A validation framework to assess performance of commercial deformable image registration in lung radiotherapy

IntroductionDeformable image registration (DIR) can play an important role in the context of adaptive radiotherapy. The AAPM Task Group 132 (TG-132) has described several quantitative measures for DIR error assessment but they can only be accurately defined when there is a ground-truth present in high-contrast regions. This work aims to set out a framework to obtain optimal results for CT-CT lung DIR in clinical setting for a commercially available system by quantifying the DIR performance in both low- and high-contrast regions.MethodsFive publicly available thorax datasets were used to assess the DIR quality. A “Ghost fiducial” method was implemented by windowing the contrast in a new feature provided by Varian Velocity v4.1. Target registration error (TRE) of the landmarks and Dice-similarity coefficient of the tumour were calculated at three different contrast settings to assess the algorithm in high- and low-contrast scenarios.ResultsFor the original unedited dataset, higher resolution DIR methods showed best performance acceptable within the recommended limit according to TG-132, when actual displacements were less than 10 mm. The relation of the actual displacement of the landmarks and TRE shows the limited capacity of the algorithm to deal with movements larger than 10 mm.ConclusionThis work found the performance of DIR methods and settings available in Varian Velocity v4.1 to be a function of contrast level as well as extent of motion. This highlights the need for multiple metrics to assess different aspects of DIR performance for various applications related to low-contrast and/or high-contrast regions.     

Dose warping performance in deformable image registration in lung

PurposeIt is unclear that spatial accuracy can reflect the impact of deformed dose distribution. In this study, we used dosimetric parameters to compare an in-house deformable image registration (DIR) system using NiftyReg, with two commercially available systems, MIM Maestro (MIM) and Velocity AI (Velocity).MethodsFor 19 non-small-cell lung cancer patients, the peak inspiration (0%)-4DCT images were deformed to the peak expiration (50%)-4DCT images using each of the three DIR systems, which included computation of the deformation vector fields (DVF). The 0%-gross tumor volume (GTV) and the 0%-dose distribution were also then deformed using the DVFs. The agreement in the dose distributions for the GTVs was evaluated using generalized equivalent uniform dose (gEUD), mean dose (D_mean), and three-dimensional (3D) gamma index (criteria: 3 mm/3%). Additionally, a Dice similarity coefficient (DSC) was used to measure the similarity of the GTV volumes.ResultsD_mean and gEUD demonstrated good agreement between the original and deformed dose distributions (differences were generally less than 3%) in 17 of the patients. In two other patients, the Velocity system resulted in differences in gEUD of 50.1% and 29.7% and in D_mean of 11.8% and 4.78%. The gamma index comparison showed statistically significant differences for the in-house DIR vs. MIM, and MIM vs. Velocity.ConclusionsThe finely tuned in-house DIR system could achieve similar spatial and dose accuracy to the commercial systems. Care must be taken, as we found errors of more than 5% for D_mean and 30% for gEUD, even with a commercially available DIR tool.     

Commissioning and validation of commercial deformable image registration software for adaptive contouring

PurposeTo report the commissioning and validation of deformable image registration(DIR) software for adaptive contouring.MethodsDIR (SmartAdapt®v13.6) was validated using two methods namely contour propagation accuracy and landmark tracking, using physical phantoms and clinical images of various disease sites. Five in-house made phantoms with various known deformations and a set of 10 virtual phantoms were used. Displacement in lateral, anterio-posterior (AP) and superior-inferior (SI) direction were evaluated for various organs and compared with the ground truth. Four clinical sites namely, brain (n = 5), HN (n = 9), cervix (n = 18) and prostate (n = 23) were used. Organs were manually delineated by a radiation oncologist, compared with the deformable image registration (DIR) generated contours. 3D slicer v4.5.0.1 was used to analyze Dice Similarity Co-efficient (DSC), shift in centre of mass (COM) and Hausdorff distances Hf_95%/avg.ResultsMean (SD) DSC, Hf_95% (mm), Hf_avg (mm) and COM of all the phantoms 1–5 were 0.84 (0.2) mm, 5.1 (7.4) mm, 1.6 (2.2) mm, and 1.6 (0.2) mm respectively. Phantom-5 had the largest deformation as compared to phantoms 1–4, and hence had suboptimal indices. The virtual phantom resulted in consistent results for all the ROIs investigated. Contours propagated for brain patients were better with a high DSC score (0.91 (0.04)) as compared to other sites (HN: 0.84, prostate: 0.81 and cervix 0.77). A similar trend was seen in other indices too. The accuracy of propagated contours is limited for complex deformations that include large volume and shape change of bladder and rectum respectively. Visual validation of the propagated contours is recommended for clinical implementation.ConclusionThe DIR algorithm was commissioned and validated for adaptive contouring.     

