Mise en place de la Maîtrise Statistique des Procédés pour les contrôles pré-traitement par imagerie portale pour la technique VMAT

National audienc

Design of experiments in medical physics: Application to the AAA beam model validation

Purpose The purpose of this study is to evaluate the usefulness of the design of experiments in the analysis of multiparametric problems related to the quality assurance in radiotherapy. The main motivation is to use this statistical method to optimize the quality assurance processes in the validation of beam models. Method Considering the Varian Eclipse system, eight parameters with several levels were selected: energy, MLC, depth, X, Y1 and Y2 jaw dimensions, wedge and wedge jaw. A Taguchi table was used to define 72 validation tests. Measurements were conducted in water using a CC04 on a TrueBeam STx, a TrueBeam Tx, a Trilogy and a 2300IX accelerator matched by the vendor. Dose was computed using the AAA algorithm. The same raw data was used for all accelerators during the beam modelling. Results The mean difference between computed and measured doses was 0.1 ± 0.5% for all beams and all accelerators with a maximum difference of 2.4% (under the 3% tolerance level). For all beams, the measured doses were within 0.6% for all accelerators. The energy was found to be an influencing parameter but the deviations observed were smaller than 1% and not considered clinically significant. Conclusion Designs of experiment can help define the optimal measurement set to validate a beam model. The proposed method can be used to identify the prognostic factors of dose accuracy. The beam models were validated for the 4 accelerators which were found dosimetrically equivalent even though the accelerator characteristics differ

A new robust statistical method for treatment planning systems validation using experimental designs

Introduction Dose computation verification is an important part of acceptance testing. The IAEA Tecdoc 1540 and 1583 suggest comparing computed dose to measurements for several beam configurations. However, this process is time-consuming and results out of tolerance are often left unexplained. Purpose To validate a treatment planning system using experimental designs which allow evaluating several parameters in a few tests selected by a robust statistical method. Materials and methods The Taguchi table L36 (211 × 312) was used to determine the 72 beams needed to test the 7 parameters chosen: energy, MLC, depth, jaw field size in X, Y1 and Y2 directions and wedge. Measurements were conducted in water using a CC04 (IBA) on a TrueBeam STx, a TrueBeam Tx, a Trilogy and a C-serie clinac (Varian). Dose was computed using the AAA algorithm (Eclipse, version 11). The same raw data was used for all accelerators during the algorithm configuration. Results The mean difference between computed and measured doses was 0.1 ±± 0.5% for all tested beams and all linacs with a maximum difference of 2.4% (under the 3% tolerance level). For all beams, the measured doses were within 0.6% for all linacs. No studied parameter led to statistically significant deviation between computed and measured doses. Conclusion Experimental design is a robust statistical method to validate an algorithm. Only 2 h of measurements were needed to evaluate 7 parameters. Furthermore, the commissioned accelerators were found dosimetrically equivalent even though the linac characteristics differ

Détermination de la limite de contrôle du taux Gamma par inférence statistique bayésienne. Application aux contrôles pré-traitement

Statistical Process Control (SPC) and control charts have been successfully implemented in radiotherapy in order to detect special causes of variation in processes and thus to increase their quality. Furthermore, the Gamma Index Pass Rate (GIPR), characterized by a Beta distribution, is a widely used parameter for analyzing measurements in medical physics. As a special attention must be given to the computation of the GIPR control limit (CL), we proposed two control charts in order to detect special causes. Measurements were realised on a TrueBeam v2.5 (Varian) equipped with an aS1200 EPID5. Statistical computations were performed with R (v.3.5.1) software. Pre-treatment quality controls compared the dose distribution acquired with the EPID with the calculated dose distribution using the GIPR metric generated by Portal Dosimetry v13.7. The following settings were used: (a) local 3 % / 3 mm (th. 30 %), (b) ROI: MLC + 5 mm. CLs calculations are illustrated in details. The first control chart had an adaptive CL based on Bayesian estimation. It was constructed without prior information on the distribution of the GIPR. The CLs were computed starting from the first arc and continuously adapted according to the recursive Bayesian principle. The second control chart had a fixed control limit computed as the quantile of the Beta distribution. A simple SPC methodology for monitoring GIPR was precisely described and CLs were calculated from the first arc. As a first step, Bayesian inference was used until a sufficient amount of data is collected and, in a second step, a fixed control limit was calculated. The strong points of this method lie in using the true GIPR’s distribution that minimize both: false positive and no detection of special causes. This methodology should be very useful for those who wish to set an individual control chart suited to the viability constraints of their radiotherapy department (prescription, dosimetry, equipment’s)

