The Production And Application Of Anthropomorphic Abdominal Phantom In Deformable Image Registration Validation

Over the years many efforts have been made in developing deformable image registration(DIR)approaches with applications in adaptive radiation therapy(ART).Yet,a more urgent question we need to answer is how to assess the uncertainty or validate the accuracy of these existing DIR approaches in the context of ART.The aim of this study is to produce an anthropomorphic abdominal deformable phantom,and provide a construction method of abdominal deformable phantom which can quantitatively evaluate the geometric accuracy and dose accumulation accuracy validation of DIR.This phantom can simulate complex anatomical structure change which is caused by the deformation of organs.The proposed phantom is an anthropomorphic abdominal phantom that consists of simulated organs including soft tissue,liver,kidney,spleen,stomach,vertebra and two metastasis tumors.All molds for the deformable organs are negative molds that are 3D printed using contours from an ovarian cancer patient.The organ molds are molded with deformable material made of different mixtures of polyvinyl chloride(PVC)and the softener dioctyl terephthalate.Soft PVC with different densities are obtained by a polynomial fitting curve that describes the relation between the CT number and PVC-softener blending ratio.The rigid vertebras are constructed by molding of white cement and cellulose pulp.The final abdominal phantom is assembled by arranging all the simulated organs inside a hollow dummy according to their anatomies,and sealed by deformable material with averaged CT number of muscle and fat.195 rubber cubes of 1mm3 are embedded uniformly inside the phantom as fiducial landmarks for DIR geometric accuracy and dose accumulation accuracy evaluation.To facilitate dosimetry evaluation,two channels are excavated in the phantom parallel to the superior-inferior direction for ionization chamber insertion.The feasibility of applying the constructed phantom for organ-wise fiducial-based DIR accuracy validation and in vivo dosimetric measurement is also performed on three DIR algorithms available in the open source DIR toolkit-DIRART.The constructed deformable material shows good elastic behavior and is stable in HU values over times,proving to be a practical material for the deformable phantom.The simulated internal organs of this deformable phantom are with the close CT numbers,three dimensional anatomical structure and spatial position relationship as the realistic organs.The CT images of real patient and phantom are highly similar and have good contrast.The contour of each organ and the position of landmarks can be distinguished clearly in the CT images.The dosimetric measurements show a good agreement with the calculated doses from the treatment planning system.Fiducial-based accuracy analysis conduct on the constructed phantom demonstrated the feasibility of applying the phantom for organ-wise DIR accuracy assessment.In this work,we have designed and constructed a 3D anthropomorphic abdominal deformable phantom.Detailed steps regarding the phantom construction including deformable materials recipe,organ molding,landmark point embedding,etc.are described.Phantom properties such as deformable material elasticity and CT number stability over time are tested,and the feasibility of applying the phantom for DIR accuracy validation is also demonstrated.As shown in the experimental results,phantom construction is not effort-demanding,and the phantom is inexpensive in material cost and do not need dedicated tools for fabrication.Any institution can create an identical or similar phantom following these steps and use it as a routine tool for the DIR quality assurance(QA).