Initial clinical experience with Epid-based in-vivo dosimetry for VMAT treatments of head-and-neck tumors

We evaluated an EPID-based in-vivo dosimetry algorithm (IVD) for complex VMAT treatments in clinical routine. 19 consecutive patients with head-and-neck tumors and treated with Elekta VMAT technique using Simultaneous Integrated Boost strategy were enrolled. In-vivo tests were evaluated by means of (i) ratio R between daily in-vivo isocenter dose and planned dose and (ii) γ-analysis between EPID integral portal images in terms of percentage of points with γ-value smaller than one (γ_%) and mean γ-values (γ_mean), using a global 3%–3 mm criteria. Alert criteria of ±5% for R ratio, γ_% < 90% and γ_mean > 0.67 were chosen. A total of 350 transit EPID images were acquired during the treatment fractions. The overall mean R ratio was equal to 1.002 ± 0.019 (1 SD), with 95.9% of tests within ±5%. The 2D portal images of γ-analysis showed an overall γ_mean of 0.42 ± 0.16 with 93.3% of tests within alert criteria, and a mean γ% equal to 92.9 ± 5.1% with 85.9% of tests within alert criteria. Relevant discrepancies were observed in three patients: a set-up error was detected for one patient and two patients showed major anatomical variations (weight loss/tumor shrinkage) in the second half of treatment. The results are supplied in quasi real-time, with IVD tests displayed after only 1 minute from the end of arc delivery. This procedure was able to detect when delivery was inconsistent with the original plans, allowing physics and medical staff to promptly act in case of major deviations between measured and planned dose.     

EPID in vivo dosimetry in RapidArc technique

Aim

The aim of the study was to estimate the dose at the reference point applying an aSi-EPID device in the course of patient treatment.

Materials and methods

The method assumes direct proportionality between EPID signal and dose delivered to the patient reference point during the treatment session. The procedure consists of treatment plan calculation for the actual patient in the arc technique. The plan was realized with an elliptic water-equivalent phantom. An ionization chamber inside the phantom measured the dose delivered to the reference point. Simultaneously, the EPID matrix measured the CU distribution. EPID signal was also registered during patient irradiation with the same treatment plan. The formula for in vivo dose calculation was based on the CU(g) function, EPID signal registered during therapy and the relation between the dose and EPID signal level measured for the phantom. In vivo dose was compared with dose planned with the treatment planning system.Irradiation was performed with a Clinac accelerator by Varian Medical Systems in the RapidArc technique. The Clinac was equipped with an EPID matrix (electronic portal image device) of aSi-1000. Treatment plans were calculated with the Eclipse/Helios system. The phantom was a Scanditronix/Wellhöfer Slab phantom, and the ionization chamber was a 0.6 ccm PTW chamber.

Results

In vivo dose calculations were performed for five patients. Planned dose at the reference point was 2 Gy for each treatment plan. Mean in vivo dose was in the range of 1.96–2.09.

Conclusions

Our method was shown to be appropriate for in vivo dose evaluation in the RapidArc technique.     

QA of dynamic MLC based on EPID portal dosimetry

PurposeDynamic 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 materialsA 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 discussionThe EPID system exhibits good long term reproducibility. The mean portal dose at the centre of a 10 × 10 cm² 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.ConclusionsThis 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.     

An assessment of a 3D EPID-based dosimetry system using conventional two- and three-dimensional detectors for VMAT