Evaluation of the Block Matching deformable registration algorithm in the field of head-and-neck adaptive radiotherapy

Background and purposeTo compare the accuracy of the Block Matching deformable registration (DIR) against rigid image registration (RIR) for head-and-neck multi-modal images CT to cone-beam CT (CBCT) registration.Material and methodsPlanning-CT and weekly CBCT of 10 patients were used for this study. Several volumes, including medullary canal (MC), thyroid cartilage (TC), hyoid bone (HB) and submandibular gland (SMG) were transposed from CT to CBCT images using either DIR or RIR. Transposed volumes were compared with the manual delineation of these volumes on every CBCT. The parameters of similarity used for analysis were: Dice Similarity Index (DSI), 95%-Hausdorff Distance (95%-HD) and difference of volumes (cc).ResultsWith DIR, the major mean difference of volumes was −1.4 cc for MC, revealing limited under-segmentation. DIR limited variability of DSI and 95%-HD. It significantly improved DSI for TC and HB and 95%-HD for all structures but SMG. With DIR, mean 95%-HD (mm) was 3.01 ± 0.80, 5.33 ± 2.51, 4.99 ± 1.69, 3.07 ± 1.31 for MC, TC, HB and SMG, respectively. With RIR, it was 3.92 ± 1.86, 6.94 ± 3.98, 6.44 ± 3.37 and 3.41 ± 2.25, respectively.ConclusionBlock Matching is a valid algorithm for deformable multi-modal CT to CBCT registration. Values of 95%-HD are useful for ongoing development of its application to the cumulative dose calculation.     

Accuracy of target delineation by positron emission tomography-based auto-segmentation methods after deformable image registration: A phantom study

PurposeDiagnostic positron emission tomography and computed tomography (PET/CT) images can be fused to the planning CT images by a deformable image registration (DIR). The aim of this study was to evaluate the standardized uptake value (SUV) and target delineation on deformed PET images.MethodsWe used a cylindrical phantom and removable inserts of four spheres (16–38 mm in diameter) and three ellipsoids with a volume equal to the 38-mm-diameter sphere (S38) in each. S38 was filled with 18F-fluorodeoxyglucose activity, and then PET/CT images were acquired. The contours of S38 were generated using original PET images by PET auto-segmentation (PET-AS) methods of (1) SUV2.5, (2) 40% of maximum SUV (SUV40%max), and (3) gradient-based (GB), and were deformed to the other inserts by DIR. We compared the volumes and the SUV_max with the generated contours using the deformed PET images.ResultsThe SUV_max was slightly decreased by DIR; the mean absolute difference was −0.10 ± 0.04. For SUV2.5 and SUV40%max, the differences in S38 volumes between the original and deformed PET images were less than 5%, regardless of deformation type. For the GB, the contoured volumes obtained from deformed PET images were larger than those of the original PET images for the deformation type of ellipsoids. When the S38 was deformed to the 16-mm-diameter sphere, the maximum volume difference was −22.8%.ConclusionsAlthough SUV fluctuations by DIR were negligible, the target delineation on deformed PET images by the GB should be carefully considered owing to the distortion of intensity profiles.     

A quantitative evaluation of deformable image registration based on MV cone beam CT images: Impact of deformation magnitudes and image modalities

