2.5% less than that in the latter one. The EPID-measured LS file can serve the same purpose as a MLC log files does for the derivation of the delivered fluence map and yet is independent of the leaf calibration. The approach also allows users who do not have access to MLC log files to probe the actual IMRT delivery and translate the information gained for dose reconstruction in adaptive radiation therapy.

A literature review of electronic portal imaging for radiotherapy dosimetry

Electronic portal imaging devices (EPIDs) have been the preferred tools for verification of patient positioning for radiotherapy in recent decades. Since EPID images contain dose information, many groups have investigated their use for radiotherapy dose measurement. With the introduction of the amorphous-silicon EPIDs, the interest in EPID dosimetry has been accelerated because of the favourable characteristics such as fast image acquisition, high resolution, digital format, and potential for in vivo measurements and 3D dose verification. As a result, the number of publications dealing with EPID dosimetry has increased considerably over the past approximately 15 years. The purpose of this paper was to review the information provided in these publications. Information available in the literature included dosimetric characteristics and calibration procedures of various types of EPIDs, strategies to use EPIDs for dose verification, clinical approaches to EPID dosimetry, ranging from point dose to full 3D dose distribution verification, and current clinical experience. Quality control of a linear accelerator, pre-treatment dose verification and in vivo dosimetry using EPIDs are now routinely used in a growing number of clinics. The use of EPIDs for dosimetry purposes has matured and is now a reliable and accurate dose verification method that can be used in a large number of situations. Methods to integrate 3D in vivo dosimetry and image-guided radiotherapy (IGRT) procedures, such as the use of kV or MV cone-beam CT, are under development. It has been shown that EPID dosimetry can play an integral role in the total chain of verification procedures that are implemented in a radiotherapy department. It provides a safety net for simple to advanced treatments, as well as a full account of the dose delivered. Despite these favourable characteristics and the vast range of publications on the subject, there is still a lack of commercially available solutions for EPID dosimetry. As strategies evolve and commercial products become available, EPID dosimetry has the potential to become an accurate and efficient means of large-scale patient-specific IMRT dose verification for any radiotherapy department.

Verification of dose delivery for a prostate sIMRT treatment using a SLIC-EPID

The current work focuses on the verification of transmitted dose maps, measured using a scanning liquid ionization chamber-electronic portal imaging device (SLIC-EPID) for a typical step-and-shoot prostate IMRT treatment using an anthropomorphic phantom at anterior-posterior (A-P), and several non-zero gantry angles. The dose distributions measured using the SLIC-EPID were then compared with those calculated in the modelled EPID for each segment/subfield and also for the corresponding total fields using a gamma function algorithm with a distance to agreement and dose difference criteria of 2.54mm and 3%, respectively.

Correction of conebeam CT values using a planning CT for derivation of the "dose of the day"

BACKGROUND AND PURPOSE

Verification of the actually delivered 3D dose distribution during each treatment fraction ("dose of the day") is the most complete and clinical relevant "in-vivo" check of an IMRT treatment. To do this, during patient treatment portal dose images are routinely acquired with our electronic portal imaging device to derive the delivered fluence map for each treatment field. In addition, a conebeam CT scan is acquired just prior to treatment to derive the patient geometry at the time of treatment. However, the use of conebeam CT scans for dose calculation is hampered by inaccuracies in the conversion of CT values to electron densities due to an enlarged scatter contribution.

MATERIALS AND METHODS

In this work, a method is described for mapping of Hounsfield Units of the planning CT to the conebeam CT scan, while accounting for non-rigidity in the anatomy, e.g. related to weight loss, in an approximate way. The method was validated for head and neck cancer patients by comparing dose distributions calculated using adjusted Hounsfield Units with a golden standard.

RESULTS AND CONCLUSIONS

The observed dose differences were less than 1% in the majority of points, and in at least 96% of the points a 3D gamma analysis resulted in gamma values of less than 1 when applying a 2%/2mm criterion, showing that this straightforward approach allows for an accurate dose calculation based on conebeam CT scans.

