## 4 Discussion  
In this study, we reported an alternative workflow for OPT in the frame of the specific CNAO case, where a non-dedicated fixed horizontal beamline and an ETS work in synergy for treatment delivery to patients affected by OM. In particular, the investigated modification of the clinical procedure concerned the phase in which the ETS is aligned in order to position the embedded fixation light along the prescribed gaze direction retrieved from the treatment plan. Currently, this procedure is performed inside the CT bunker with the aid of a dedicated in-house developed marker-tracking device [17,18]. We proposed a method envisaging this procedure to be performed in a dedicated non-clinical room equipped with a general purpose infra-red optical-tracking device as a way to limit the occupancy of the CT bunker with not strictly clinical procedures. For comparing the conventional versus the alternative proposed method, we merged a dataset consisting of the retrospective data coming from patients already treated at CNAO and simulated cases with clinical-like irradiation geometries, and ETS alignment was performed under the two conditions. When aligning the ETS in the dedicated room, certain limitations related to the lower flexibility of the OTS installed in the CT room were overcome by exploiting the more flexible three-TVC configuration of the commercial infrared tracking device. The comparison gave raise to similar performance in ETS alignment and corresponding gaze direction definition with respect to the nominal prescription under the two investigated conditions. In general, the residuals (nominal vs calculated) on the polar coordinate were greater than those on the azimuthal coordinate for both methods (clinical and novel). The differences we found were within or slightly higher than the clinical threshold of attention (1\({}^{\circ}\)), both when the retrospective patient data were considered alone and when the dataset was augmented with simulated cases. The deviations come from inaccuracies in optical-tracking camera calibration, residuals in reference system mapping and intrinsic accuracy of the robotic manipulator used for ETS alignment. It is worth recalling that the effects of these deviations are checked and eventually compensated for at the time of treatment by a point-based registration between reference and current positions of the implanted fiducial clips around the lesion in CT images. For this purpose, before and during irradiation, X-rays are used to verify the correct alignment of the eye by comparing the actual configuration of the tantalum clips with the planning reference. A clinical operator manually detects the clips' center in the radiographic images and a commercial software (VeriSuite 1.8, Medcom GmbH, 2012)  
Figure 10: Boxplot of statistical test for the difference in polar (**a**) and azimuthal angles (**b**) between Group 1 and Group 2, including 40 patients. The \(p\)-value for the rank-sum test indicates that there are no significant differences in the distribution of scores between the two methods. estimates a correction vector based on the distance between the clips' center in the nominal configuration and the actual configuration. A three-DOF correction vector is consequently applied to the treatment chair aiming at minimizing setup deviations. Among the reasons causing the detected deviations, optical-tracking intrinsic accuracy and calibration residuals influencing the overall 3D markers' localization accuracy are the ones that we consider more relevant. As a matter of fact, Table 4 shows the 3D localization accuracy of calibration residuals of the general purpose OTS used in the laboratory for ETS alignment and of the optical system installed in the CT room (OTS-CT). Data were obtained by localizing a known marker configuration with known coordinates in agreement with the quality assurance (QA) daily procedure adopted at CNAO [27].