Abstract Book

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ESTRO 37

to perform MR-only simulation, there are few requirements that need to be met, including: (a) synthetic CT images generated from single or multiple MR with high geometric and dosimetric accuracy (b) MR images with sufficient soft tissue contrast for contouring both target and normal structures and (c) 2D DRRs or 3D reference images with sufficient bone, soft tissue, and/or implanted fiducial visualization to guide image-based patient setup. Although methods for performing various steps for MR-only treatment planning have been developed, actual clinical implementation and clinical workflows are still in its infancy. Multiple checks and QA processes need to be implemented at various stages to streamline clinical processes using MR images alone. Use of multiple MR datasets for contouring, synthetic CT generation as well as fiducial identification has resulted in a strong need for an organized workflow to streamline inter-sequence registrations as well as automatic image layouts for contouring by physicians. The goal of this session is to review clinical implementation, QA approaches, challenges and workflows for MR-only simulation, planning and treatment localization and address future improvements. SP-0662 MOSkin detectors for in vivo measurement of rectal wall dose in prostate SBRT boosts J. Lehmann 1 1 Calvary Mater Newcastle, Department of Radiation Oncology, Newcastle- NSW, Australia Abstract text Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detectors have been used for in vivo dosimetry for brachytherapy and external beam therapy for several decades now. MOSFETs inherent advantages as detectors include their small size and their dose rate independence. MOSFET based dosimetry systems have been improved to overcome some of the detector’s limitations, like temperature and angular dependences. Rectal retractors have been employed to allow for safe dose escalation in external beam prostate cancer treatments. The Rectafix rectal sparing device was used in the “PROstate Multicentre External beam radioTHErapy Using Stereotactic boost” (PROMETHEUS) study for the delivery of the boost treatments. Patients in this study received two boost fractions of 9.5-10 Gy Stereotactic Body Radiation Therapy (SBRT), each delivered with dual arc volumetric modulated arc therapy (VMAT). For twelve study patients a dual “MOSkin” MOSFET detector was attached to the anterior surface of the Rectafix during their SBRT treatment. Time resolved measurements were collected by reading out the detectors at a rate of 1 Hz throughout the dose delivery. The measured dose was compared to the planned dose exported from the treatment planning system using the detector position of the day, determined with Cone Beam Computed Tomography (CBCT) imaging. The average difference between the measured and the planned doses over the whole course of treatment for all arcs measured was 9.7% with a standard deviation of 3.6%. The cumulative MOSkin reading was lower than the total planned dose for 64% of the arcs measured. The average difference between the final measured and final planned doses for all arcs measured was 3.4% of the final planned dose, with a standard deviation of 10.3%. The study proved the feasibility of real time measurements of rectal dose during SBRT VMAT treatment of the prostate. It confirmed the difficulty of Symposium: In vivo dosimetry and online dose verification

measurements in a high dose gradient. The effort involved and the found dose differences highlight the importance of a clear clinical indication for any in vivo dosimetry and the need for action limits. As used here, in vivo measurements can be a useful additional safety measure and dose confirmation when introducing a new treatment technique. SP-0663 Scintillation detectors for in vivo dose validation in brachytherapy K. Tanderup 1 1 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Abstract text Treatment verification is of specific importance in brachytherapy due to hypofractionation and steep dose gradients. Furthermore, the brachytherapy workflow includes many manual processes, which increases the risk of radiation events, as most misadministrations and near misses are related to human errors. As likelihood and consequences of errors are considered more prominent in brachytherapy than in EBRT, it is a paradox that treatment verification technologies are less developed in brachytherapy. Preliminary results from surveys within GEC-ESTRO activities indicate that less than 10% of clinics perform in vivo dosimetry, while the majority of clinics are interested, if a relevant system had been available. Novel detector technology is currently opening the field of real-time in vivo dosimetry, which can improve the capacity of error detection significantly. Innovations in in vivo dosimetry are currently driven by application of real- time source tracking during treatment delivery through dose rate measurements combined with error detection algorithms. Real-time in vivo dosimetry requires dose rate detectors and currently two promising concepts are being explored: 1) small point detectors in close proximity to the source and 2) EPID panels. Many types of point detectors are available, but the interest in scintillating crystals is growing with the request for small detectors. A number of detector materials are being explored and key characteristics such as reproducibility, sensitivity, energy dependence, and stability over time are being mapped out in the search of detectors which can make in vivo dosimetry most sensitive to errors and which can function optimally in the clinical workflow. SP-0664 EPID-based 3D in vivo dosimetry for SBRT lung and spine – Values and Challenges E. Van Uytven 1 , P. McCowan 2 , T. VanBeek 3 , B.M. McCurdy 2 1 University of Manitoba, Dept of medical physics- Dept of physics and Astronomy, Winnipeg- Manitoba, Canada 2 CancerCare Manitoba, Medical Physics, Winnipeg, Canada 3 CancerCare Manitoba, Medical Physics, WIniipeg, Canada In 2014, CancerCare Manitoba (CCMB) began performing 3D in vivo dose verification of for all fractions of stereotactic body radiation treatments (SBRTs). Findings over a 31-month period were analyzed in detail and reported recently in the Red Journal [1] and will be discussed. The values and challenges associated with clinical implementation of an EPID-based in vivo dosimetry program and on-going sustainable operation will also be discussed. While pre-treatment quality assurance (QA) systems provide verification of linac deliverability and output, in vivo dose verification methods can provide verification of treatment efficacy to the patient. Methods & Materials 100 lung and 15 spine treatments provided a total of 602 fractions of measured data. During treatment delivery, transmission aS1000 EPID images were acquired in cine Abstract text Introduction