Verification of helical tomotherapy delivery using autoassociative kernel regressiona)

Quality assurance (QA) is a topic of major concern in the field of intensity modulated radiation therapy (IMRT). The standard of practice for IMRT is to perform QA testing for individual patients to verify that the dose distribution will be delivered to the patient. The purpose of this study was to develop a new technique that could eventually be used to automatically evaluate helical tomotherapy treatments during delivery using exit detector data. This technique uses an autoassociative kernel regression (AAKR) model to detect errors in tomotherapy delivery. AAKR is a novel nonparametric model that is known to predict a group of correct sensor values when supplied a group of sensor values that is usually corrupted or contains faults such as machine failure. This modeling scheme is especially suited for the problem of monitoring the fluence values found in the exit detector data because it is able to learn the complex detector data relationships. This scheme still applies when detector data are summed over many frames with a low temporal resolution and a variable beam attenuation resulting from patient movement. Delivery sequences from three archived patients (prostate, lung, and head and neck) were used in this study. Each delivery sequence was modified by reducing the opening time for random individual multileaf collimator (MLC) leaves by random amounts. The errof and error-free treatments were delivered with different phantoms in the path of the beam. Multiple autoassociative kernel regression (AAKR) models were developed and tested by the investigators using combinations of the stored exit detector data sets from each delivery. The models proved robust and were able to predict the correct or error-free values for a projection, which had a single MLC leaf decrease its opening time by less than 10 msec. The model also was able to determine machine output errors. The average uncertainty value for the unfaulted projections ranged from 0.4% to 1.8% of the detector signal. The low model uncertainty indicates that the AAKR model is extremely accurate in its predictions and also suggests that the model may be able to detect errors that cause the fluence to change by less than 2%. However, additional evaluation of the AAKR technique is needed to determine the minimum detectable error threshold from the compressed helical tomotherapy detector data. Further research also needs to explore applying this technique to electronic portal imaging detector data.

Daily monitoring of linear accelerator beam parameters using an amorphous silicon EPID

An amorphous silicon EPID has been investigated to test its suitability as a daily check device for linac output and to provide daily monitoring of beam profile parameters such as flatness, symmetry, field size and wedge factor. Open and wedged 6 and 8 MV photon beams were collected on a daily basis for a period of just over a year and analysed in software to determine daily values of these parameters. Daily output results gave agreement between EPID measured dose and ion chamber measurements with a standard deviation of 0.65%. Step changes in flatness, symmetry and field size were readily detected by the EPID and could be correlated with adjustments made on service days and QC sessions. The results could also be used to assess the long term beam stability. Recalibration of the EPID required new baseline values of the parameters to be set. Wedge factors measured at one collimator angle proved stable but sensitive to changes in beam steering. The EPID proved to be a useful daily check device for linac output which can simultaneously be used for daily monitoring of beam profiles and field sizes.

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.

Developments in electronic portal imaging systems

Verification of geometric accuracy at the time of treatment delivery has always been a necessary part of the radiotherapy process. Since the introduction of conformal and intensity-modulated radiotherapy, the consequences of patient positioning errors are more serious. Portal imaging has played a large part in fulfilling the need for improved geometric accuracy. This review examines how portal imaging has progressed through the development and evolution of electronic portal imaging devices (EPIDs). Changes in technology, including the current commercial systems, and how image quality has changed are presented. The clinical usage of EPIDs and the technological innovations being devised for further improvements in image quality and systems are considered.

Dosimetric pre-treatment verification of IMRT using an EPID; clinical experience

BACKGROUND AND PURPOSE

In our clinic a QA program for IMRT verification, fully based on dosimetric measurements with electronic portal imaging devices (EPID), has been running for over 3 years. The program includes a pre-treatment dosimetric check of all IMRT fields. During a complete treatment simulation at the linac, a portal dose image (PDI) is acquired with the EPID for each patient field and compared with a predicted PDI. In this paper, the results of this pre-treatment procedure are analysed, and intercepted errors are reported. An automated image analysis procedure is proposed to limit the number of fields that need human intervention in PDI comparison.

MATERIALS AND METHODS

Most of our analyses are performed using the gamma index with 3% local dose difference and 3mm distance to agreement as reference values. Scalar parameters are derived from the gamma values to summarize the agreement between measured and predicted 2D PDIs. Areas with all pixels having gamma values larger than one are evaluated, making decisions based on clinically relevant criteria more straightforward.

