ESTRO 37 Abstract book

S346

ESTRO 37

SP-0661 Clinical implementation and clinical experience with MR only workflows N. Tyagi 1 1 MSKCC, Medical Physics, New York- NY, USA

appear in very similar fashion. Recently, our group has introduced an automatic method to localize fiducials based on the fiducial's distinct distortion of the local magnetic field, which can be detected on phase images. The accuracy of this method is comparable to CT based The superior soft tissue of MRI over CT greatly facilitates the critical step in the simulation process: the tumor and OAR delineation. Currently, CT is still the master image modality as it provides the information on electron density and bony anatomy. Nowadays, thanks to innovations in MR technology and image processing, this is no longer the case. Accurate electron density maps and reference images can be obtained with MRI in a reliable manner. Thus, from an MRI perspective, the traditionally largest technical obstacles to allow MRI to become the sole imaging modality for treatment simulation has been overcome. It is up to radiotherapy clinics to start using MR-only simulation to improve treatment quality, patient comfort and logistics. SP-0660 Different methods of creating pseudo-CT images J. Jonsson 1 1 University Umea Norrlands Universitetssjukhus, Radiation Sciences, Umea, Sweden Abstract text Ever since the introduction of MR guided target definition for radiotherapy, the notion of MR-only based radiotherapy has been present. Since the target delineation is based on MR images, MR-only based radiotherapy would be favorable to simplify the workflow, as well as to reduce the risk of systematic errors being introduced into the treatment from image registration between the MR and planning CT. A major obstacle to overcome before such a workflow can become reality, is to be able to perform heterogeneity corrected dose calculations without the CT image. While the CT directly reflects the attenuation properties of the patient, the MR image correlates with the proton density and the magnetic relaxation of the imaged tissue. The typical example of the problem is the signal ambiguity between bone and air – while they both appear dark in an MR image, their attenuation coefficients are vastly different. This has led many research groups to investigate methods of constructing attenuation maps, or “pseudo CTs”, from MR data. There has been a plethora of methods presented in the literature, ranging from the very simple approach of assigning the entire patient anatomy the attenuation property of water, to much more involved conversions. The different methods can be categorized into two main approaches; voxel based and atlas based conversions. The voxel based methods mainly use the image intensities in the MR images for conversions – the spatial location of the voxels plays little or no role in the assignment of attenuation properties. Conversely, the atlas based methods mainly depend on the spatial location of the voxels to decide what attenuation property should be assigned to the pCT. There are also hybrid methods, where both voxel and atlas based methods are employed to reach the final assignment of attenuation. Most methods presented in the literature produce clinically acceptable results for photon dose calculations. In general, the voxel based methods are specific to the sequences which the models are trained on, but are general with regards to the patient population. In contrast, atlas based methods are more insensitive to input image data but specific to the patient population. In this lecture, the different methods will be discussed and exemplified, providing typical results and potential difficulties with the different conversion strategies. localization. Conclusions

Abstract text MRI is increasingly used in external beam radiotherapy planning for target and normal structure delineation because of superior soft tissue contrast. The interobserver variability in defining target is significantly reduced with MR as compared to CT for many disease sites. Recently, MR simulation platforms have been introduced, including flat table tops, external laser systems, and radiation therapy specific scanning protocols that are further enabling the use of MRI as the primary imaging modality for radiotherapy planning. MRI as a primary imaging modality for treatment planning is preferred based on several major advantages, including minimizing dosimetric errors introduced by mis- registration with the planning CT or changes in anatomy between the two scans, improving efficiency, and reducing redundant imaging and patient costs. However, 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 Symposium: In vivo dosimetry and online dose verification