An improved abdominal phantom for intrafraction image guidance validation.

A dynamically compressible phantom of the human abdomen that simulates organ motion with breathing is being developed for possible testing of image-gated beam delivery in radiotherapy. The polyvinyl chloride plastisol (PVCP) phantom features a cavity that can contain a deformable normoxic polyacrylamide gel (nPAG) dosimeter that is intended for use with MRI to provide dosimetric data. The phantom has been improved by the inclusion of new components that are more realistic anatomically and exhibit CT values similar to those of the tissues they mimic. Component organs were made from 3D-printed molds developed from CT contours of a real patient and their radiodensities adjusted by varying the mass ratios of the PVCP hardener and softener during manufacture. To make the phantom more compatible with ultrasound imaging a graphite scatterer was mixed into some of the phantom components to produce a background speckle pattern. This provided contrast between the body and a moving anatomical target intended for motion tracking. Phantom insert motion magnitude and repeatibility was assessed using CT by imaging two phantom inserts, one containing fiducial markers and the other containing iodinated gelatin, at the same position after repeated cycles of deformation. The maximum motion of a phantom fiducial at the position of the phantom treatment target was found to be 12.2 mm. The phantom design resulted in dosimeter motion with a point-to-point repatability within 0.3 mm on average and contour repeatability resulting in Dice coefficients exceeding 0.98 on average.