RESULTS

In 270 patients, the pre-treatment checks revealed four clinically relevant errors. Calculation of statistics for a group of 75 patients showed that the patient-averaged mean gamma value inside the field was 0.43 +/- 0.13 (1SD) and only 6.1 +/- 6.8% of pixels had a gamma value larger than one. With the proposed automated image analysis scheme, visual inspection of images can be avoided in 2/3 of the cases.

CONCLUSION

EPIDs may be used for high accuracy and high resolution routine verification of IMRT fields to intercept clinically relevant dosimetric errors prior to the start of treatment. For the majority of fields, PDI comparison can fully rely on an automated procedure, avoiding excessive workload.

Fast, daily linac verification for segmented IMRT using electronic portal imaging

PURPOSE

Intensity modulated radiotherapy (IMRT) requires dedicated quality assurance (QA). Recently, we have published a method for fast (1-2 min) and accurate linac quality control for dynamic multileaf collimation, using a portal imaging device. This method is in routine use for daily leaf motion verification. The purpose of the present study was to develop an equivalent procedure for QA of IMRT with segmented (static) multileaf collimation (SMLC).

MATERIALS AND METHODS

The QA procedure is based on measurements performed during 3- to 8-month periods at Elekta, Siemens and Varian accelerators. On each measurement day, images were acquired for a field consisting of five 3 x 22 cm(2) segments. These 10 monitor unit (MU) segments were delivered in SMLC mode, moving the leaves from left to right. Deviations of realized leaf gap widths from the prescribed width were analysed to study the leaf positioning accuracy. To assess hysteresis in leaf positioning, the sequential delivery of the SMLC segments was also inverted. A static 20 x 20 cm(2) field was delivered with exposures between 1 and 50 MU to study the beam output and beam profile at low exposures. Comparisons with an ionisation chamber were made to verify the EPID dose measurements at low MU. Dedicated software was developed to improve the signal-to-noise ratio and to correct for image distortion.

RESULTS AND CONCLUSIONS

The observed long-term leaf gap reproducibility (1 standard deviation) was 0.1 mm for the Varian, and 0.2 mm for the Siemens and the Elekta accelerators. In all cases the hysteresis was negligible. Down to the lowest MU, beam output measurements performed with the EPID agreed within 1+/-1% (1SD) with ionisation chamber measurements. These findings led to a fast (3-4 min) procedure for accurate, daily linac quality control for SMLC.

A method for deconvolution of integrated electronic portal images to obtain incident fluence for dose reconstruction

A method to convert integrated electronic portal imaging device (EPID) images to fluence for the purpose of reconstructing the dose to a phantom is investigated here for simple open fields. Ultimately, the goal is to develop a method to reconstruct the dose to patients. The EPID images are transformed into incident intensity fluence by spatial filtering with a deconvolution kernel. The kernel uses a general mathematical form derived from a Monte Carlo calculation of the point spread function of an EPID. The deconvolution kernel is fitted using a downhill search algorithm that minimizes the difference between the reconstructed dose and the dose measured in water. The beam profile “horns” that are removed by the EPID calibration procedure are restored to the resulting images by direct multiplication using the measured in‐air off‐axis ratio. Applying the fitted kernel to an EPID image provides the incident fluence for that beam. This beam fluence is then entered into an independent dose calculation algorithm for phantom or patient dose reconstruction. The phantom dose was computed to an accuracy of 2.0% of the dmaxdose at one standard deviation. The method is general and can possibly be applied to any EPID equipped with an integration mode. We demonstrate the application of the fitted kernel in two clinical IMRT cases.

PACS numbers: 87.52.Df, 87.53.Bn, 87.53.Dq, 87.56.Fc, 87.66.Pm

Improving IMRT quality control efficiency using an amorphous silicon electronic portal imager