Charge sharing between pixels in the spectral Medipix2 x-ray detector

This paper gives an overview of the Medipix2 x-ray detector and its use in medical imaging, with the MARS-CT scanner (MARS, Medipix All Resolution System) as an example. The Medipix2 chip is a photon counting pixel detector with the ability of energy discrimination. It was developed at CERN and is composed of a sensor layer bump bonded to electronics layer. It has 256x256 pixels, each one covering an area of 55x55µm². Furthermore, every pixel can be read out separately. The MARS-CT scanner uses these properties to scan biological objects obtaining multi-energy (spectral) x-ray images with high contrast between materials and high spatial resolution. Charge sharing is the phenomenon by which the electron-hole charge cloud, induced in the sensor layer by an absorbed photon, is detected by a cluster of neighbouring pixels. Each pixel in the cluster generates a signal corresponding to its fraction of the cloud, so the detector will record several photons each of lower energies. This effect has to be considered with the use of Medipix2, because of its small pixels and the hybrid architecture. The effect was measured and a simulation modelled with the aim to reconstruct the spectrum removing the distortion of the detection process

Optimizing the effective spot size and the dosimetric leaf gap of the AcurosXB algorithm for VMAT treatment planning

PURPOSE: The aim of this study is to provide and test a new methodology to adjust the AcurosXB beam model for VMAT treatment plans. METHOD: The effective target spot size of the AcurosXB v15 algorithm was adjusted in order to minimize the difference between calculated and measured penumbras. The dosimetric leaf gap (DLG) was adjusted using the asynchronous oscillating sweeping gap tests defined in the literature and the MLC transmission was measured. The impact of the four parameters on the small field output factors was assessed using a design of experiment methodology. Patient quality controls were performed for the three beam models investigated including two energies and two MLC models. RESULTS: Effective target spot sizes differed from the manufacturer recommendations and strongly depended on the MLC model considered. DLG values ranged from 0.7 to 2.3 mm and were found to be larger than the ones based on the sweeping gap tests. All parameters were found to significantly influence the calculated output factors, especially for the 0.5 cm × 0.5 cm field size. Interactions were also identified for fields smaller than 2 cm × 2 cm, suggesting that adjusting the parameters on the small field output factors should be done with caution. All patient quality controls passed the universal action limit of 90%. CONCLUSION: The methodology provided is simple to implement in clinical practice. It was validated for three beam models covering a large variety of treatment types and localizations

Comparison of five dose calculation algorithms in a heterogeneous media using design of experiment

Purpose Design of experiments (DoE) provides a methodology to reveal the influence of input values on the measured output with a limited number of trials. The purpose of this study was to describe how DoE can be used to evaluate the performances of several dose calculation systems in heterogeneous media, including algorithms like Pencil Beam (PB), Anisotropic Analytical Algorithm (AAA), Acuros XB (AXB), Monte Carlo (MC) and Collapsed Cone Volume (CCV). Method This study was carried out using a CIRS Model 002LFC IMRT Thorax Phantom customized with a water-equivalent heterogeneity inside the lung. The calculated dose distributions were compared to Gafchromic® EBT3 film measurements. The beam configurations were selected using DoE to study the influence of five parameters simultaneously (energy, collimator angulation, gantry angulation, X and Y jaws) and to optimize the number of experiments. An analysis of variance was performed over the entire irradiation field and over various regions of interest (tumour, shadow of tumour and lungs). Results DoE enabled to quantify and determine the statistically significant factors, leading to an evaluation of the dose calculation systems in the lung case. The resulting scoring could be as follow (from best to worst): AXB_Dm, CCV, AXB_Dw, XVMC_Dm, XVMC_Dw, AAA and last PB. Differences between the algorithms were specially observed in the tumour and the shadow regions. Conclusion DoE is a robust statistical method to compare several dose calculation systems. The various analyses lead to the conclusion that AXB handled more accurately most of the situations investigated in heterogeneous media