PurposeTo provide a 3D dosimetric evaluation of a commercial portal dosimetry system using 2D/3D detectors under ideal conditions using VMAT.MethodsA 2D ion chamber array, radiochromic film and gel dosimeter were utilised to provide a dosimetric evaluation of transit phantom and pre-treatment ‘fluence’ EPID back-projected dose distributions for a standard VMAT plan. In-house 2D and 3D gamma methods compared pass statistics relative to each dosimeter and TPS dose distributions.ResultsFluence mode and transit EPID dose distributions back-projected onto phantom geometry produced 2D gamma pass rates in excess of 97% relative to other tested detectors and exported TPS dose planes when a 3%, 3 mm global gamma criterion was applied. Use of a gel dosimeter within a glass vial allowed comparison of measured 3D dose distributions versus EPID 3D dose and TPS calculated distributions. 3D gamma comparisons between modalities at 3%, 3 mm gave pass rates in excess of 92%. Use of fluence mode was indicative of transit results under ideal conditions with slightly reduced dose definition.Conclusions3D EPID back projected dose distributions were validated against detectors in both 2D and 3D. Cross validation of transit dose delivered to a patient is limited due to reasons of practicality and the tests presented are recommended as a guideline for 3D EPID dosimetry commissioning; allowing direct comparison between detector, TPS, fluence and transit modes. The results indicate achievable gamma scores for a complex VMAT plan in a homogenous phantom geometry and contributes to growing experience of 3D EPID dosimetry.     

2D EPID dose calibration for pretreatment quality control of conformal and IMRT fields: A simple and fast convolution approach

PurposeThis work presents an original algorithm that converts the signal of an electronic portal imaging device (EPID) into absorbed dose in water at the depth of maximum.MethodsThe model includes a first image pre-processing step that accounts for the non-uniformity of the detector response but also for the perturbation of the signal due to backscatter radiation. Secondly, the image is converted into absorbed dose to water through a linear conversion function associated with a dose redistribution kernel. These two computation parameters were modelled by correlating the on-axis EPID signal with absorbed dose measurements obtained on square fields by using an ionization chamber placed in water at the depth of maximum dose. The accuracy of the algorithm was assessed by comparing the dose determined from the EPID signal with the dose derived by the treatment planning system (TPS) using the ϒ-index. These comparisons were performed on 8 conformal radiotherapy treatment fields (3DCRT) and 18 modulated fields (IMRT).ResultsFor a dose difference and a distance-to-agreement set to 3% of the maximum dose and 2 mm respectively, the mean percentage of points with a ϒ-value less than or equal to 1 was 99.8% ± 0.1% for 3DCRT fields and 96.8% ± 2.7% for IMRT fields. Moreover, the mean gamma values were always less than 0.5 whatever the treatment technique.ConclusionThese results confirm that our algorithm is an accurate and suitable tool for clinical use in a context of IMRT quality assurance programmes.     

Set-up error validation with EPID images: Measurements vs Egs_cbct simulation

AimIn this study, the egs_cbct code’s ability to replicate an electronic portal imaging device (EPID) is explored.BackgroundWe have investigated head and neck (H&N) setup verification on an Elekta Precise linear accelerator. It is equipped with an electronic portal imaging device (EPID) that can capture a set of projection images over different gantry angles.Methods and materialsCone-beam computed tomography (CBCT) images were reconstructed from projection images of two different setup scenarios. Projections of an Anthropomorphic Rando head phantom were also simulated by using the egs_cbct Monte Carlo code for comparison with the measured projections.Afterwards, CBCT images were reconstructed from this data. Image quality was evaluated against a metric defined as the image acquisition interval (IAI). It determines the number of projection images to be used for CBCT image reconstruction.ResultsFrom this results it was established that phantom shifts could be determined within 2 mm and rotations within one degree accuracy using only 20 projection images (IAI = 10 degrees). Similar results were obtained with the simulated data.ConclusionIn this study it is demonstrated that a head and neck setup can be verified using substantially fewer projection images. Bony landmarks and air cavities could still be observed in the reconstructed Rando head phantom. The egs_cbct code can be used as a tool to investigate setup errors without tedious measurements with an EPID system.     

