Virtual Spacer Implantation Platform Based on Finite Element Method for Simulation and Planning Patient-Specific Spacer Placement

Abstract

In recent years, due to major advances in radiotherapy (RT) delivery systems, more attention is paid to integrating RT into cancer management. Efficacy of advanced RT techniques like stereotactic body radiation therapy is shown for lung, liver, prostate and pancreas cancer. Use of RT, however, can be limited due to proximity of organs at risk (OARs). Previous studies show dosimetric benefits of spacers, namely dose escalation and increasing OAR sparing, to make separation between target and OARs for prostate and recent studies for pancreas. Yet, dosimetric benefits of placement of spacer highly depend on location and shape of spacer implanted. Accordingly, purpose of our study is to reduce geometric uncertainties of spacer placement by presenting a simulation platform that generates an optimized plan for spacer implantation. Pre- and post-spacer paired computed tomography (CT) are acquired. Gross target volume (GTV) and OARs are delineated by physician, as well as, spacer in the post-injection CT. Data is anonymized and fed into our simulation platform to create finite element model (FEM) of spacer placement process and simulate using finite element analysis (FEA). In this study, location of spacer is determined by post-injection CT for validation. To do so, pre- and post-injection CTs are registered by a rigid registration method while bones and later head of pancreas and prostate are used as landmark. To ensure the accuracy and convergence of FEA, anatomical and physical properties of organs are considered as a set of patient-specific boundary conditions. Finally, result of FEA is interpreted as deformations of 3D CT images and structures. To evaluate performance of simulation, we utilized overlap volume histogram (OVH) thresholds, L3cc and L9cc defined as overlapping tumor and OAR volume at 3mm and 9mm expansion of GTV, while introducing Distance Angular Histogram (DAH) to assess spatial performance of our method. 4 cases of hydrogel spacer are included in this study: 2 prostate and 2 pancreas cases. Applicable to any type of spacer, we used hydrogel cases for validation due to considerably higher level of biomechanical complexity. OVH thresholds (L3cc and L9cc) are similar for virtual and post injection cases, and considerably lower compared to pre-injection (table_1). Further, DAH shows that distribution of distance between tumor and OAR is more than a simple shift, and thus, closer to reality. Although spacer can improve RT outcome, its use is impeded by uncertainties associated with placement procedure. Our virtual spacer platform is aimed to help planning placement of spacer and obviate uncertainties of using it. Future studies will focus on improving performance and integration to real-time ultrasound guided procedure.Tabled 1Abstract 2678; TableOVH MetricL3mmL9mmSitesPancreasProstatePancreasProstateCasesABCDABCDPre-Injection0.0050.015000.0700.1600.0200.126Post-Injection00000.0140.0140.0050.004Virtual Spacer00000.0190.0120.0010.005 Open table in a new tab