To facilitate training and planning for surgical procedures such as prostate brachytherapy, we are developing new models for needle insertion and radioactive seed implantation in soft tissues. We describe a new 2D dynamic FEM model based on a reduced set of scalar parameters such as needle friction, sharpness, and velocity, and a 7-phase insertion sequence where the FEM mesh is updated to maintain element boundaries along the needle shaft. The computational complexity of our model grows linearly with the number of elements in the mesh and achieves 24 frames per second for 1250 triangular elements on a 750 Mhz PC. We use the simulator to characterize the sensitivity of seed placement error to surgical and biological parameters. Results indicate that seed placement error is highly sensitive to surgeon-controlled parameters such as needle position, sharpness, and friction, and less sensitive to patient-specific parameters such as tissue stiffness and compressibility.
Figure 1: Simulation of needle insertion based on an ultrasound image of a human prostate cancer patient. Frame (a) outlines the prostate (in green) and the target implant location (small white dot) which is fixed in the world frame. Our simulation places a radioactive seed (large green disc) at this location (d). After needle extraction and tissue retraction, the placement error, the distance between the target and resulting seed location shown in (f), is 30% of the width of the prostate. Needle plans that compensate for tissue deformation can reduce placement errors like these that damage healthy tissue and fail to kill cancerous cells.