ESTRO 2020 Abstract Book

S1088 ESTRO 2020

entire ring when measured with cross-section 90 degrees to the long axis of the catheter. An MRI receive coil placed on an HDR brachytherapy trocar (Figure 1C) was used to improve signal strength and ease location of the silicone during scanning. A calibration curve (Figure 1D) was obtained relating the longitudinal relaxation time of silicone oxygen sensor to oxygen partial pressure. Conclusion Oxygen sensitive silicone placed on the external surface of the brachytherapy catheter offers the ability to make multiple measurements of tissue oxygen axially along the catheter. Use of an intra-lumen MRI receive coil for high signal-to-noise ratio avoids the need to localize the silicone volume of interest. Acknowledgements: This work is supported by a Bridge Project Expansion Grant (The Koch Institute for Integrative Cancer Research/Dana Farber Cancer Center) and Frontier Research Program Grant (Koch Institute for Integrative Cancer Research). J. Tokuda is supported by the Image Guided Therapy Center (NIH P41EB015898). G. Ekchian is supported by the Koch Institute Quinquennial Cancer Research Fellowship and the Kavanaugh Translational Innovation Fellowship.

Conclusion Obtained measurements reveal the need for dose adaptation and that model-based algorithms should be considered for planning complex HDR surface brachytherapy cases. Radiochromic film is an efficient in- vivo dosimeter that can be used in dosimetry of these cases and consequent adaptive re-planning. OC-1033 Oxygen Sensing HDR-Brachytherapy Catheters with an Intra-Lumen MRI Receive Coil R. Cormack 1 , G. Ekchian 2 , J. Tokuda 3 , C. Frangieh 4 , L. Lee 1 , M. Cima 5 1 Brigham and Women's Hospital, Radiation Oncology, Boston, USA ; 2 Massachusetts Institute of Technology, Koch Institute for Integrative Cancer Research, Cambridge, USA ; 3 Brigham and Women's Hospital, Radiology, Boston, USA ; 4 Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, Cambridge, USA ; 5 Massachusetts Institute of Technology, Materials Science and Engineering, Cambridge, USA Purpose or Objective Oxygen monitoring in oncology has long been of interest to clinicians and researchers due to the pronounced and widespread negative impact of tumor hypoxia on patient outcomes. Hypoxia induced radiation resistance is well established in cancers including cervical, prostate, and head and neck. MR-guided HDR-brachy offers the ability for hyper-localized dose escalation to overcome treatment resistance in low oxygen tumor sub-volumes but the lack of a clinically available direct and quantitative oxygen sensor has prevented the clinical implementation of these techniques. We report on the incorporation of our MRI- measurable oxygen sensitive silicone 1 on the external surface of an HDR-brachy catheter. This will offer a direct and quantitative measurement of tissue oxygen, at multiple tissue locations, without the need for an additional invasive step. Additionally, as the oxygen sensitive silicone is attached to the catheter it will remain at a fixed location until the catheters are removed at the end of treatment, maintaining the ability to co-locate oxygen measurements with the means for dose escalation Oxygen sensitive silicone rings were cured in molds of appropriate dimensions and placed on the outside surface of the catheter. Catheters containing silicone were imaged using a 3T Siemens MRI. The longitudinal relaxation time was measured with a custom MRI receive coil placed in the inner lumen of the catheter using a turbo spin echo inversion recovery pulse sequence. Results Oxygen sensitive silicone was successfully placed on the external surface of the catheter and allows for customizable spacing and dimensions (Figure 1A). Multiple silicone depots can be visualized in a single cross-sectional scan along the length of the catheter (Figure 1B) or the during each fraction. Material and Methods

OC-1034 Parallel imaging compressed sensing for prostate MRI without an endorectal coil: a prospective study J. Sanders 1 , S. Frank 2 , A. Venkatesan 3 , T. Bathala 3 , C. Tang 2 , R. Kudchadker 4 , T. Bruno 2 , M. Pagel 5 , J. Ma 1 1 University of Texas MD Anderson Cancer Center, Imaging Physics, Houston, USA ; 2 University of Texas MD Anderson Cancer Center, Radiation Oncology, Houston, USA ; 3 University of Texas MD Anderson Cancer Center, Diagnostic Radiology, Houston, USA ; 4 University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA ; 5 University of Texas MD Anderson Cancer Center, Cancer Systems Imaging, Houston, USA Purpose or Objective To prospectively investigate parallel imaging and compressed sensing (PICS) in postimplant prostate brachytherapy MRI without an endorectal coil (ERC). Material and Methods Arbitrary k-space sampling was incorporated into a 3D fully balanced steady-state free precession pulse sequence (CISS). Five k-space sampling patterns were investigated including fully sampled, random, pseudo-random Poisson disk, and both single- and multi-phase encoding GRAPPA accelerations.