Review of strategies for MRI based reconstruction of endocavitary and interstitial applicators in brachytherapy of cervical cancer

Brachytherapy plays an essential role in the curative intent management of locally advanced cervical cancer. The introduction of the magnetic resonance (MR) as a preferred image modality and the development of new type of applicators with interstitial components have further improved its benefits.The aim of this work is to review the current status of one important aspect in the cervix cancer brachytherapy procedure, namely catheter reconstruction.MR compatible intracavitary and interstitial applicators are described. Considerations about the use of MR imaging (MRI) regarding appropriate strategies for applicator reconstruction, technical requirements, MR sequences, patient preparation and applicator commissioning are included.It is recommendable to perform the reconstruction process in the same image study employed by the physician for contouring, that is, T2 weighted (T2W) sequences. Nevertheless, a clear identification of the source path inside the catheters and the applicators is a challenge when using exclusively T2W sequences. For the intracavitary component of the implant, sometimes the catheters may be filled with some substance that produces a high intensity signal on MRI. However, this strategy is not feasible for plastic tubes or titanium needles, which, moreover, induce magnetic susceptibility artifacts. In these situations, the use of applicator libraries available in the treatment planning system (TPS) is useful, since they not only include accurate geometrical models of the intracavitary applicators, but also recent developments have made possible the implementation of the interstitial component. Another strategy to improve the reconstruction process is based on the incorporation of MR markers, such as small pellets, to be used as anchor points.Many institutions employ computed tomography (CT) as a supporting image modality. The registration of CT and MR image sets should be carefully performed, and its uncertainty previously assessed. Besides, an important research work is being carried out regarding the use of ultrasound and electromagnetic tracking technologies

IMPROVING REALTIME 3-D TRACKING OF HIGH DOSE RATE RADIATION SOURCE USING A FLAT PANEL DETECTOR

Previous research 1 on this subject tracked the presumed exact path the HDR source would follow in real-time, during breast brachytherapy treatments in other to ensure accurate dose delivery and effectively confirm actual source position. As a continuation, this research has three objectives. Firstly, we will extract information from patient DICOM file which will be used to perform evaluations, then we will establish communication between our C program and the new Varex Paxscan flat panel detector (FPD). Finally, we will try to embed our C codes into a MATLAB graphical user interface (GUI) This research will attempt to improve the overall existing system in several ways including, code optimization and trying a sample simulation of the process in MATLAB guide app, to check the quality of the new design. Finally, all the algorithms will be integrated into the user-friendly GUI, such that its operation can be implemented easily. The FPD is used to obtain images resulting from the exit radiation of the HDR source, emerging from an organized matrix of markers. The images are processed using in-built functions in MATLAB to obtain projection coordinates, and marker coordinates. Each marker along with its projection constitutes a line in 3D. Using the mathematical solution for near intersection of two 3D lines, N-markers will produce N*(N-1)/2 points of intersection and their mean will produce a more precise source position. The changes in this position as well as the time interval between these changes will be used to confirm the accuracy of our treatment system using the standalone monitoring system built in this research. In the previous study the accuracy of source position detection using the FPD was found to be in sub-millimeter. This study which uses a new FPD with improved features is used to confirm that, but our focus here is improvement of the previous work, as stated earlier

Suivi électromagnétique en curiethérapie à haut débit de dose : performance et rôle de la technologie

