Patient-Specific Biomechanical Diaphragm-Ribs Respiratory Motion Model for Radiation Therapy

Respiratory-induced organ motion is a technical challenge to radiation therapy for lung cancer. Breathing is controlled by two independent muscles: the thorax and diaphragm muscles. The modeling of their action constitutes an important step for the respiratory motion model. The amplitude of the diaphragm forces and ribs displacement are patient-specific and depends on geometrical and physiological characteristics of the patient. This article presents a patient-specific bio-mechanical model (PSBM) of the diaphragm, as well as ribs kinematics. To determine the appropriate values of specific diaphragm forces for each patient, during a whole respiratory cycle, inverse finite element (FE) analysis methodology has been implemented to match the experimental results to the FE simulation results. Ribs kinematics extracted and calculated directly from 4D Computed Tomography (CT) scan images. We have investigated the effect of element type, finite deformation and elasticity on the accuracy and computation time. The results demonstrate that the proposed FE model including ribs kinematics can accurately predict the diaphragm motion with an average surface error in diaphragm/lungs contact region less than $2.2\pm 2.1mm$ . This constitutes first steps for biomechanical patient-specific of the respiratory system modeling to pilot lungs and lung tumor motion for External Beam Radiation Therapy (EBRT).