Background and purposeTo evaluate the impact of deformation magnitude and image modality on deformable-image-registration (DIR) accuracy using Halcyon megavoltage cone beam CT images (MVCBCT).Materials and methodsPlanning CT images of an anthropomorphic Head phantom were aligned rigidly with MVCBCT and re-sampled to achieve the same resolution, denoted as pCT. MVCBCT was warped with twenty simulated pre-known virtual deformation fields (T_i, i = 1–20) with increasing deformation magnitudes, yielding warped CBCT (wCBCT). The pCT and MVCBCT were registered to wCBCT respectively (Multi-modality and Uni-modality DIR), generating deformation vector fields V_i and V_i′ (i = 1–20). V_i and V_i′ were compared with T_i respectively to assess the DIR accuracy geometrically. In addition, V_i, T_i, and V_i′ were applied to pCT, generating deformed CT (dCT_i), ground-truth CT (G_i) and deformed CT′ (dCT_i′) respectively. The Hounsfield Unit (HU) on these virtual CT images were also compared.ResultsThe mean errors of vector displacement increased with the deformation magnitude. For deformation magnitudes between 2.82 mm and 7.71 mm, the errors of uni-modality DIR were 1.16 mm ~ 1.73 mm smaller than that of multi-modality (p = 0.0001, Wilcoxon signed rank test). DIR could reduce the maximum signed and absolute HU deviations from 70.8 HU to 11.4 HU and 208 HU to 46.2 HU respectively.ConclusionsAs deformation magnitude increases, DIR accuracy continues to deteriorate and uni-modality DIR consistently outperformed multi-modality DIR. DIR-based adaptive radiotherapy utilizing the noisy MVCBCT images is only conditionally applicable with caution.     

Validation of a deformable image registration produced by a commercial treatment planning system in head and neck

In recent years one of the areas of interest in radiotherapy has been adaptive radiation therapy (ART), with the most efficient way of performing ART being the use of deformable image registration (DIR). In this paper we use the distances between points of interest (POIs) in the computed tomography (CT) and the cone beam computed tomography (CBCT) acquisition images and the inverse consistence (IC) property to validate the RayStation treatment planning system (TPS) DIR algorithm. This study was divided into two parts: Firstly the distance-accuracy of the TPS DIR algorithm was ascertained by placing POIs on anatomical features in the CT and CBCT images from five head and neck cancer patients. Secondly, a method was developed for studying the implication of these distances on the dose by using the IC. This method compared the dose received by the structures in the CT, and the structures that were quadruply-deformed. The accuracy of the TPS was 1.7 ± 0.8 mm, and the distance obtained with the quadruply-deformed IC method was 1.7 ± 0.9 mm, i.e. the difference between the IC method multiplied by two, and that of the TPS validation method, was negligible. Moreover, the IC method shows very little variation in the dose-volume histograms when comparing the original and quadruply-deformed structures. This indicates that this algorithm is useful for planning adaptive radiation treatments using CBCT in head and neck cancer patients, although these variations must be taken into account when making a clinical decision to adapt a treatment plan.     

Evaluation of four-dimensional cone beam computed tomography ventilation images acquired with two different linear accelerators at various gantry speeds using a deformable lung phantom

We evaluated four-dimensional cone beam computed tomography (4D-CBCT) ventilation images (VI_CBCT) acquired with two different linear accelerator systems at various gantry speeds using a deformable lung phantom.The 4D-CT and 4D-CBCT scans were performed using a computed tomography (CT) scanner, an X-ray volume imaging system (Elekta XVI) mounted in Versa HD, and an On-Board Imager (OBI) system mounted in TrueBeam. Intensity-based deformable image registration (DIR) was performed between peak-exhale and peak-inhale images. VI_CBCT- and 4D-CT-based ventilation images (VI_CT) were derived by DIR using two metrics: one based on the Jacobian determinant and one on changes in the Hounsfield unit (HU). Three different DIR regularization values (λ) were used for VI_CBCT. Correlations between the VI_CBCT and VI_CT values were evaluated using voxel-wise Spearman’s rank correlation coefficient (r).In case of both metrics, the Jacobian-based VI_CBCT with a gantry speed of 0.6 deg/sec in Versa HD showed the highest correlation for all the gantry speeds (e.g., λ = 0.05 and r = 0.68). Thus, the r value of the Jacobian-based VI_CBCT was greater or equal to that of the HU-based VI_CBCT. In addition, the ventilation accuracy of VI_CBCT increased at low gantry speeds.Thus, the image quality of VI_CBCT was affected by the change in gantry speed in both the imaging systems. Additionally, DIR regularization considerably influenced VI_CBCT in both the imaging systems. Our results have the potential to assist in designing CBCT protocols, incorporating VI_CBCT imaging into the functional avoidance planning process.     