La curiethérapie à haut débit de dose est un traitement anti-cancer utilisé pour différents sites tels les cancers gynécologiques, la prostate, le sein, la tête et le cou. La technique consiste à déposer de la dose de radiation près ou à l'intérieur de la tumeur. Différentes étapes composent ce traitement et des erreurs peuvent s'introduire dans chacune d'entres elles. Par le passé, plusieurs études ont utilisé la dosimétrie in vivo pour détecter et éliminer certaines erreurs de la chaîne. Cette pratique n'est pas uniformisée, puisqu'aucune solution commerciale n'existe sur le marché actuellement. En plus de la dosimétrie, des systèmes de suivi élecromagnétique ont aussi prouvé qu'ils pouvaient être utilisés pour la détection d'erreurs avant le traitement. Ce projet de doctorat explore des erreurs possibles dans la chaîne de traitement et propose le suivi électromagnétique comme étant une solution pour éviter celles-ci. Dans cette thèse, la fabrication d'un dosimètre et l'utilisation du suivi électromagnétique dans le cadre de la curiethérapie à haut débit de dose y sont traitées. Tout d'abord, une étude rétrospective a été complétée pour faire ressortir les performances requises d'un dosimètre in vivo utilisé pendant des traitements du cancer de la prostate. Les positions d'arrêts et les temps d'arrêts de tous les patients ont été extraits des fichiers de chaque traitement. Un dosimètre virtuel a été positionné dans un des cathéters de traitement pour chacun des patients. Une comparaison des temps d'arrêts et des positions d'arrêt sa été complétée pour l'identification des cathéters. Il a été démontré qu'une précision de 1 mm sur la distance source-dosimètre serait idéale. Pour la cohorte de patient utilisée, les temps d'arrêts sont de meilleurs discriminants que les positions d'arrêts. Une précision temporelle de 0,1 s serait idéale. Par la suite, une sélection du capteur électromagnétique pour la construction d'un dosimètre intégrant le suivi électromagnétique de sa position a été réalisée. La dépendance angulaire et la distance capteur-scintillateur ont aussi été étudiées. Parmi les capteurs disponibles, celui possédant le plus petit effet sur la réponse du scintillateur a été utilisé. La reconstruction d'un prototype d'un applicateur blindé pour les cancers gynécologiques a été faite à l'aide du suivi électromagnétique (EM). L'erreur moyenne du capteur sélectionné pour cette étude était de 0,17 mm lorsqu'il se trouvait à 250 mm du générateur de champ. Aucune différence significative sur la mesure n'a été observée à proximité du blindage de cet applicateur. Le suivi EM a aussi été testé lorsqu'il était intégré dans un câble de vérification d'un projecteur de source (Flexitron, Elekta Brachytherapy, Veenendaal, Pays-Bas). Les coordonnées de reconstructions ont été prises lors de la rétraction du câble de vérification. Une comparaison des reconstructions avec différentes vitesses de rétraction du câble a été faite. Les décalages de 5 mm ont tous été identifiés avec une vitesse de reconstruction de 10 cm/s. Néanmoins, il faut une vitesse maximale de 2,5 cm/s pour détecter les décalages de 1 mm. Deux dosimètres avec suivi EM ont été construits, soit un avec une fibre scintillante de plastique(BCF-60) et l'autre avec un scintillateur inorganique (ZnSe:O). Les dosimètres construits ont été calibrés. Les mesures de dose ont été faites en respectant les conditions de diffusion complète et ont été comparées avec le formalisme du TG-43. Par rapport à ce dernier, le dosimètre organique avait une différence de 1,7± 0,2 % alors que l'inorganique possédait une différence de 1,5± 0,7 %pour des distances source-dosimètre allant de 8 mm à 60 mm. Une étude de détection d'erreur a été accomplie. Un gain maximal de 24,0 % est observé pour les déplacements latéraux de 0,5 mm pour le dosimètre inorganique lorsque le suivi EM est utilisé, tandis qu'un gain maximal pour les déplacements longitudinaux (0,5 mm) de 17,4 % a été montré pour ce même scintillateur. Les différents résultats de ce projet quantifient les gains ainsi que les perpectives que l'ajout du suivi EM apportent à la curiethérapie HDR et justifient son introduction dans ce domaine.High-dose-rate (HDR) brachytherapy is a cancer treatment used for various sites such as gynecological, prostate, breast, head and neck cancers. The technique consists in delivering a dose of radiation by having one or multiple sources in close proximity or with in a tumor. This is a multi-step process and errors can happen at any step during its execution. Several studies have used in vivo dosimetry to detect and avoid possible errors. This practice is not standardized, as no commercial solution currently exists on the market. In addition to dosimetry, electromagnetic (EM) tracking systems have also proven to be useful for the detection of some pre-treatment errors. This thesis explores errors that can occur during the treatment process and suggests a solution based on electromagnetic tracking. The construction of a dosimeter and the use of an EM tracking will be studied in the context of high-dose-rate (HDR) brachytherapy. First, a retrospective study was completed to highlight the required performance of an in vivo dosimeter during prostate cancer treatments. Dwell positions and dwell times for all patients were extracted from each treatment file. A virtual dosimeter was positioned in one of the treatment catheters for each patient. A comparison of each dwell times and dwell positions was completed for the identification of catheters. It has been shown that an accuracy of 1 mm would be ideal on the source-dosimeter distance. For the studied patient cohort, dwell times are better discriminators than dwell positions. This study showed that it is important to avoid placing the dosimeter near the center of the implant. Then, a selection of components for the construction of the dosimeter was performed. Among the available sensors, the one with the smallest impact on the scintillator response was chosen for the work quantifying the gain of EM tracking. The angular dependence and the sensor-scintillator distance were also studied. The reconstruction of a shielded applicator prototype for gynecological cancer was made using EM tracking. An average error of the selected sensor for this study was 0.17 mm when it was 250 mm from the field generator. EM tracking was also tested when integrated into the check cable of an after loader (Flexitron, Elekta Brachytherapy, Veenendaal, Netherlands). The reconstruction coordinates were taken during retraction of the check cable. A comparison of the reconstructions for different cable speeds was made. From this study, the speed for a linear path reconstruction is recommended at 5 cm/s. The 5-mm shifts were all identified with a reconstruction speed of 10 cm/s. Nevertheless, a maximum speed of 2.5 cm/s was needed to detect 1-mm shifts. Two dosimeters were constructed, one with a plastic scintillating fiber (BCF-60) and one with an inorganic scintillator(ZnSe:O). All dose measurements were made in full scatter conditions and were compared with the TG-43 formalism. Compared to the latter, the organic dosimeter had a difference of 1.7± 0.2 % while the inorganic had a difference of 1.5± 0.7 % over an interval of 8 mm to 60 mm from the source. An error detection study was performed and a comparison was made to determine the gain provided by the EM tracking. A maximum gain of 24.0 % was observed with a lateral displacement of 0.5 mm for the inorganic dosimeter. For longitudinal displacements (0.5 mm), a maximum gain of 17.4 % was shown for this same scintillator. The different results obtained in this project will quantify the performance for the construction of an in vivo dosimeter for HDR brachytherapy. The gains from the addition of EM monitoring to HDR brachytherapy will justify its use in this field

