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In this paper, we present needle insertion forces and motion trajectories measured during actual brachytherapy needle insertion while implanting radioactive seeds in the prostate glands of 20 different patients. The needle motion was captured using ultrasound images and a 6 degree-of-freedom electromagnetic-based position sensor. Needle velocity was computed from the position information and the corresponding time stamps. From in vivo data we found the maximum needle insertion forces to be about 15.6 and 8.9 N for 17 gauge (1.47 mm) and 18 gauge (1.27 mm) needles, respectively. Part of this difference in insertion forces is due to the needle size difference (17G and 18G) and the other part is due to the difference in tissue properties that are specific to the individual patient. Some transverse forces were observed, which are attributed to several factors such as tissue heterogeneity, organ movement, human factors in surgery, and the interaction between the template and the needle. However, theses insertion forces are significantly responsible for needle deviation from the desired trajectory and target movement. Therefore, a proper selection of needle and modulated velocity (translational and rotational) may reduce the tissue deformation and target movement by reducing insertion forces and thereby improve the seed delivery accuracy. The knowledge gleaned from this study promises to be useful for not only designing mechanical/robotic systems but also developing a predictive deformation model of the prostate and real-time adaptive controlling of the needle.

Pubic arch interference (PAI) often caused inadequate prostate coverage during transperineal brachytherapy using all parallel needles. In this paper, a hybrid implantation approach is presented in which additional angulated needles can be used to avoid PAI. This approach can be applied in prostate brachytherapy using a robotic assisted device. To examine the feasibility of this approach, volume data from three prostate seed implant patients were selected, which represent small, medium, and large prostates. As the blocking area was artificially increased simulating pubic arch overlap, the dosimetry outcomes and ratio of number of angulated needles to the total number of needles were analyzed. The hybrid dosimetry broke down when blocking is over 42%. As the percent of blocking increased, the ratio of the number of angulated needles to the total number of needles increased, while the dosimetry outcomes only had a slight trend of worsening. When close to the breakdown point, the dosimetry outcomes worsen drastically. Therefore, for moderate PAI the hybrid dosimetry is feasible.