Geometric inaccuracy and co-registration errors for CT,DynaCT and MRI images used in robotic stereotactic radiosurgery treatment planning

PurposeTo measure the combined errors due to geometric inaccuracy and image co-registration on secondary images (dynamic CT angiography (dCTA), 3D DynaCT angiography (DynaCTA), and magnetic resonance images (MRI)) that are routinely used to aid in target delineation and planning for stereotactic radiosurgery (SRS).MethodsThree phantoms (one commercial and two in-house built) and two different analysis approaches (commercial and MATLAB based) were used to quantify the magnitude of geometric image distortion and co-registration errors for different imaging modalities within CyberKnife’s MultiPlan treatment planning software. For each phantom, the combined errors were reported as a mean target registration error (TRE). The mean TRE’s for different intramodality imaging parameters (e.g., mAs, kVp, and phantom set-ups) and for dCTA, DynaCTA, and MRI systems were measured.ResultsOnly X-ray based imaging can be performed with the commercial phantom, and the mean TRE ± standard deviation values were large compared to the in-house analysis using MATLAB. With the 3D printed phantom, even drastic changes in treatment planning CT imaging protocols did not greatly influence the mean TRE (<0.5 mm for a 1 mm slice thickness CT). For all imaging modalities, the largest mean TRE was found on DynaCT, followed by T2-weighted MR images (albeit all <1 mm).ConclusionsThe user may overestimate the mean TRE if the commercial phantom and MultiPlan were used solely. The 3D printed phantom design is a sensitive and suitable quality assurance tool for measuring 3D geometric inaccuracy and co-registration errors across all imaging modalities.     

Validation of a method for “dose of the day” calculation in head-neck tomotherapy by using planning ct-to-MVCT deformable image registration

PurposeThe aim of this study was to test the feasibility and dosimetric accuracy of a method that employs planning CT-to-MVCT deformable image registration (DIR) for calculation of the daily dose for head and neck (HN) patients treated with Helical Tomotherapy (HT).MethodsFor each patient, the planning kVCT (CTplan) was deformably registered to the MVCT acquired at the 15th therapy session (MV15) with a B-Spline Free Form algorithm using Mattes mutual information (open-source software 3D Slicer), resulting in a deformed CT (CTdef). On the same day as MVCT15, a kVCT was acquired with the patient in the same treatment position (CT15). The original HT plans were recalculated both on CTdef and CT15, and the corresponding dose distributions were compared; local dose differences <2% of the prescribed dose (DD2%) and 2D/3D gamma-index values (2%-2 mm) were assessed respectively with Mapcheck SNC Patient software (Sun Nuclear) and with 3D-Slicer.ResultsOn average, 87.9% ± 1.2% of voxels were found for DD2% (on average 27 slices available for each patient) and 94.6% ± 0.8% of points passed the 2D gamma analysis test while the 3D gamma test was satisfied in 94.8% ± 0.8% of body’s voxels.ConclusionsThis study represents the first demonstration of the dosimetric accuracy of kVCT-to-MVCT DIR for dose of the day computations. The suggested method is sufficiently fast and reliable to be used for daily delivered dose evaluations in clinical strategies for adaptive Tomotherapy of HN cancer.     

Influence of different urinary bladder filling levels and controlling regions of interest selection on deformable image registration algorithms

PurposeEvaluation of Raystation ANAtomically CONstrained Deformation Algorithm (ANACONDA) performance to different urinary bladder filling levels in male pelvis anatomic site varying the controlling Regions Of Interest (ROIs).MethodsDifferent image datasets were obtained with ImSimQA (Oncology System Limited, Shrewsbury, UK) to evaluate ANACONDA performances (RaySearch Laboratories, Stockholm, Sweden). Deformation vector fields were applied to a synthetic man pelvis and a real patient computed tomography (CT) dataset (reference CTs) resulting in deformed CTs (target CTs) with various bladder filling levels. Different deformable image registrations (DIRs) were generated between each target CTs and reference CTs varying the controlling ROIs subset. Deformed ROIs were mapped from target CT to reference CT and then compared to reference ROIs. Evaluation was performed by Dice Similarity Coefficient (DSC), Correlation Coefficient (CC), Mean Distance to Agreement (MDA), maximum Distance to Agreement (maxDA) and with the introduction of global DSC (global_DSC) and global CC (global_CC) parameters.ResultsIn both synthetic and real patient CT cases, DSC scored less than 0.75 and MDA greater than 3 mm when no ROIs or only bladder were exploited as controlling ROI. DSC and CC increased by increasing the number of controlling ROIs selected whereas, an opposite behavior was observed for MDA and maxDA.ConclusionsANACONDA performances can be influenced by bladder filling fluctuation if no controlling ROIs are selected. Global_DSC and global_CC are useful parameters to quantitatively compare DIR algorithms. DIR performances improve by increasing the number of controlling ROIs selected, reaching a saturation level after a defined ROIs subset selection.     

