How Linear Elastic Material Properties Affect Lung Tumor Motion and Deformation: A Finite Element Analysis

Abstract

Radiation therapy (RT) is a common method for treating lung cancer; however, targeting the moving tumor while sparing the surrounding healthy lung tissue is challenging. One of the methods proposed is tracking the tumor at the time of treatment and using multi leaf collimators (MLCs) to precisely target the tumor shape. One of the most accurate ways to predict the position and shape of the tumor is simulating the respiratory process at the time of RT delivery. However, an efficient and accurate way to find patient-specific material characteristics of lung and tumor as input for the simulation is still to be found. In this work, we have i) studied the effect that lung and tumor material properties have on tumor displacement and deformation and ii) found out that a simple, linear, elastic model may be sufficient for examining the effect of the material properties on the tumor displacement and deformation. A model was constructed in a simulation environment. STL meshes of a lung and a tumor were imported and set up to have separate material property values assigned for elastic modulus, Poisson's ratio, and density. Boundary conditions on the lung included a fixed surface located at the top of the lung and a distributed boundary load on the lower surfaces of the lung. The boundary load provided a 2.5cm displacement of the lower surfaces to imitate the diaphragm motion during the inhalation phase of the breathing cycle. Lung density showed no effect on tumor movement or deformation. Tumor displacement differences as a function of material property values ranged from 0.0043-1.53cm. The lung elastic modulus had the greatest effect on tumor displacement, with displacements ranging from 0.041- 1.53cm, or a range of approximately 1.5cm. Lung Poisson's ratio values affected displacement with a maximum displacement range of 0.1942cm. Tumor elastic modulus showed an insignificant effect on displacement of the tumor, with displacement ranges up to 0.0043cm. Tumor displacement was also insensitive to tumor Poisson's ratio values with a maximum range of displacement of 0.0005cm. We found out that a simple, linear, elastic model was useful for examining the effect of the material properties on the tumor displacement and deformation. Tumor properties and Poisson's ratio of the lung did not affect the tumor displacement and deformation significantly; however, having a patient-specific approximated value for the lung elastic modulus is required for optimal modeling of the process. Implementation of MLC-based tracking in RT, although an effective technique, is complex and time-consuming. Finding simplifications that make this more cost- and time-efficient can potentially increase the accuracy of delivery techniques, especially for lung cancer. Improvements that can be easily adopted to the RT workflow can improve overall therapy outcomes and quality of life for more patients.