# Risk Management for the Reliability of Robotic Assisted Treatment of Non-resectable Liver Tumors
## 4 Discussion  
Since targeted HDR brachytherapy and targeted chemotherapeutic agent delivery do not offer a therapeutic solution by today's standards, the implementation of a technical solution (which minimizes the drawbacks of the medical procedure) may be valuable for the medical community. Based on the estimated hazards of the medical task and their occurrence risks (see Table 1), the authors proposed the HeRo concept (which resulted from an AHP and a multiphase QFD) and estimated the residual risks thereafter (see Table 4). One question still stands, which is \"do the benefits of the medical procedure outweigh the residual risks?\" The authors attempt to answer this from a technical point of view (since there are no relevant medical data about the therapeutic index of HDR brachytherapy and targeted chemotherapy for non-resectable HCC, one can only assume that these therapies would do more good than harm due to their positive results in other areas of the body). The safety aspect regarding HeRo robotic system exploitation refers to the mechanical aspects of the robot and the control of the robot. Since the HeRo robotic system is composed of one gantry mechanism and one spherical mechanism, it follows that the robotic system has no singularities in the workspace. Moreover, throughout the QFD analysis, the correlation between various characteristics (e.g., accuracy, stiffness, etc.) was emphasized. Consequently, choosing technical solutions that increase accuracy (such as linear and circular guides and high-quality actuation solutions) has a positive impact on the safety as well. The mechanical solutions together with state-of-the-art automation solutions and sensors should lead to the development of a reliable experimental model for the HeRo concept. The two modules of the HeRo parallel robotic system operate \"mirrored\" relative to each other, a fact that provides multiple advantages. A variety of insertion points and trajectories may be achieved since the insertion instruments (for the needle and I-US probe) may be mounted on either Module 1 or Module 2 of the robotic system. Furthermore, the simple design with decoupled motions has the advantage that it allows optimal technical maintenance of the robotic system since the robot becomes easy to assemble or disassemble. From the three ways in which RCM manipulation may be achieved (see Section 2.1.1), following the systematic development of HeRo, the authors chose a mechanical constraint using a spherical mechanism which is located outside the patient body. Consequently, the RCM will be fully constrained, and using this approach should imply lower overall errors (with respect to other technical approaches). The errors for this case are correlated with the radius of the sphere. The advantage of a larger radius is that it increases the orientation precision. The disadvantage is that the mechanism may suffer from component elasticity, vibrations, and mechanical stress. However, the circular guides used for the HeRo design are commercially available (well calibrated and with well-known mechanical characteristics), and the elasticity of the material is insignificant. As previously stated, assuming a 1\({}^{\circ}\) angular error and 1 mm linear error results in (combined) errors of 2.5 mm and 5 mm at 50 mm depth and 200 mm depth, respectively (not taking into account any needle deflection) [5], and according to [4], these values are not acceptable for the targeted treatments. Figure 12 illustrates point clouds to show the error distribution for the mentioned values in Cartesian coordinates (where the points within the point clouds are not due to a random distribution but rather computed with incrementally numerical data for the input). To evaluate the accuracy of the gantry mechanism is straightforward since there is a one-to-one dependency between the Cartesian position of the RCM point and the values of the gantry mechanism actuators.