Characterization of Physical and Dosimetric Aspects of SRS HyperArc Treatment Technique

HyperArc is a radiation therapy planning technique using stereotactic radiosurgical doses delivered via single isocenter volumetric modulated arcs (VMAT) for multi-lesion brain cancer. Due to the nature of these types of treatments it is imperative that the proper quality assurance is completed to ensure the safety of the patient. Geometric limits were assessed using a dose volume histogram (DVH) and known rotational errors of theoretical lesions of different dimensions and lengths from the isocenter. Dosimetric comparisons were evaluated using an ion chamber and Gafchromic EBT-XD film and portal dosimetry. The conclusions from these investigations were that patient rotation will result in a loss of target coverage, which is emphasized for smaller targets and isocenter placement farther away from the lesion. The use of a six degree of freedom (6DoF) couch for set-up and surface tracking during beam delivery reliably positions patients to avoid positioning errors that would degrade treatment outcomes. HyperArc is a user-friendly clinical tool that can be confidently used in a clinical setting when the limitations are well known to the user

Dosimetric investigation of image-guided radiotherapy for prostate cancer using cone-beam computed tomography

Objectives: 1. To survey the current practice of image-guided radiotherapy (IGRT) for prostate cancer in the United Kingdom. 2. To validate a practical dose calculation strategy on cone-beam computed tomography (CBCT) 3. To assess the effect of CBCT verification imaging frequency on actual dose delivered to target volume and organs at risk during a course of image-guided radiotherapy for prostate cancer. 4. To compare the dosimetric effects of reduction of CTV-PTV margin with daily imaging. / Material and Methods: 59 radiotherapy centres in the United Kingdom were included in an online survey of IGRT practice. The survey covered details of verification strategy during prostate radiotherapy, with specific questions on imaging frequency. A validation study of the CBCT dose calculation strategy was evaluated on 37 fractions using Bland-Altman plots. The study technique was compared to the density-override technique. A pilot comparison of CTV coverage with bone matching to soft tissue matching was performed. For the principal dosimetric analysis, 844 cone-beam CT (CBCT) images from 20 patients undergoing radical prostate radiotherapy were included. Patients received a dose of 74 Gy in 37 fractions using 7-field intensity-modulated radiotherapy. Target volume and organs at risk were contoured manually on each CBCT image. A daily online CBCT verification schedule was compared with a protocol of day 1-3 followed by weekly imaging. 3 mm, 5 mm, and 7 mm CTV-PTV margins were compared for daily imaging. / Results: CBCT is the principal verification imaging modality in the UK for prostate cancer, used by 66% of centres. There is no consensus on optimal imaging schedule, with 2 main strategies used. These are the daily online schedule and the day 1-3 followed by weekly schedule. Use of CBCT contours on planning CT showed good agreement with the density-override technique, provided multifield IMRT was used. There were clear drops in target coverage if a bone match strategy was used in comparison to soft tissue matching. 90% of patients had improved target coverage with daily online in comparison to weekly online soft tissue match. A median of 37 fractions achieved CTV coverage with daily imaging compared with 34 fractions with a weekly online protocol. 80% of patients had a reduction in rectal dose with the daily protocol. Margin reduction to 5 mm with adequate target coverage was feasible with daily imaging. / Conclusions: Daily online CBCT verification improves CTV coverage and reduces rectal dose during IGRT for prostate cancer. Daily CBCT imaging allows reduction of CTV-PTV margin for radiotherapy

Proof-of-Concept For Converging Beam Small Animal Irradiator

The Monte Carlo particle simulator TOPAS, the multiphysics solver COMSOL., and several analytical radiation transport methods were employed to perform an in-depth proof-ofconcept for a high dose rate, high precision converging beam small animal irradiation platform. In the first aim of this work, a novel carbon nanotube-based compact X-ray tube optimized for high output and high directionality was designed and characterized. In the second aim, an optimization algorithm was developed to customize a collimator geometry for this unique Xray source to simultaneously maximize the irradiator’s intensity and precision. Then, a full converging beam irradiator apparatus was fit with a multitude of these X-ray tubes in a spherical array and designed to deliver converged dose spots to any location within a small animal model. This aim also included dose leakage calculations for estimation of appropriate external shielding. The result of this research will be the blueprints for a full preclinical radiation platform that pushes the boundaries of dose localization in small animal trials

On the investigation of a novel x-ray imaging techniques in radiation oncology

Radiation therapy is indicated for nearly 50% of cancer patients in Australia. Radiation therapy requires accurate delivery of ionising radiation to the neoplastic tissue and pre-treatment in situ x-ray imaging plays an important role in meeting treatment accuracy requirements. Four dimensional cone-beam computed tomography (4D CBCT) is one such pre-treatment imaging technique that can help to visualise tumour target motion due to breathing at the time of radiation treatment delivery. Measuring and characterising the target motion can help to ensure highly accurate therapeutic x-ray beam delivery. In this thesis, a novel pre-treatment x-ray imaging technique, called Respiratory Triggered 4D cone-beam Computed Tomography (RT 4D CBCT), is conceived and investigated. Specifically, the aim of this work is to progress the 4D CBCT imaging technology by investigating the use of a patient’s breathing signal to improve and optimise the use of imaging radiation in 4D CBCT to facilitate the accurate delivery of radiation therapy. These investigations are presented in three main studies: 1. Introduction to the concept of respiratory triggered four dimensional conebeam computed tomography. 2. A simulation study exploring the behaviour of RT 4D CBCT using patientmeasured respiratory data. 3. The experimental realisation of RT 4D CBCT working in a real-time acquisitions setting. The major finding from this work is that RT 4D CBCT can provide target motion information with a 50% reduction in the x-ray imaging dose applied to the patient

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

Applications of Monte Carlo Methods in Biology, Medicine and Other Fields of Science

This volume is an eclectic mix of applications of Monte Carlo methods in many fields of research should not be surprising, because of the ubiquitous use of these methods in many fields of human endeavor. In an attempt to focus attention on a manageable set of applications, the main thrust of this book is to emphasize applications of Monte Carlo simulation methods in biology and medicine