ESTRO 2020 Abstract Book

S398 ESTRO 2020

deformation range while providing complex motion profiles and enhancing target features for both mono- and multi-modality registration. Material and Methods The deformable insert simulates a compressible tumor model within a healthy organ or tissue housing made from a custom molded low density open-cell polyurethane foam. A gap between the foam and insert housing in the S- I direction allows for partially decoupled rigid target motion from target deformation. Open cell foams, flexible rubbers, and elastic hydrogels were evaluated for deformability, T1w and T2w contrast, and CT signal to replace the liquid filled tumor shaped void. Internal structure such as boundary layers, crosshairs, and vasculature are also evaluated. Scans were acquired using 3D T1 Vibe and TrueFISP MRI in the coronal plane on a Siemens 3T MAGNETOM Prisma, balanced gradient-echo MR tracking sequence on a 1.5T Elekta Unity, and 90 keV and 120 keV CT on GE eXplore CT120 and Phillips Brilliance Big Bore systems. Results The tumor shaped void exhibits a 30 x 40 mm ellipsoid shape and a 7.5 mm offset in the L-R or A-P direction (assembly dependent). Hollow axial fiducials are filled with 5 mm ceramic spheres, providing 6 points for EPID/CT co-registration of tumor position and deformation, fig 1. The molded foam body has a modified hexagonal shape, providing six channels for minimizing return path turbulent flow of the aqueous based contrast solution within the insert during motion. The MR contrast solution exhibits a T1 of approx. 577 ms and T2 of approx. 67 ms. A motion range of 2.5 cm and deformation range of 1.5 cm was demonstrated for a total movement of 4 cm at a frequency of 10 bpm, fig 2, with +/- 30 deg rotation. Varying complex and physiological deformation profiles show hysteresis typical of respiratory motion. Numerous silicone rubbers display vivid MR contrast with CT visibility.

Conclusion In this work we provide a first demonstration of deformable MLC tracking on the 1.5T MRI-linac platform. Using a novel deformable motion phantom we could demonstrate a good correlation between the MLC aperture deformation and the target deformation. Ongoing work focuses on improving the phantom by filling the hollow fiducials with radiation absorbing material, which would allow for easier visualization on the EPID, enabling end-to- end latency estimations. This is also expected to result in more precise deformation estimates. OC-0707 A physiological 4D deformable phantom with complex motion profiles for MR and CT guided RT QA M. Perrin 1 , N. Hartman 1 , K.I. Penev 1 , J. Dietrich 1 , P.T.S. Borman 2 , C. Zachiu 2 , M.F. Fast 2 , E. Barberi 1 1 Modus Medical Devices Inc., Research and Development, London, Canada ; 2 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective The use of MRgRT has made real-time intrafraction imaging of tumor motion possible through its superior soft-tissue contrast. With this comes the need to benchmark MRgRT QA protocols, ensuring geometric uncertainties are quantified and compared with estimates produced from tracking algorithms. Previously, a prototype 4D motion phantom insert for MR, capable of repetitive physiological deformation, was implemented. This work evaluates advancements to the current 4D deformable motion phantom insert design, and is focused on increasing

Fig

1