Radiomics analysis of EPID measurements for patient positioning error detection in thyroid associated ophthalmopathy radiotherapy

PurposeElectronic portal imaging detector (EPID)-based patient positioning verification is an important component of safe radiotherapy treatment delivery. In computer simulation studies, learning-based approaches have proven to be superior to conventional gamma analysis in the detection of positioning errors. To approximate a clinical scenario, the detectability of positioning errors via EPID measurements was assessed using radiomics analysis for patients with thyroid-associated ophthalmopathy.MethodsTreatment plans of 40 patients with thyroid-associated ophthalmopathy were delivered to a solid anthropomorphic head phantom. To simulate positioning errors, combinations of 0-, 2-, and 4-mm translation errors in the left–right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were introduced to the phantom. The positioning errors-induced dose differences between measured portal dose images were used to predict the magnitude and direction of positioning errors. The detectability of positioning errors was assessed via radiomics analysis of the dose differences. Three classification models—support vector machine (SVM), k-nearest neighbors (KNN), and XGBoost—were used for the detection of positioning errors (positioning errors larger or smaller than 3 mm in an arbitrary direction) and direction classification (positioning errors larger or smaller than 3 mm in a specific direction). The receiver operating characteristic curve and the area under the ROC curve (AUC) were used to evaluate the performance of classification models.ResultsFor the detection of positioning errors, the AUC values of SVM, KNN, and XGBoost models were all above 0.90. For LR, SI, and AP direction classification, the highest AUC values were 0.76, 0.91, and 0.80, respectively.ConclusionsCombined radiomics and machine learning approaches are capable of detecting the magnitude and direction of positioning errors from EPID measurements. This study is a further step toward machine learning-based positioning error detection during treatment delivery with EPID measurements.     

In-vivo EPID dosimetry for IMRT and VMAT based on through-air predicted portal dose algorithm

We have adapted the methodology of Berry et al. (2012) for Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) treatments at a fixed source to imager distance (SID) based on the manufacturer’s through-air portal dose image prediction algorithm. In order to fix the SID a correction factor was introduced to account for the change in air gap between patient and imager. Commissioning data, collected with multiple field sizes, solid water thicknesses and air gaps, were acquired at 150 cm SID on the Varian aS1200 EPID. The method was verified using six IMRT and seven VMAT plans on up to three different phantoms. The method’s sensitivity and accuracy were investigated by introducing errors. A global 3%/3 mm gamma was used to assess the differences between the predicted and measured portal dose images. The effect of a varying air gap on EPID signal was found to be significant – varying by up to 30% with field size, phantom thickness, and air gap. All IMRT plans passed the 3%/3 mm gamma criteria by more than 95% on the three phantoms. 23 of 24 arcs from the VMAT plans passed the 3%/3 mm gamma criteria by more than 95%. This method was found to be sensitive to a range of potential errors. The presented approach provides fast and accurate in-vivo EPID dosimetry for IMRT and VMAT treatments and can potentially replace many pre-treatment verifications.     

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

Aim

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

Detection of setup uncertainties with 3D surface registration system for conformal radiotherapy of breast cancer

Aim

To investigate the clinical application of a technique for patient set-up verification in breast cancer radiotherapy based on a 3D surface image registration system.

Background

Accurate and reproducible patient set-up is a prerequisite to correctly deliver fractionated radiotherapy. Various approaches are available to verify and correct patient setup for 3D image acquisition in a radiation treatment room.

Materials and methods

The study analyzed the setup reproducibility of 15 patients affected by breast cancer and candidates for conformal radiotherapy by using the AlignRT system (VisionRT, London, UK). At the initial setup, electronic portal imaging device (EPID) images were compared with Digitally Reconstructed Radiographs (DRRs) and a reference three-dimensional (3D) surface image was obtained by AlignRT. Surface images were acquired prior to every subsequent setup procedure. The systematic and random errors along longitudinal and vertical directions were measured and compared for the two systems.

Results

The procedure for surface registration, image acquisition and comparison with the reference image took less than 1 min on average. The T test for systematic error showed no significant difference between the 2 verification systems along the longitudinal (p = 0.69) and vertical (p = 0.67) axes. The T-test for random error showed a significant difference between the 2 systems along the vertical axis (p = 0.05).

Conclusion

AlignRT is fast, simple, non-invasive and seems to be reliable in detecting patient setup errors. Our results suggest that it could be used to assess the setup reproducibility for breast cancer patients.     