An amorphous silicon electronic portal imaging device (EPID) has been investigated to determine its usefulness and efficiency for performing linear accelerator quality control checks specific to step and shoot intensity modulated radiation therapy (IMRT). Several dosimetric parameters were measured using the EPID: dose linearity and segment to segment reproducibility of low dose segments, and delivery accuracy of fractions of monitor units. Results were compared to ion chamber measurements. Low dose beam flatness and symmetry were tested by overlaying low dose beam profiles onto the profile from a stable high-dose exposure and visually checking for differences. Beam flatness and symmetry were also calculated and plotted against dose. Start-up reproducibility was tested by overlaying profiles from twenty successive two monitor unit segments. A method for checking the MLC leaf calibration was also tested, designed to be used on a daily or weekly basis, which consisted of summing the images from a series of matched fields. Daily images were coregistered with, then subtracted from, a reference image. A threshold image showing dose differences corresponding to > 0.5 mm positional errors was generated and the number of pixels with such dose differences used as numerical parameter to which a tolerance can be applied. The EPID was found to be a sensitive relative dosemeter, able to resolve dose differences of 0.01 cGy. However, at low absolute doses a reproducible dosimetric nonlinearity of up to 7% due to image lag/ghosting effects was measured. It was concluded that although the EPID is suitable to measure segment to segment reproducibility and fractional monitor unit delivery accuracy, it is still less useful than an ion chamber as a tool for dosimetric checks. The symmetry/flatness test proved to be an efficient method of checking low dose profiles, much faster than any of the alternative methods. The MLC test was found to be extremely sensitive to sudden changes in MLC calibration but works best with a composite reference image consisting of an average of five successive days' images. When used in this way it proved an effective and efficient daily check of MLC calibration. Overall, the amorphous silicon EPID was found to be a suitable device for IMRT QC although it is not recommended for dosimetric tests. Automatic procedures for low monitor unit profile analysis and MLC leaf positioning yield considerable time-savings over traditional film techniques.

Maximum MLC opening effect in dynamic delivery of IMRT: leaf-positional analysis

The analysis of dynamic multileaf collimator (MLC) positions for the delivered intensity-modulated radiotherapy (IMRT) plans is crucial in that it may capture dose delivery problems otherwise difficult to observe and quantify in the conventional dosimetric measurements with film or with an ionization chamber. In some IMRT systems, delivery of IMRT fields starts with a maximum MLC opening (roughly the shape of the target in the beam's-eye view) and then proceeds to the subsequent dynamic MLC subfields. No irradiation is required in going from the initial segment (maximum opening) to the next one, and theoretically, no dose should be delivered in that initial moment. However, due to a finite sampling time of the MLC controller, the finite speed of the MLC, and a finite leaf tolerance, there may be some dose delivered between the first and the second segment. The amount of the excess dose is higher for larger dose rates and for a smaller number of the total monitor units per IMRT field. The magnitude of the dose errors could be in the order of a few percent. Effects similar to the maximum MLC opening may occur in other situations as well, for instance, when leaves are forced to move over large distances in a short time. Confounding this are dose errors due to the uncertainty in the MLC transmission. The analysis of the actual leaf positions recorded in the dynamic MLC log file is helpful in differentiating between the two types of errors and in determining the optimal dynamic MLC delivery parameters.

Use of an amorphous silicon electronic portal imaging device for multileaf collimator quality control and calibration

Multileaf collimator (MLC) calibration and quality control is a time-consuming procedure typically involving the processing, scanning and analysis of films to measure leaf and collimator positions. Faster and more reliable calibration procedures are required for these tasks, especially with the introduction of intensity modulated radiotherapy which requires more frequent checking and finer positional leaf tolerances than previously. A routine quality control (QC) technique to measure MLC leaf bank gain and offset, as well as minor offsets (individual leaf position relative to a reference leaf), using an amorphous silicon electronic portal imaging device (EPID) has been developed. The technique also tests the calibration of the primary and back-up collimators. A detailed comparison between film and EPID measurements has been performed for six linear accelerators (linacs) equipped with MLC and amorphous silicon EPIDs. Measurements of field size from 4 to 24 cm with the EPID were systematically smaller than film measurements over all field sizes by 0.4 mm for leaves/back-up collimators and by 0.2 mm for conventional collimators. This effect is due to the gain calibration correction applied by the EPID, resulting in a 'flattening' of primary beam profiles. Linac dependent systematic differences of up to 0.5 mm in individual leaf/collimator positions were also found between EPID and film measurements due to the difference between the mechanical and radiation axes of rotation. When corrections for these systematic differences were applied, the residual random differences between EPID and film were 0.23 mm and 0.26 mm (1 standard deviation) for field size and individual leaf/back-up collimator position, respectively. Measured gains (over a distance of 220 mm) always agreed within 0.4 mm with a standard deviation of 0.17 mm. Minor offset measurements gave a mean agreement between EPID and film of 0.01+/-0.10 mm (1 standard deviation) after correction for the tilt of the EPID and small rotational misalignments between leaf banks and the back-up collimators used as a reference straight edge. Reproducibility of EPID measurements was found to be very high, with a standard deviation of <0.05 mm for field size and <0.1 mm for individual leaf/collimator positions for a 10x10 cm2 field. A standard set of QC images (three field sizes defined both by leaves only and collimators only) can be acquired in less than 20 min and analysed in 5 min.

