Poster - Thur Eve - 05: Safety systems and failure modes and effects analysis for a magnetic resonance image guided radiation therapy system.

INTRODUCTION: An online Magnetic Resonance guided Radiation Therapy (MRgRT) system is under development. The system is comprised of an MRI with the capability of travel between and into HDR brachytherapy and external beam radiation therapy vaults. The system will provide on-line MR images immediately prior to radiation therapy. The MR images will be registered to a planning image and used for image guidance. With the intention of system safety we have performed a failure modes and effects analysis. METHODS: A process tree of the facility function was developed. Using the process tree as well as an initial design of the facility as guidelines possible failure modes were identified, for each of these failure modes root causes were identified. For each possible failure the assignment of severity, detectability and occurrence scores was performed. Finally suggestions were developed to reduce the possibility of an event. RESULTS/DISCUSSION: The process tree consists of nine main inputs and each of these main inputs consisted of 5 - 10 sub inputs and tertiary inputs were also defined. The process tree ensures that the overall safety of the system has been considered. Several possible failure modes were identified and were relevant to the design, construction, commissioning and operating phases of the facility. The utility of the analysis can be seen in that it has spawned projects prior to installation and has lead to suggestions in the design of the facility.

Poster - Thur Eve - 21: ROC analysis in patient specific quality assurance.

INTRODUCTION: Many institutions rely on a patient specific measurement for IMRT/VMAT patient QA. In diagnostic imaging, radiologists use Receiver Operator Curves (ROC) to help quantify the value of a diagnostic imaging test. The purpose of this work is to investigate the value or ROC methodology for patient specific IMRT QA. METHODS AND MATERIALS: Beam fluences for 34 prostate IMRT patients were analyzed using gamma analysis. For half of these, measurements were done using the planned beam fluences. For the rest, perturbations to the MLC leaf positions were introduced. Gamma analysis was then used to measure fluence differences. Assuming that the unperturbed fluencies were positive measurements, distributions of true positive and false negatives were calculated. RESULTS: For poorly performing beam delivery systems the choice of γ-DTA criterion has little effect on test sensitivity and specificity. The AUC is increased by about 10% for high performance beam delivery systems. For a 3%/3mm γ-DTA condition, ideal cut off values are reasonably independent of MLC performance. At a tighter γ-DTA condition of 2%/2mm, then the optimal sensitivity and specificity of the test is more dependent on MLC performance. DISCUSSION: For a pass-fail test such as the γ-DTA map is, it is important to choose an optimal cut off value to maximize the sensitivity and specificity of the test. ROC methodology allows users to follow a prescriptive method to obtain ideal cut-off values for gamma analysis, and to assess improvements in sensitivity and specificity for higher performing beam delivery system.

SU-D-213CD-06: Workflow and Safety Systems of a Linac-MR Sim-Brachytherapy MRgRT™ Facility.

PURPOSE: To develop the operational workflow and safety systems of a magnetic resonance-guided radiotherapy system (MRgRT™), which comprises an MR scanner on rails that travels between a linac vault, MR simulation room and brachytherapy suite. METHODS: To develop a safe and streamlined clinical workflow, we conducted a comprehensive process review based on a layered approach to overall MRgRT safety that included i) facility design, (ii) workflow iii) system design and interlocks and iv) policies and procedures. We applied existing guidelines for MR and radiation safety, and employed system-level failure modes and effects analyses to design the MRgRT facility and clinical procedures. RESULTS: In the MRgRT system configuration, the MR and treatment systems are physically decoupled and used independently requiring novel administration of existing MR and radiation guidelines. A key element for the safe operation of the moving MR unit is the concept that all three rooms represent zone 4 areas (American College of Radiology guidelines). Using this concept, we applied MR guidelines to develop safe procedures for the overall suite, including screening of all persons entering the suite in zone 2 and control of ferromagnetic materials. We generated a clinical workflow that ensures expedient and safe transition between MR imaging and treatment delivery in both the linac and brachytherapy rooms. In addition, we designed emergency protocols for MRgRT, which helped drive requirements for the facility and system design, e.g., need for an accessible MR-safe stretcher. CONCLUSIONS: We designed the first comprehensive description of the MRgRT workflow, interlocking systems and safety procedures. With this layered approach to safety, we addressed critical aspects regarding safe operation and workflow for the system and provided multiple redundancies for key processes. Coupled with customized staff training, the proposed design ensures the safe operation of the MRgRT facility. This work has received research personnel support from IMRIS.

Patient dosimetry for hybrid MRI-radiotherapy systems.