EPID-based in vivo dosimetry for stereotactic body radiotherapy of non-small cell lung tumors: Initial clinical experience

PurposeEPID-based in vivo dosimetry (IVD) has been implemented for stereotactic body radiotherapy treatments of non-small cell lung cancer to check both isocenter dose and the treatment reproducibility comparing EPID portal images.Methods15 patients with lung tumors of small dimensions and treated with volumetric modulated arc therapy were enrolled for this initial experience. IVD tests supplied ratios R between in vivo reconstructed and planned isocenter doses. Moreover a γ-like analysis between daily EPID portal images and a reference one, in terms of percentage of points with γ-value smaller than 1, P_γ<1, and mean γ-values, γ_mean, using a local 3%–3 mm criteria, was adopted to check the treatment reproducibility. Tolerance levels of 5% for R ratio, P_γ<1 higher than 90% and γ_mean lower than 0.67 were adopted.ResultsA total of 160 EPID images, two images for each therapy session, were acquired during the treatment of the 15 patients. The overall mean of the R ratios was equal to 1.005 ± 0.014 (1 SD), with 96.9% of tests within ± 5%. The 2 D image γ-like analysis showed an overall γ_mean of 0.39 ± 0.12 with 96.1% of tests within the tolerance level, and an average P_γ<1 value equal to 96.4 ± 3.6% with 95.4% of tests with P_γ<1 > 90%. Paradigmatic discrepancies were observed in three patients: a set-up error and a patient morphological change were identified thanks to CBCT image analysis whereas the third discrepancy was not fully justified.ConclusionsThis procedure can provide improved patient safety as well as a first step to integrate IVD and CBCT dose recalculation.     

In silico investigation of factors affecting the MV imaging performance of a novel water-equivalent EPID

PurposeA Geant4 model of a novel, water-equivalent electronic portal imaging device (EPID) prototype for radiotherapy imaging and dosimetry utilising an array of plastic scintillating fibres (PSFs) has been developed. Monte Carlo (MC) simulations were performed to quantify the PSF-EPID imaging performance and to investigate design aspects affecting performance for optimisation.MethodsUsing the Geant4 model, the PSF-EPID’s imaging performance for 6 MV photon beams was quantified in terms of its modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE). Model parameters, including fibre dimensions, optical cladding reflectivity and scintillation yield, were varied to investigate impact on imaging performance.ResultsThe MC-calculated DQE(0) for the reference PSF-EPID geometry employing 30 mm fibres was approximately nine times greater than values reported for commercial EPIDs. When using 10 mm long fibres, the PSF-EPID DQE(0) was still approximately three times greater than that of a commercial EPID. Increased fibre length, cladding reflectivity and scintillation yield produced the greatest decreases in NPS and increases in DQE.ConclusionsThe potential to develop an optimised next-generation water-equivalent EPID with MV imaging performance at least comparable to commercial EPIDs has been demonstrated. Factors most important for optimising prototype design include fibre length, cladding reflectivity and scintillation yield.     

Comparison of two different EPID-based solutions performing pretreatment quality assurance: 2D portal dosimetry versus 3D forward projection method

PurposeThe aim of this paper is to characterize two different EPID-based solutions for pre-treatment VMAT quality assurance, the 2D portal dosimetry and the 3D projection technique. Their ability to catch the main critical delivery errors was studied.MethodsMeasurements were performed with a linac accelerator equipped with EPID aSi1000, Portal Dose Image Prediction (PDIP), and PerFRACTION softwares. Their performances were studied simulating perturbations of a reference plan through systematic variations in dose values and micromultileaf collimator position. The performance of PDIP, based on 2D forward method, was evaluated calculating gamma passing rate (%GP) between no-error and error-simulated measurements. The impact of errors with PerFRACTION, based on 3D projection technique, was analyzed by calculating the difference between reference and perturbed DVH (%ΔD). Subsequently pre-treatment verification with PerFRACTION was done for 27 patients of different pathologies.ResultsThe sensitivity of PerFRACTION was slightly higher than sensitivity of PDIP, reaching a maximum of 0.9. Specificity was 1 for PerFRACTION and 0.6 for PDIP. The analysis of patients’ DVHs indicated that the mean %ΔD was (1.2 ± 1.9)% for D2%, (0.6 ± 1.7)% for D95% and (−0.0 ± 1.2)% for Dmean of PTV. Regarding OARs, we observed important discrepancies on DVH but that the higher dose variations were in low dose area (<10 Gy).ConclusionsThis study supports the introduction of the new 3D forward projection method for pretreatment QA raising the claim that the visualization of the delivered dose distribution on patient anatomy has major advantages over traditional portal dosimetry QA systems.     