Stereotactic arc therapy for small elongated tumors using cones and collimator jaws; dosimetric and planning aspects

Stereotactic arc treatment of small intracranial tumors is usually performed with arcs collimated by circular cones, resulting in treatment volumes which are basically spherical. For nonspherical lesions this results in a suboptimal dose distribution. Multiple isocenters may improve the dose conformity for these lesions, at the cost of large overdosages in the target volume. To achieve improved dose conformity as well as dose homogeneity, the linac jaws (with a minimum distance of 1.0 cm to the central beam axis) can routinely be used to block part of the circular beams. The purpose of this study was to investigate the feasibility of blocking cones with diameters as small as 1.0 cm and a minimum distance between the jaw and the central beam axis of 0.3 cm. First, the reproducibility in jaw positioning and resulting dose delivery on the treatment unit were assessed. Second, the accuracy of the TPS dose calculation for these small fields was established. Finally, clinically applied treatment plans using nonblocked cones were compared with plans using the partially blocked cones for several treatment sites. The reproducibility in dose delivery on our Varian Clinac 2300 C/D machines on the central beam axis is 0.8% (1 SD). The accuracy of the treatment planning system dose calculation algorithm is critically dependent on the used fits for the penumbra and the phantom scatter. The average deviation of calculated from measured dose on the central beam axis is -1.0%+/-1.4% (1 SD), which is clinically acceptable. Partial cone blocking results in improved dose distributions for elongated tumors, such as vestibular schwannoma and uveal melanoma. Multiple isocenters may be avoided. The technique is easy to implement and requires no additional workload.

Verification of patient position and delivery of IMRT by electronic portal imaging

BACKGROUND AND PURPOSE

The purpose of the work presented in this paper was to determine whether patient positioning and delivery errors could be detected using electronic portal images of intensity modulated radiotherapy (IMRT).

PATIENTS AND METHODS

We carried out a series of controlled experiments delivering an IMRT beam to a humanoid phantom using both the dynamic and multiple static field method of delivery. The beams were imaged, the images calibrated to remove the IMRT fluence variation and then compared with calibrated images of the reference beams without any delivery or position errors. The first set of experiments involved translating the position of the phantom both laterally and in a superior/inferior direction a distance of 1, 2, 5 and 10 mm. The phantom was also rotated 1 and 2 degrees . For the second set of measurements the phantom position was kept fixed and delivery errors were introduced to the beam. The delivery errors took the form of leaf position and segment intensity errors.

RESULTS

The method was able to detect shifts in the phantom position of 1 mm, leaf position errors of 2 mm, and dosimetry errors of 10% on a single segment of a 15 segment IMRT step and shoot delivery (significantly less than 1% of the total dose).

CONCLUSIONS

The results of this work have shown that the method of imaging the IMRT beam and calibrating the images to remove the intensity modulations could be a useful tool in verifying both the patient position and the delivery of the beam.

SIFT: A method to verify the IMRT fluence delivered during patient treatment using an electronic portal imaging device

PURPOSE

Radiotherapy patients are increasingly treated with intensity-modulated radiotherapy (IMRT) and high tumor doses. As part of our quality control program to ensure accurate dose delivery, a new method was investigated that enables the verification of the IMRT fluence delivered during patient treatment using an electronic portal imaging device (EPID), irrespective of changes in patient geometry.