A 3D US Guidance System for Permanent Breast Seed Implantation: Development and Validation

Permanent breast seed implantation (PBSI) is a promising breast radiotherapy technique that suffers from operator dependence. We propose and have developed an intraoperative 3D ultrasound (US) guidance system for PBSI. A tracking arm mounted to a 3D US scanner registers a needle template to the image. Images were validated for linear and volumetric accuracy, and image quality in a volunteer. The tracking arm was calibrated, and the 3D image registered to the scanner. Tracked and imaged needle positions were compared to assess accuracy and a patient-specific phantom procedure guided with the system. Median/mean linear and volumetric error was ±1.1% and ±4.1%, respectively, with clinically suitable volunteer scans. Mean tracking arm error was 0.43mm and 3D US target registration error ≤0.87mm. Mean needle tip/trajectory error was 2.46mm/1.55°. Modelled mean phantom procedure seed displacement was 2.50mm. To our knowledge, this is the first reported PBSI phantom procedure with intraoperative 3D image guidance

High dose rate brachytherapy treatment verification using a flat panel detector

High dose rate (HDR) brachytherapy treatments are usually delivered in large dose fractions and have the clinical advantage of highly conformal dose distributions due to the steep dose gradient produced by the 192Ir source. The routine use of 3D imaging for treatment planning enables clinical teams to finely optimise the dose distribution around the defined target while limiting dose to the surrounding organs at risk. A significant challenge in brachytherapy is to ensure the dose is delivered to the patient as planned, which can be challenging due to factors that impact the accuracy of dose delivery. Due to the high degree of manual processes in brachytherapy, the relative risk of treatment delivery error is high when compared to other radiotherapy modalities. Additionally, interstitial and intracavitary brachytherapy suffer from anatomical motion and swelling due to catheter (or applicator) implant trauma. There are two fundamental ways to verify a HDR brachytherapy treatment delivery: (i) verify the source dwell positions and times are as per the treatment plan, or (ii) perform a measurement in vivo with a dosimeter. Although reported in many small patient studies, in vivo dosimetry (IVD) has many limitations (e.g. detector position uncertainty and limited sampling) making the interpretation of results for treatment verification difficult. These challenges may be the reason for the limited routine application of IVD as a treatment verification technique. Since the treatment plan is a planned set of dwell positions and times, the former approach has the potential to verify the dose over the entire treatment volume. This thesis addresses the need to improve the methodology for treatment verification in HDR brachytherapy. This work aims to establish a verification technique that can be used routinely in the clinical environment, not impact the patient and provide data that can be confidently interpreted to verify the entire treatment delivery. To achieve this, a novel approach to treatment verification was investigated, avoiding the challenges of directly measuring dose. Measurements of the source position, during the treatment delivery (source tracking) were made, enabling direct comparison with the treatment plan for verification. Additionally, a method to establish a structured approach for performing this treatment verification process was accomplished, with the objective to enable routine use and widespread uptake of this process. The overall goal of this novel verification approach was to improve the quality of treatment delivery and patient safety in HDR brachytherapy. To investigate this new approach, a flat panel detector (FPD) was employed as the measurement device. The detector, originally designed for use as an electronic portal imaging device, was characterised for use with an 192Ir brachytherapy source. The FPD response and the image acquisition timing were investigated to demonstrate its capability for this work. The images of the response to the 192Ir source, acquired with the FPD, were interpreted by a range of algorithms, extracting metrics that could be correlated with the position of the source. A concept for integration into a clinical environment was developed, by placing the FPD in the treatment couch, immediately below the target volume. The potential for the brachytherapy implant to displace due to anatomical and other influences was addressed by performing pre-treatment image verification. An imaging geometry was established, allowing registration with the treatment plan, enabling identification and quantification of implant displacement (in the treatment bunker) immediately prior to treatment delivery. The relationship established between the measurement frame of reference and the treatment plan permitted direct comparison of the measured dwell positions with the planned dwell positions for verification of treatment delivery. Treatment delivery metrics were developed to detect the occurrence of a treatment error, and based on the unique signature of the error, identification of the error source was possible. This concept of treatment verification was transferred into the clinical environment and a patient measurement was performed to understand the challenges of clinical implementation. The FPD responded to the 192Ir source, for a range of clinically relevant distances (20 to 200 mm) away from the FPD despite the low dose rates and the changing photon spectrum. The image acquisition time was one image capture every 1.8 seconds, and although not designed for this application, the FPD was adequate to perform this proof of principle work. Using a range of algorithms, the images acquired by the FPD were processed to determine the source position. It was determined that a centre of mass approach was the most accurate method (x and y s.d. 0.3 and 0.1 mm, up to 200 mm from the FPD imaging plane) to determine source position in the 2D plane of the FPD. The influence of inhomogeneities and finite phantom geometry were quantified relative to their influence on the accuracy of determining the source position when applied in a clinical scenario. A structured approach to pre-treatment imaging was developed, with a robust method to perform a 3D reconstruction of the implant in the treatment bunker using a ‘shift image’ technique. A registration between the treatment planning system (TPS) and the measurement space (FPD) was established allowing quantitative evaluation of the implant changes since treatment planning imaging. Pre-treatment imaging was capable of identifying catheter displacements in the order of 2.0 mm with a confidence of 95%. Identification of a treatment delivery error was possible with the use of metrics that when combined define an error ‘signature’ that suggest the source of the error. The absolute relationship between the measurement space and the TPS allow error trapping to identify errors that would otherwise go undetected, for example an incorrect channel length definition error. This verification approach was applied successfully in a clinical setting. Pre-treatment imaging allowed confirmation that the implanted catheters had not significantly displaced prior to treatment and source tracking results confirmed the treatment was delivered as planned. The clinical implementation had minimal impact on the workflow, increasing the patient setup (and imaging) time by only 15 minutes while not adding any additional time to the radiation dose delivery portion of the treatment. This initial work using a FPD for treatment verification in HDR brachytherapy has highlighted the benefits of this approach. This novel approach provides multiple layers of verification, including pre-treatment imaging (in 2D and 3D) to identify potential sources of error prior to treatment delivery. Source tracking, in conjunction with pre-treatment imaging, provides quantitative verification of the entire treatment delivery, currently not possible with other methods. This approach establishes a new standard of verification which has the potential to improve the quality of treatment delivery and improves patient safety in HDR brachytherapy

A simulation study of BrachyShade, a shadow-based internal source tracking system for HDR prostate brachytherapy

This paper presents a simulation study of BrachyShade, a proposed internal source-tracking system for real time quality assurance in high dose rate prostate brachytherapy. BrachyShade consists of a set of spherical tungsten occluders located above a pixellated silicon photodetector. The source location is estimated by minimising the mean squared error between a parametric model of the shadow image and acquired images of the shadows projected on the detector plane. A novel algorithm is finally employed to correct the systemic error resulting from Compton scattering in the medium. The worst-case error obtained with BrachyShade for a 13.5 ms image acquisition is less than 1.3 mm in the most distant part of the treatment volume, while for 75% of source locations an error of less than 0.42 mm was achieved