Accuracy of a dose-area product compared to an absorbed dose to water at a point in a 2 cm diameter field

International audiencePurpose: Graphite calorimeters with a core diameter larger than the beam can be used to establish dosimetric references in small fields. The dose-area product (DAP) measured can theoretically be linked to an absorbed dose at a point by the determination of a profile correction. This study aims at comparing the DAP-based protocol to the usual absorbed dose at a point protocol in a 2 cm diameter field for which both references exist. Methods: Two calorimeters were used, respectively, with a sensitive volume of 0.6 cm (for the absorbed dose at a point measurement) and 3 cm diameter (for the DAP measurement). Profile correction was calculated from a 2D dose mapping using three detectors: a PinPoint chamber, a synthetic diamond, and EBT3 films. A specific protocol to read EBT3 films was implemented and the dose-rate and energy dependences were studied to assure a precise measurement, especially in the penumbra and out-of-field regions. Results: EBT3 films were found independent on dose rates over the range studied but showed a strong under-response (18%) at low energies. Depending on the dosimeter used for calculating the profile correction, a deviation of 0.8% (PinPoint chamber), 0.9% (diamond), or 1.9% (EBT3 films) was observed between the calibration coefficient derived from DAP measurements and the one directly established in terms of absorbed dose to water at a point. Conclusions: The DAP method can currently be linked to the classical dosimetric reference system based in an absorbed dose at a point only with a confidence interval of 95% (k = 2). None of the detectors studied can be used to determine an absorbed dose to water at a point from a DAP measurement with an uncertainty smaller than 1.2%. (C) 2016 American Association of Physicists Medicine

32 Comparison of the dose calculation algorithms in a heterogeneous media using experimental design

International audienceIntroductionThis study compares the performances of different dose calculation algorithms in heterogeneous medium. Then, the properties of two algorithms on clinical cases are considered for the thoracic cases in conformal 3D radiotherapy (RC3D).Methods(1) A study on the CIRS phantom was performed by comparing the dose distributions measured with Gafchromic EBT3® with those calculated by the AAA [Analytical Anisotropic Algorithm v13, Varian], AXB [Acuros XB v13.7, Varian], PB [Pencil Beam v4.5.5, Brainlab], MC [Monte Carlo v4.5.5, Brainlab] and CCV [Collapsed Cone Volume v2.0.1, Mobius Medical System]. The performances of these different algorithms were tested using an experimental design of Taguchi Math Eq to demonstrate the influence of the following factors, in 18 tests: energy, collimator angle, gantry angle, X and Y field size.A comparison by subtracting the dose distributions between those measured and computed was achieved for the 18 tests. (2) The impact of the selection of the algorithm was performed on treatment plans initially calculated with AAA, on a set of 15 patients with thoracic cancer in RC3D. By keeping the same UM number, these treatment plans were compared in terms of coverage of the 95% PTV, a conformity index, doses to the PTV and organs at risk. If necessary, the planimetry was modified in AXB (weighting, filtering and normalization point) in order to validate the treatment plan clinically.Results(1) In heterogeneous medium, the average dose differences between the measured and calculated doses are, respectively, for the AXB, AAA, CCV, MC and PB: 0.01.8%, 1.22.2%, −0.12.0%, 1.21.9% and 7.04.1%. (2) For treatment plans with similar MU numbers, an average dose difference of −1.2% in the level of the 95% coverage of the PTV and a modification of conformity index from 1.10 (AAA) to 0.76 (AXB) are obtained, attesting to a lower coverage of the PTV with AXB, related to a difference in the evaluation of heterogeneities. Once these treatment plans were modified, the resulting planimetry was similar to the clinically validated one, with a mean difference of MU greater than 1.5% against treatment plans calculated with AAA.ConclusionsThe best performances are shown with the MC and the AXB algorithms. The experimental design allows reliable comparison of the algorithms with 18 tests. The PEX highlights bigger deviations for small field modelling. Regarding the dosimetry study, AXB can be used for RC3D thoracic cancer treatment. A complementary study for the same localization is ongoing for the VMAT treatment technique