The isolated perfused rat liver: standardization of a time-honoured model

For many years, the isolated perfused rat liver (IPRL) model has been used to investigate the physiology and pathophysiology of the rat liver. This in vitro model provides the opportunity to assess cellular injury and liver function in an isolated setting. This review offers an update of recent developments regarding the IPRL set-up as well as the viability parameters that are used, with regards to liver preservation and ischaemia and reperfusion mechanisms.A review of the literature was performed into studies regarding liver preservation or liver ischaemia and reperfusion. An overview of the literature is given with particular emphasis on perfusate type and volume, reperfusion pressure, flow, temperature, duration of perfusion, oxygenation and on applicable viability parameters (liver damage and function).The choice of IPRL set-up depends on the question examined and on the parameters of interest. A standard technique is cannulation of the portal vein, bile duct and caval vein with pressure-controlled perfusion at 20 cm H2O (15 mmHg) to reach a perfusion flow of approximately 3 mL/min/g liver weight. The preferred perfusion solution is Krebs-Henseleit buffer, without albumin. The usual volume is 150-300 cm3, oxygenated to a pO2 of more than 500 mmHg. The temperature of the perfusate is maintained at 37 degrees C. Standardized markers should be used to allow comparison with other experiments.     

Role of “the frame cycle time” in portal dose imaging using an aS500-II EPID

IntroductionThis paper evaluates the role of an acquisition parameter, the frame cycle time “FCT”, in the performance of an aS500-II EPID.Materials and methodsThe work presented rests on the study of the Varian EPID aS500-II and the image acquisition system 3 (IAS3). We are interested in integrated acquisition using asynchronous mode. For better understanding the image acquisition operation, we investigated the influence of the “frame cycle time” on the speed of acquisition, the pixel value of the averaged gray-scale frame and the noise, using 6 and 15 MV X-ray beams and dose rates of 1–6 Gy/min on 2100 C/D Linacs.ResultsIn the integrated mode not synchronized to beam pulses, only one parameter the frame cycle time “FCT” influences the pixel value. The pixel value of the averaged gray-scale frame is proportional to this parameter. When the FCT <55 ms (speed of acquisition V_f/s > 18 frames/s), the speed of acquisition becomes unstable and leads to a fluctuation of the portal dose response. A timing instability and saturation are detected when the dose per frame exceeds 1.53 MU/frame. Rules were deduced to avoid saturation and to optimize this dosimetric mode.ConclusionThe choice of the acquisition parameter is essential for the accurate portal dose imaging.     

Exploration of temporal stability and prognostic power of radiomic features based on electronic portal imaging device images

PurposeWe aimed to explore the temporal stability of radiomic features in the presence of tumor motion and the prognostic powers of temporally stable features.MethodsWe selected single fraction dynamic electronic portal imaging device (EPID) (n = 275 frames) and static digitally reconstructed radiographs (DRRs) of 11 lung cancer patients, who received stereotactic body radiation therapy (SBRT) under free breathing. Forty-seven statistical radiomic features, which consisted of 14 histogram-based features and 33 texture features derived from the graylevel co-occurrence and graylevel run-length matrices, were computed. The temporal stability was assessed by using a multiplication of the intra-class correlation coefficients (ICCs) between features derived from the EPID and DRR images at three quantization levels. The prognostic powers of the features were investigated using a different database of lung cancer patients (n = 221) based on a Kaplan-Meier survival analysis.ResultsFifteen radiomic features were found to be temporally stable for various quantization levels. Among these features, seven features have shown potentials for prognostic prediction in lung cancer patients.ConclusionsThis study suggests a novel approach to select temporally stable radiomic features, which could hold prognostic powers in lung cancer patients.     

