A virtual sensor is developed for the online estimation of needle tip deflection during permanent interstitial brachytherapy needle insertion. Permanent interstitial brachytherapy is an effective, minimally invasive, and patient friendly cancer treatment procedure. The deflection of the needles used in the procedure, however, undermines the treatment efficiency and, therefore, needs to be minimized. Any feedback control technique to minimize the needle deflection will require feedback of this quantity, which is not easy to provide. The proposed virtual sensor for needle deflection incorporates a force/torque sensor, mounted at the base of the needle that always remains outside the patient. The measured forces/torques are used by a mathematical model, developed based on mechanical needle properties. The resulting estimation of tip deflection in real time during needle insertion is the main contribution of this paper. The proposed approach solely relies on the measured forces and torques without a need for any other invasive/noninvasive sensing devices. A few mechanical models have been introduced previously regarding the way the forces are composed along the needle during insertion; we will compare our model to those approaches in terms of accuracy. In order to conduct experiments to verify the deflection model, a custom-built, 2-DOF robotic system for needle insertion is developed and discussed. This system is a prototype of an intelligent, hand-held surgical assistant tool that incorporates the virtual sensor proposed in this paper. 1. Introduction Permanent interstitial brachytherapy is a cancer treatment procedure, in which radioactive seeds are implanted in tissue (e.g., the prostate, see Figure 1) in order to eliminate the cancer from inside. This procedure has emerged as an effective, minimally invasive, patient friendly, and cost-effective treatment option. For maximum treatment efficiency, the seeds have to be placed at exact locations inside and around the tumor that are determined in the preoperative planning stage. The radioactive seeds are initially loaded inside special needles. Intraoperatively, the seed-carrying needles are manually advanced toward planned locations where the seeds are deposited. Figure 1: View of a prostate brachytherapy procedure as it is currently performed. Two critical assumptions in the above procedure are that the needles will remain parallel across the entire length of their insertion in tissue and that the tissue will not deform as the needles penetrate it. However, in practice, neither assumption holds well,
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