Method of evaluating respiratory induced organ motion by vector volume histogram

PurposePublished organ motion data have been collected from measurements of a limited number of points within the organ, the centroid, or the edge of the organ. These are derived from the spatial characteristics of respiratory induced motion; however, this approach does not consider non-rigid organ deformation. We propose a novel quantitative method for evaluating respiratory induced organ motion using Deformable Image Registration (DIR).MethodTwo phases from a 4-dimensional computed tomography (4D CT) dataset at maximum inspiration and expiration were each taken from five patients. The left and right lungs, esophagus, stomach, spinal cord, and liver were manually contoured in the end-expiration phase. The hybrid deformable registration algorithm of the RayStation treatment planning system (TPS) was used to deform the end-expiration phase to the end-inspiration phase. From this, the deformation vector field (DVF) was calculated. DVFs consist of DVF_LR (left-right), DVF_AP (anterior-posterior), and DVF_SI (superior-inferior) as separate files. We calculated the vector volume histogram (VVH) and L_max (maximum absolute vector of the organ) to evaluate every vector for each individual organ. We also measured respiratory organ motion from the position of the organ centroid in two phases.ResultsVVH enabled us to find the absolute distance and volume of the organ contributing to motion points on the curve. Organ motion using the centroid method was smaller than L_max using VVH. Using the centroid method, it is difficult to evaluate the deformable organ motion.ConclusionVVH may be a useful technique in evaluating organ volumetric change during respiratory organ motion.     

Comprehensive quality assurance phantom for the small animal radiation research platform (SARRP)

PurposeTo develop and test the suitability and performance of a comprehensive quality assurance (QA) phantom for the Small Animal Radiation Research Platform (SARRP).Methods and materialsA QA phantom was developed for carrying out daily, monthly and annual QA tasks including: imaging, dosimetry and treatment planning system (TPS) performance evaluation of the SARRP. The QA phantom consists of 15 (60 × 60 × 5 mm³) kV-energy tissue equivalent solid water slabs. The phantom can incorporate optically stimulated luminescence dosimeters (OSLD), Mosfet or film. One slab, with inserts and another slab with hole patterns are particularly designed for image QA.ResultsOutput constancy measurement results showed daily variations within 3%. Using the Mosfet in phantom as target, results showed that the difference between TPS calculations and measurements was within 5%. Annual QA results for the Percentage depth dose (PDD) curves, lateral beam profiles, beam flatness and beam profile symmetry were found consistent with results obtained at commissioning. PDD curves obtained using film and OSLDs showed good agreement. Image QA was performed monthly, with image-quality parameters assessed in terms of CBCT image geometric accuracy, CT number accuracy, image spatial resolution, noise and image uniformity.ConclusionsThe results show that the developed QA phantom can be employed as a tool for comprehensive performance evaluation of the SARRP. The study provides a useful reference for development of a comprehensive quality assurance program for the SARRP and other similar small animal irradiators, with proposed tolerances and frequency of required tests.     

Free-to-use DIR solutions in radiotherapy: Benchmark against commercial platforms through a contour-propagation study