Effectiveness of the No action level protocol for head & neck patients – Time considerations

BackgroundThe No Action Protocol (NAL) was used to diminish the systematic set-up error. Recently, owing to the development of image registration technologies, the on-line positioning control is more often used. This method significantly reduces the CTV–PTV margin at the expense of the lengthening of a treatment session. The efficiency of NAL in decreasing the total treatment time for Head&Neck patients was investigated.MethodsResults of set-up control of 30 patients were analyzed. The set-up control was carried out on-line. For each patient and each fraction, the set-error and the time needed for making the set-up control procedure were measured. Next, retrospectively, the NAL was applied to this data. The number of initial errors (without interventions) and after NAL protocol were compared in terms of errors larger than 3 and 4 mm. The average and total time used for portal control was calculated and compared.ResultsThe number of setup errors in the posterior-anterior, inferior-superior, and right–left directions ≥3 mm and ≥4 mm were 98, 79, and 91 sessions and 44, 38 and 30 sessions out of 884 sessions. After NAL protocol the number of errors ≥3 mm and ≥4 mm decreased to 84, 57, and 39 sessions and 31, 15 and 10 sessions, respectively. The average time needed for one set-up control was 5.1 min. NAL protocol allows saving 4049 min for the whole group.ConclusionsFor locations where the random set-up errors are small, the NAL enables a very precise treatment of patients. Implementation of this protocol significantly decreases the total treatment time.     

A new test set-up for skull fracture characterisation

Skull fracture is a frequently observed type of severe head injury. Historically, a variety of impact test set-ups and techniques have been used for investigating skull fracture. The most frequently used are the free-fall technique, the guided fall or drop tower set-up and the piston-driven impactor set-up. This document proposes a new type of set-up for cadaver head impact testing which combines the strengths of the most frequently used techniques and devices. The set-up consists of two pendulums, which allow for a 1 degree of freedom rotational motion. The first pendulum is the impactor and is used to strike the blow. The head is attached to the second pendulum using a polyester resin. Local skull deformation and impact force are measured with a sample frequency of 65 kHz. From these data, absorbed energy until skull fracture is calculated. A set-up evaluation consisting of 14 frontal skull and head impact tests shows an accurate measurement of both force and local skull deformation until fracture of the skull. Simplified mechanical models are used to analyse the different impacting techniques from literature as well as the new proposed set-up. It is concluded that the proposed test set-up is able to accurately calculate the energy absorbed by the skull until fracture with an uncertainty interval of 10%. Second, it is concluded that skull fracture caused by blunt impact occurs before any significant motion of the head. The two-pendulum set-up is the first head impact device to allow a well-controlled measurement environment without altering the skull stress distribution.     

Development of sludge filterability test to assess the solids removal potential of a sludge bed

A qualitative sludge characterisation technique called "sludge filterability technique" has been developed. This technique enables the determination of the sludge potential for the physical removal of solids, weighing the effect of different process parameters on solids removal and identifying the mechanisms of solids removal in an upflow anaerobic sludge bed system. In this paper guidelines for conducting the test are given and a "standardised" set-up is presented. The experimental set-up and protocol are simple and the results can be obtained in a period as short as a few hours. A sludge sample is added to an upflow reactor incubated at 4 degrees C, to limit gas production, washed with an anaerobically pre-treated and suspended solids free wastewater to remove solids washed out from the sludge, and then fed with a model substrate, prepared from fish meal with a standard procedure. Several experimental runs were conducted to validate and optimise the technique. The results showed that the technique is reliable, workable and reproducible.