Verification of Gynaecological Brachytherapy Treatments Using an End-to-End Phantom

High dose rate brachytherapy allows the delivery of radiation internally, with high-dose gradients creating a conformal distribution. The inherent drawback of this treatment exists within small uncertainties producing a large impact on safety and efficacy. Applicator displacement was ret-rospectively simulated for 29 cervical cancer treatments to determine a critical shift in applicator position. A 2 mm shift in the anterior and posterior directions was detrimental to the bladder and rectum, respectively and a 4 mm shift in all directions caused a critical reduction in HR-CTV cover-age. These findings indicate the importance of quality assurance practices that mitigate applicator displacement. Furthermore, the source localisation accuracy required for cervical brachytherapy was quantified. HDR gynaecological brachytherapy relies on 3D imaging, contouring, precise reconstruc-tion of applicator position and transfer of data to the afterloading device. To evaluate this process an end-to-end phantom was developed, which consists of a component that houses gynaecological applicators and the Magic Plate 987 (MP987), created by the Centre of Medical and Radiation Physics, University of Wollongong. The 21 × 22.5 cm2 silicon diode array facilitates source tracking at clinically relevant depths. A characterisation of the MP987 for HDR source tracking has been performed, producing an error in dwell time and position of 0.1s and 0.25 mm respectively, for dwell times greater than 5 s. Source tracking accuracy is a function of both dwell time and distance from detector to source. The End-to-end phantom has verified both vaginal and cervical treatments. For a vaginal treat-ment, the mean residual in dwell position is within (0.24 ± 0.01) mm for all directions, with the difference in dwell time being (0.10 ± 0.01) s. Catheter swap, indexer length and activity miscalibration errors were all detected within the vaginal therapy end-to-end test. Validation of the End-to-end phantom for a cervical brachytherapy treatment produced a mean difference of (3.49 ± 0.57) mm,(4.74 ± 0.77) mm, (6.14± 1) mm in the X, Y and Z directions respectively, with a dwell time differ-ence of (0.19 ± 0.03) s. The localisation accuracy achieved is below the critical displacement value established within the treatment planning study. Improvement in co-registration and Z localisation methodologies will provide better outcomes for cervical cases. The End-to-end phantom successfully verifies the procedure for HDR gynaecological brachytherapy treatments, enabling safe and effective patient care

A simulation study of BrachyShade, a shadow-based internal source tracking system for HDR prostate brachytherapy

© 2018 Commonwealth of Australia. Department of Education and Training, and Department of Industry, Innovation and Science.. This paper presents a simulation study of BrachyShade, a proposed internal source-tracking system for real time quality assurance in high dose rate prostate brachytherapy. BrachyShade consists of a set of spherical tungsten occluders located above a pixellated silicon photodetector. The source location is estimated by minimising the mean squared error between a parametric model of the shadow image and acquired images of the shadows projected on the detector plane. A novel algorithm is finally employed to correct the systemic error resulting from Compton scattering in the medium. The worst-case error obtained with BrachyShade for a 13.5 ms image acquisition is less than 1.3 mm in the most distant part of the treatment volume, while for 75% of source locations an error of less than 0.42 mm was achieved

A tool to automatically analyze electromagnetic tracking data from high dose rate brachytherapy of breast cancer patients

During High Dose Rate Brachytherapy (HDR-BT) the spatial position of the radiation source inside catheters implanted into a female breast is determined via electromagnetic tracking (EMT). Dwell positions and dwell times of the radiation source are established, relative to the patient's anatomy, from an initial X-ray-CT-image. During the irradiation treatment, catheter displacements can occur due to patient movements. The current study develops an automatic analysis tool of EMT data sets recorded with a solenoid sensor to assure concordance of the source movement with the treatment plan. The tool combines machine learning techniques such as multi-dimensional scaling (MDS), ensemble empirical mode decomposition (EEMD), singular spectrum analysis (SSA) and particle filter (PF) to precisely detect and quantify any mismatch between the treatment plan and actual EMT measurements. We demonstrate that movement artifacts as well as technical signal distortions can be removed automatically and reliably, resulting in artifact-free reconstructed signals. This is a prerequisite for a highly accurate determination of any deviations of dwell positions from the treatment plan