A novel geometry has been proposed for a hybrid magnetic resonance imaging (MRI)-linac system in which a 6 MV linac is mounted on the open end of a biplanar, low field (0.2 T) MRI magnet on a single gantry that is free to rotate around the patient. This geometry creates a scenario in which the magnetic field vector remains fixed with respect to the incident photon beam, but moves with respect to the patient as the gantry rotates. Other proposed geometries are characterized by a radiation source rotating about a fixed cylindrical magnet where the magnetic field vector remains fixed with respect to the patient. In this investigation we simulate the inherent dose distribution patterns within the two MRI-radiation source geometries using PENELOPE and EGSnrc Monte Carlo radiation transport codes with algorithms implemented to account for the magnetic field deflection of charged particles. Simulations are performed in phantoms and for clinically realistic situations. The novel geometry results in a net Lorentz force that remains fixed with respect to the patient (in the cranial-caudal direction) and results in a cumulative influence on dose distribution for a multiple beam treatment scenario. For a case where patient anatomy is reasonably homogeneous (brain plan), differences in dose compared to a conventional (no magnetic field) case are minimal for the novel geometry. In the case of a lung plan where the inhomogeneous patient anatomy allows for the magnetic field to have significant influence on charged particle transport, larger differences occur in a predictable manner. For a system using a fixed cylindrical geometry and higher magnetic field (1.5 T), differences from the case without a magnetic field are significantly greater.

Sci-Thurs PM: Delivery-08: Investigation of the source of RF noise from a modulator for an MR-linac project.

INTRODUCTION: The next significant step in the advancement of IGRT is the integration of an MRI with a linac. The MRI-linac will provide images with exquisite soft tissue contrast in real-time during treatment. A possible problem associated with the proposed integration is the RF noise generated by the linac. This noise could interfere with the received signals of the MRI producing deleterious effects in the image quality. The work herein is concerned with understanding the processes involved in the RF noise production and the magnitude and frequency of this RF noise in the modulator of a linac. MATERIALS/METHODS: A software programming environment, MultiSIM, was used to model the electronic components of a modulator. Several Current and Voltage waveforms from the modulator were measured with an oscilloscope and compared with the corresponding results from the modulator model for validation. Finally, RF noise generated by the modulator was measured using field probes, which permits the frequency components of the measured and simulated modulator waveforms to be compared with the measured RF noise. RESULTS/DISCUSSION: The modeled PFN charging current and voltage, and klystron current show good agreement with measurements, with the exception of the tail of the klystron voltage signal. Once the model has been validated in both the time and frequency domains, future work will entail predicting pulse shape changes when, and if, modifications to the modulator are made. Specifically, modifications will be made which shift and/or reduce the RF noise in the frequency range of interest for a 0.2T MRI.

Sci-Fri AM: YIS-09: The effect of magnetic interference on a coupled MR-linac system: Optimization of 3D FEM linac model.

PURPOSE: The coupling of a 0.2T bi-planar Magnetic Resonance Imager and medical linear accelerator (linac) is proposed to provide real-time Image Guided Radiotherapy. This coupling necessitates the linac to be within the fringe fields of the bi-planar magnets causing magnetic interference. The design and optimization of the minimum required shielding is necessary to reduce the fringe field magnitudes to a point where a clinically useful radiation beam is produced. METHOD: A first step to designing shielding is the full 3D radio-frequency modeling of the linac waveguide using the Finite Element Method. Various optimizations were performed on the linac model in order to achieve a desired resonant frequency, π/2 phase shift per cavity and other desired properties. An accelerating cavity (AC) and coupling cavity (CC) was first optimized in 3D to have identical resonant frequencies before the full 3D model was generated. RESULTS: In order to increase the capture efficiency of the injected electrons, the electric field in the first AC was reduced by shifting the first CC towards the gun end of the linac. The input waveguide AC dimensions were adjusted to account of the additional coupling iris and the last full AC had its gap length decreased. CONCLUSION: This work is the first step to determining the minimum magnetic shielding required to produce a clinically useful radiation beam from a coupled MR-Linac system. The fully optimized 3D model more accurately calculates the electric and magnetic field values since it includes the effects of coupling.

Sci-Fri PM: Planning-02: MRI-based radiation treatment planning for an MRI-linac system.

At Cross Cancer Institute, we are investigating a novel MRI-linac system consisting of a bi-planar 0.2 T permanent magnet coupled with a 6 MV Linac. The system can freely revolve axially around the patient to deliver dose from any desired angle. For such a system, the radiation treatment planning procedure is expected to rely on the MR images only, i.e. MRI Simulation. Replacing the current CT/CT+MRI-based RTP procedure with MRI Simulation will eliminate the need for the planning CT scanning sessions (no additional x-ray exposure) and consequently the image fusion between MRI and planning CT. In this work, we propose a comprehensive MRI-based RTP procedure for an MRI-Linac system. Specifically, the method consists of a) data acquisition, b) analysis and correction of image artifacts caused by the scanner-related and patient-induced distortions, c) segmentation of organ structures relevant to dosimetric calculations (e.g. soft tissue, bone, air), d) conversion of MR images into CT-like images by assigning bulk electron density values to organ contours defined at step c), e) dose calculations in external magnetic field, and f) plan evaluation. Monte Carlo simulations were performed to determine the linac-MRI scanner's magnetic field induced effects on the dose deposited patterns using patient data. Specifically, we investigated the dosimetric differences between the corresponding MRI-based RT plans simulated at zero and 0.2 T. We found that the maximum percent differences for brain studies were within 4%. Most of these differences occurred at the inferior field edge and superficially at beam exits.

A video-based contour acquisition system.

A contour acquisition system using a video camera to view a patient is outlined. A calibration technique is derived and presented in detail. Results of contour measurements are shown to be accurate to within 2 mm. Difficulties with the technique are discussed and improvements to the method are suggested.