METHODS AND MATERIALS

Each IMRT treatment field is split into a static field and a modulated field, which are delivered in sequence. Images are acquired for both fields using an EPID. The portal dose image obtained for the static field is used to determine changes in patient geometry between the planning CT scan and the time of treatment delivery. With knowledge of these changes, the delivered IMRT fluence can be verified using the portal dose image of the modulated field. This method, called split IMRT field technique (SIFT), was validated first for several phantom geometries, followed by clinical implementation for a number of patients treated with IMRT.

RESULTS

The split IMRT field technique allows for an accurate verification of the delivered IMRT fluence (generally within 1% [standard deviation]), even if large interfraction changes in patient geometry occur. For interfraction radiological path length changes of 10 cm, deliberately introduced errors in the delivered fluence could still be detected to within 1% accuracy. Application of SIFT requires only a minor increase in treatment time relative to the standard IMRT delivery.

CONCLUSIONS

A new technique to verify the delivered IMRT fluence from EPID images, which is independent of changes in the patient geometry, has been developed. SIFT has been clinically implemented for daily verification of IMRT treatment delivery.

Dosimetric IMRT verification with a flat-panel EPID

A convolution-based calibration procedure has been developed to use an amorphous silicon flat-panel electronic portal imaging device (EPID) for accurate dosimetric verification of intensity-modulated radiotherapy (IMRT) treatments. Raw EPID images were deconvolved to accurate, high-resolution 2-D distributions of primary fluence using a scatter kernel composed of two elements: a Monte Carlo generated kernel describing dose deposition in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Relative fluence profiles measured with the EPID are in very good agreement with those measured with a diamond detector, and exhibit excellent spatial resolution required for IMRT verification. For dosimetric verification, the EPID-measured primary fluences are convolved with a Monte Carlo kernel describing dose deposition in a solid water phantom, and cross-calibrated with ion chamber measurements. Dose distributions measured using the EPID agree to within 2.1% with those measured with film for open fields of 2 x 2 cm2 and 10 x 10 cm2. Predictions of the EPID phantom scattering factors (SPE) based on our scatter kernels are within 1% of the SPE measured for open field sizes of up to 16 x 16 cm2. Pretreatment verifications of step-and-shoot IMRT treatments using the EPID are in good agreement with those performed with film, with a mean percent difference of 0.2 +/- 1.0% for three IMRT treatments (24 fields).

An accurate calibration method of the multileaf collimator valid for conformal and intensity modulated radiation treatments

Because for IMRT treatments the required accuracy on leaf positioning is high, conventional calibration methods may not be appropriate. The aim of this study was to develop the tools for an accurate MLC calibration valid for conventional and IMRT treatments and to investigate the stability of the MLC. A strip test consisting of nine adjacent segments 2 cm wide, separated by 1 mm and exposed on Kodak X-Omat V films at Dmax depth, was used for detecting leaf-positioning errors. Dose profiles along the leaf-axis were taken for each leaf-pair. We measured the dose variation on each abutment to quantify the relative positioning error (RPE) and the absolute position of the abutment to quantify the absolute positioning error (APE). The accuracy of determining the APE and RPE was 0.15 and 0.04 mm, respectively. Using the RPE and the APE the MLC calibration parameters were calculated in order to obtain a flat profile on the abutment at the correct position. A conventionally calibrated Elekta MLC was re-calibrated using the strip test. The stability of the MLC and leaf-positioning reproducibility was investigated exposing films with 25 adjacent segments 1 cm wide during three months and measuring the standard deviation of the RPE values. A maximum shift over the three months of 0.27 mm was observed and the standard deviation of the RPE values was 0.11 mm.

Quantitative measurement of MLC leaf displacements using an electronic portal image device