PurposeA contour propagation study has been conducted to benchmark three algorithms for Deformable Image Registration (DIR) freely available online against well-established commercial solutions.MethodsElastiX, BRAINS and Plastimach, available as modules in the open source platform 3DSlicer, were tested as the recent AAPM Task group 132 guidelines proposes. The overlap of the DIR-mapped ROIs in four computational anthropomorphic phantoms was measured. To avoid bias every algorithm was left to run without any human interaction nor particular registration strategy. The accuracy of the algorithms was measured using the Dice Similarity Coefficient (DSC) and Mean Distance to Conformity (MDC) metrics. The registration quality was compared to the recommended geometrical accuracy suggested by AAPM TG132 and to the results of a large population-based study performed with commercial DIR solutions.ResultsThe considered free-to-use DIR solutions generally meet acceptable accuracy and good overlap (DSC > 0.85). Mild failures (DSC < 0.75) were detected only for the smallest structures. In case of extremely severe deformations acceptable accuracy was not met (MDC > 3 mm). The morphing capability of the tested algorithms equals those of commercial systems when the user interaction is avoided. Underperformances were detected only in cases where a specific registration strategy is mandatory to obtain a satisfying match.ConclusionsAll of the considered algorithms show performances not inferior to previously published data and have the potential to be good candidates for use in the clinical routine. The results and conclusions only apply to the considered phantoms and should not be considered to be generally applicable and extendable to patient cases.     

Development of an x-ray-opaque-marker system for quantitative phantom positioning in patient-specific quality assurance

PurposeWe developed an x-ray-opaque-marker (XOM) system with inserted fiducial markers for patient-specific quality assurance (QA) in CyberKnife (Accuray) and a general-purpose linear accelerator (linac). The XOM system can be easily inserted or removed from the existing patient-specific QA phantom. Our study aimed to assess the utility of the XOM system by evaluating the recognition accuracy of the phantom position error and estimating the dose perturbation around a marker.MethodsThe recognition accuracy of the phantom position error was evaluated by comparing the known error values of the phantom position with the values measured by matching the images with target locating system (TLS; Accuray) and on-board imager (OBI; Varian). The dose perturbation was evaluated for 6 and 10 MV single-photon beams through experimental measurements and Monte Carlo simulations.ResultsThe root mean squares (RMSs) of the residual position errors for the recognition accuracy evaluation in translations were 0.07 mm with TLS and 0.30 mm with OBI, and those in rotations were 0.13° with TLS and 0.15° with OBI. The dose perturbation was observed within 1.5 mm for 6 MV and 2.0 mm for 10 MV from the marker.ConclusionsSufficient recognition accuracy of the phantom position error was achieved using our system. It is unnecessary to consider the dose perturbation in actual patient-specific QA. We concluded that the XOM system can be utilized to ensure quantitative and accurate phantom positioning in patient-specific QA with CyberKnife and a general-purpose linac.     

Incorrect dosimetric leaf separation in IMRT and VMAT treatment planning: Clinical impact and correlation with pretreatment quality assurance

PurposeDynamic treatment planning algorithms use a dosimetric leaf separation (DLS) parameter to model the multi-leaf collimator (MLC) characteristics. Here, we quantify the dosimetric impact of an incorrect DLS parameter and investigate whether common pretreatment quality assurance (QA) methods can detect this effect.Methods16 treatment plans with intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) technique for multiple treatment sites were calculated with a correct and incorrect setting of the DLS, corresponding to a MLC gap difference of 0.5 mm. Pretreatment verification QA was performed with a bi-planar diode array phantom and the electronic portal imaging device (EPID). Measurements were compared to the correct and incorrect planned doses using gamma evaluation with both global (G) and local (L) normalization. Correlation, specificity and sensitivity between the dose volume histogram (DVH) points for the planning target volume (PTV) and the gamma passing rates were calculated.ResultsThe change in PTV and organs at risk DVH parameters were 0.4–4.1%. Good correlation (>0.83) between the PTV_mean dose deviation and measured gamma passing rates was observed. Optimal gamma settings with 3%L/3 mm (per beam and composite plan) and 3%G/2 mm (composite plan) for the diode array phantom and 2%G/2 mm (composite plan) for the EPID system were found. Global normalization and per beam ROC analysis of the diode array phantom showed an area under the curve <0.6.ConclusionsA DLS error can worsen pretreatment QA using gamma analysis with reasonable credibility for the composite plan. A low detectability was demonstrated for a 3%G/3 mm per beam gamma setting.     

