Development of a 6 Degrees of Freedom Prostate Brachytherapy Robot with Integrated Gravity Compensation System

Low-dose rate prostate brachytherapy is one of the most widely used radiotherapy techniques for early stage cancer treatment due to its high benefits and low side effects. This method consists in manually inserting small radioactive seeds within the prostate through the perineum under ultrasound image guidance. This implantation is often inaccurate leading to mis-distributed dosimetry. Different robotic devices were proposed to improve the accuracy of seeds implantation. However, most of them are not well suitable for clinical conditions, especially considering bulk and size. Thus, in this paper we developed a 6 degrees of freedom compact and lightweight co-manipulated robot for prostate brachytherapy easy to install in the operating room thanks to its parallel design. The proposed robotic architecture impacts the accuracy of seeds placement along the gravity axis. Therefore, a gear spring mechanism is added into each leg of the robotic system in order to minimize the gravitational torques. The spring stiffness required to compensate gravity forces was calculated by determining the kinematic model and establishing the approximate perfect balancing condition. The final robot fits in a cube of 300×300×300 mm3 and permits to the needle to reach any point within the prostate in a singularity-free workspace superior to 55×55×150 mm3 with an average torque reduction rate of 82%.