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Accurate needle insertion in soft, inhomogeneous tissue has been a major concern in several recent studies involving robot-assisted percutaneous therapies. In procedures that involve multiple needle insertions such as transrectal ultrasound guided prostate brachytherapy, it is important to reduce tissue deformation before puncture and during insertion. In order to reduce this deformation, we have studied the effect of different trajectories for a 2-DOF robot performing needle insertion in soft tissue. We have compared tissue deformation and infinitesimal force per tissue displacement for different trajectories. According to the results of our experiments, infinitesimal force per displacement is a useful parameter for online trajectory update. Our proposed position/force controller is shown to provide considerable improvement in performance with regard to tissue deformation before puncture.