The success of an IMRT treatment relies on the positioning accuracy of the MLC (multileaf collimator) leaves for both step-and-shoot and dynamic deliveries. In practice, however, there exists no effective and quantitative means for routine MLC QA and this has become one of the bottleneck problems in IMRT implementation. In this work we present an electronic portal image device (EPID) based method for fast and accurate measurement of MLC leaf positions at arbitrary locations within the 40 cm x 40 cm radiation field. The new technique utilizes the fact that the integral signal in a small region of interest (ROI) is a sensitive and reliable indicator of the leaf displacement. In this approach, the integral signal at a ROI was expressed as a weighted sum of the contributions from the displacements of the leaf above the point and the adjacent leaves. The weighting factors or linear coefficients of the system equations were determined by fitting the integral signal data for a group of pre-designed MLC leaf sequences to the known leaf displacements that were intentionally introduced during the creation of the leaf sequences. Once the calibration is done, the system can be used for routine MLC leaf positioning QA to detect possible leaf errors. A series of tests was carried out to examine the functionality and accuracy of the technique. Our results show that the proposed technique is potentially superior to the conventional edge-detecting approach in two aspects: (i) it deals with the problem in a systematic approach and allows us to take into account the influence of the adjacent MLC leaves effectively; and (ii) it may improve the signal-to-noise ratio and is thus capable of quantitatively measuring extremely small leaf positional displacements. Our results indicate that the technique can detect a leaf positional error as small as 0.1 mm at an arbitrary point within the field in the absence of EPID set-up error and 0.3 mm when the uncertainty is considered. Given its simplicity, efficiency and accuracy, we believe that the technique is ideally suitable for routine MLC leaf positioning QA.

Leaf trajectory verification during dynamic intensity modulated radiotherapy using an amorphous silicon flat panel imager

An independent verification of the leaf trajectories during each treatment fraction improves the safety of IMRT delivery. In order to verify dynamic IMRT with an electronic portal imaging device (EPID), the EPID response should be accurate and fast such that the effect of motion blurring on the detected moving field edge position is limited. In the past, it was shown that the errors in the detected position of a moving field edge determined by a scanning liquid-filled ionization chamber (SLIC) EPID are negligible in clinical practice. Furthermore, a method for leaf trajectory verification during dynamic IMRT was successfully applied using such an EPID. EPIDs based on amorphous silicon (a-Si) arrays are now widely available. Such a-Si flat panel imagers (FPIs) produce portal images with superior image quality compared to other portal imaging systems, but they have not yet been used for leaf trajectory verification during dynamic IMRT. The aim of this study is to quantify the effect of motion distortion and motion blurring on the detection accuracy of a moving field edge for an Elekta iViewGT a-Si FPI and to investigate its applicability for the leaf trajectory verification during dynamic IMRT. We found that the detection error for a moving field edge to be smaller than 0.025 cm at a speed of 0.8 cm/s. Hence, the effect of motion blurring on the detection accuracy of a moving field edge is negligible in clinical practice. Furthermore, the a-Si FPI was successfully applied for the verification of dynamic IMRT. The verification method revealed a delay in the control system of the experimental DMLC that was also found using a SLIC EPID, resulting in leaf positional errors of 0.7 cm at a leaf speed of 0.8 cm/s.

IMRT: delivery techniques and quality assurance

Intensity modulated radiotherapy (IMRT) is a major development in the delivery of radiation therapy that has the potential to improve patient outcome by reducing morbidity or increasing local tumour control. Delivery techniques include those based on purpose built devices and treatment machines together with those utilizing the capabilities of computer controlled multileaf collimators which are more widely available. The complexity of IMRT techniques demands a high level of quality control both in the operation of the equipment and in the delivery of treatment to individual patients. The purpose of this paper is therefore to review the techniques available, concentrating on the use of multileaf collimators, and to consider the necessary quality control requirements for clinical application. It demonstrates that the technology is mature and sufficiently well understood so that IMRT can be safely implemented in the general clinical environment rather than being limited to application in the research environment.

Two-dimensional measurement of photon beam attenuation by the treatment couch and immobilization devices using an electronic portal imaging device

In our institution, an individualized dosimetric quality assurance protocol for intensity modulated radiotherapy (IMRT) is being implemented. This protocol includes dosimetric measurements with a fluoroscopic electronic portal imaging device (EPID) for all IMRT fields while the patient is being irradiated. For some of the first patients enrolled in this protocol, significant beam attenuation by (carbon fiber) components of the treatment couch was observed. To study this beam attenuation in two-dimensional, EPID images were also acquired in absence of the patient, both with and without treatment couch and immobilization devices, as positioned during treatment. For treatments of head and neck cancer patients with a 6 MV photon beam, attenuation of up to 15% was detected. These findings led to the development of new tools and procedures for planning and treatment delivery to avoid underdosages in the tumor.