Commissioning Monte Carlo algorithm for robotic radiosurgery using cylindrical 3D-array with variable density inserts

IntroductionTo commission the Monte Carlo (MC) algorithm based model of CyberKnife robotic stereotactic system (CK) and evaluate the feasibility of patient specific QA using the ArcCHECK cylindrical 3D-array (AC) with Multiplug inserts (MP).ResultsFour configurations were used for simple beam setup and two for patient QA, replacing water equivalent inserts by lung. For twelve collimators (5–60 mm) in simple setup, mean (SD) differences between MC and RayTracing algorithm (RT) of the number of points failing the 3%/1 mm gamma criteria were 1(1), 1(3), 1(2) and 1(2) for the four MP configurations. Tracking fiducials were placed within AC for patient QA. Single lung insert setup resulted in mean gamma-index 2%/2 mm of 90.5% (range [74.3–95.9]) and 82.3% ([66.8–94.5]) for MC and RT respectively, while 93.5% ([86.8–98.2]) and 86.2% ([68.7–95.4]) in presence of largest inhomogeneities, showing significant differences (p < 0.05).DiscussionAfter evaluating the potential effects, 1.12 g/cc PMMA and 0.09 g/cc lung material assignment showed the best results. Overall, MC-based model showed superior results compared to RT for simple and patient specific testing, using a 2%/2 mm criteria. Results are comparable with other reported commissionings for flattening filter free (FFF) delivery. Further improvement of MC calculation might be challenging as Multiplan has limited material library.ConclusionsThe AC with Multiplug allowed for comprehensive commissioning of CyberKnife MC algorithm and is useful for patient specific QA for stereotactic body radiation therapy. MC calculation accuracy might be limited due to Multiplan’s insufficient material library; still results are comparable with other reported commissioning measurements using FFF beams.     

An audit of a hospital-based Doppler ultrasound quality control protocol using a commercial string Doppler phantom

Results from a four-year audit of a Doppler quality assurance (QA) program using a commercially available Doppler string phantom are presented. The suitability of the phantom was firstly determined and modifications were made to improve the reliability and quality of the measurements. QA of Doppler ultrasound equipment is very important as data obtained from these systems is used in patient management. It was found that if the braided-silk filament of the Doppler phantom was exchanged with an O-ring rubber filament and the velocity range below 50 cm/s was avoided for Doppler quality control (QC) measurements, then the maximum velocity accuracy (MVA) error and intrinsic spectral broadening (ISB) results obtained using this device had a repeatability of 18 ± 3.3% and 19 ± 3.5%, respectively. A consistent overestimation of the MVA of between 12% and 56% was found for each of the tested ultrasound systems. Of more concern was the variation of the overestimation within each respective transducer category: MVA errors of the linear, curvilinear and phased array probes were in the range 12.3–20.8%, 32.3–53.8% and 27–40.7%, respectively. There is a dearth of QA data for Doppler ultrasound; it would be beneficial if a multicentre longitudinal study was carried out using the same Doppler ultrasound test object to evaluate sensitivity to deterioration in performance measurements.     

A novel method for dose distribution registration using fiducial marks made by a megavoltage beam in film dosimetry for intensity-modulated radiation therapy quality assurance

PurposePhotographic film is widely used for the dose distribution verification of intensity-modulated radiation therapy (IMRT). However, analysis for verification of the results is subjective. We present a novel method for marking the isocenter using irradiation from a megavoltage (MV) beam transmitted through slits in a multi-leaf collimator (MLC).MethodsWe evaluated the effect of the marking irradiation at 500 monitor units (MU) on the total transmission through the MLC using an ionization chamber and Radiochromic Film. Film dosimetry was performed for quality assurance (QA) of IMRT plans. Three methods of registration were used for each film: marking by irradiating with an MV beam through slits in the MLC (MLC-IC); marking with a fabricated phantom (Phantom-IC); and a subjective method based on isodose lines (Manual). Each method was subjected to local γ-analysis.ResultsThe effect of the marking irradiation on the total transmission was 0.16%, as measured by a ionization chamber at a 10-cm depth in a solid phantom, while the inter-leaf transmission was 0.3%, determined from the film. The mean pass rates for each registration method agreed within ±1% when the criteria used were a distance-to-agreement (DTA) of 3 mm and a dose difference (DD) of 3%. For DTA/DD criteria of 2 mm/3%, the pass rates in the sagittal plane were 96.09 ± 0.631% (MLC-IC), 96.27 ± 0.399% (Phantom-IC), and 95.62 ± 0.988% (Manual).ConclusionThe present method is a versatile and useful method of improving the objectivity of film dosimetry for IMRT QA.