Discussion 68

In this study, a multi-camera OTS was integrated for the first time in the IOERT procedure, specifically into the radiance TPS, in order to further evaluate the actual dose distribution in the patient. We assessed the accuracy of the IOERT applicator pose in an end-to-end experiment resembling real treatment scenarios that were selected according to the most common intraoperative radiation therapy (IORT) sites (breast 31.8%, abdomen 29% and rectum 19.6% [18], this tendency has recently been confirmed by European expert institutions [22]) (Figure 25). Our results included

several sources of error involved in the real process: calibration of the optical tools, intrinsic optical system limitations and point-to-point registration with user interaction. The IOERT multidisciplinary staff and the use of several surgical support devices might easily obstruct the line-of-sight between tracked objects and the OTS if few cameras are used, thus seriously degrading tracker performance. Our solution makes use of more cameras to overcome these occlusion problems, as they are arranged around the working volume. In [81], the authors reported the use of the Vicon MX visual-tracking system (Vicon Motion Systems, USA) with five near-IR cameras on the walls of a single photon emission computed tomography (SPECT) room. This study showed sub-millimetric accuracy for motion tracking during cardiac SPECT imaging, with every optical marker on the patient localised at least 84% of the time, and emphasised the flexibility of camera placement to improve marker viewing.

Figure 25. Frequency of IORT sites in Europe (ISIORT-Europe 2004 survey). GI (gastrointestinal). Source [18].

As the pointer is a key tool in this experiment, we first estimated its accuracy (between 1.7 and 2.2 mm). In [72], the authors reported that the accuracy of the location of individual optical markers with OTSs (typically 0.25 mm) is better than when using tool tips (up to 2 mm), in accordance with our results. Although the use of a pointer to manually select reference positions could seem suboptimal, it is essential in practice for registration and navigation in certain scenarios, such as the tumour bed, where placing a marker might not be feasible.

Regarding the articulated arm that attached the applicator to the CT scanner bed, we demonstrated that the applicator was held steady throughout the process, including the CT acquisition, since errors before and after the acquisition were statistically equivalent.

The physical-to-image space registration was performed with an FRE of 1.3 ± 0.4 mm. This result includes inter-operator variability. FRE was also calculated for registrations with optical markers, which are not affected by pointer error, and proved to be 0.8 ± 0.2 mm. This latter error was slightly better, although both results are within the slice thickness of the CT images. Both solutions could be used in the IOERT scenarios we describe, although the pointer and the metallic markers (or other anatomical landmarks) are a more flexible solution, given the limitations in the radio-surgical treatment theatre. The physical-to-image space registration for this solution was obtained using six metallic markers. In external beam radiation therapy (EBRT), the patient is commonly positioned by aligning three skin markers (minimum number to establish spatial concordance) with in-room lasers [82]. If one of these three markers is mislocated, the registration becomes biased. In our case, we would recommend the use of a minimum number of four markers. A large FRE in any marker may indicate a local displacement in that area, and in that case the best three markers could be selected for the alignment. Another source of error is the uncertainty in the localization of the marker centroids in the CT image. However, decreasing voxel size might not significantly improve the results as the registration error is also affected by the pointer accuracy, and consequently the increase in radiation dose for the patient would not be justified.

The integration of the OTS in the IOERT planning system is illustrated in the figures that show the fusion of the actual applicator and the virtual applicator. Our accuracy results for the applicator pose could be evaluated using the recommendations from

Task Group 147 (TG–147, commissioned by the Therapy Committee and the Quality

Assurance and Outcomes Improvement Subcommittee of the AAPM to study the localization accuracy with non-radiographic methods such as IR systems in EBRT [73]). According to TG–147, when the phantom used to perform the daily quality assurance is positioned at the treatment isocentre based on the room lasers, the non-

radiographic system should localise the phantom within 2 mm of that location [73]. In our experiment, the accuracy of the applicator pose was below 2 mm in position (mean error of the bevel centre) and 2º in orientation (mean error of the bevel axis and the longitudinal axis), which corresponds to 1.8 mm (the arc length along the bevel of the 7-cm–diameter applicator [83]). These measurements are small when compared to the diameter of the IOERT applicator (7 cm), and within the acceptable range proposed in the TG–147 recommendation. The fact that the positioning error of the applicator is slightly higher with the metallic markers than with the optical ones suggests that this discrepancy could be due to pointer error. This difference (0.5 mm on average) is similar to the FRE between both types of markers. However, the coordinates of the metallic and optical markers in the physical space were obtained following a different methodology, namely, by placing the pointer on the surface of each metallic marker and by assessing the centroid of each optical marker. Therefore, the discrepancy may also be attributed to the procedure of the marker location in the physical space, since the radius of the metallic markers (0.75 mm) is close to the difference in the positioning error of the applicator.

Our results represent a major step towards the use of this approach in IOERT. However, some issues still need to be addressed. Although the experiment described makes use of one post for every camera, the final set-up in the IOERT treatment room could take advantage of a tubular structure hanging from the ceiling to which the cameras could be easily attached in positions similar to those evaluated. Moreover, the rotation of the cameras would be adjusted in order to better cover the FOV of the IOERT treatment. The clinical installation would be checked by means of repeating the experiments described in this work. The whole setting (the phantom and the applicator) would be carefully transferred from the IOERT treatment room to the CT simulator room by means of a CT compatible transport bed [84] in order to acquire the gold standard. Accuracy of the applicator pose would be similar in the IOERT room, except for the occlusion problems caused by, for instance, operating theatre personnel and anaesthetic and laparoscopic instruments. This limitation has been partially evaluated because our results were obtained in a scenario in which the operator blocked the view of some cameras while using the pointer. Degradation of tracker performance in the

IOERT treatment room is likely to be low, since the applicator is correctly tracked if its rigid body is well designed and at least three optical markers are seen by at least two out of the eight cameras in the OTS (stereo vision). In fact, the cameras are placed surrounding the scenario in order to reduce occlusions. Nevertheless, a figure of merit of the tracking process would be useful for the interactive assessment of the data collected by the multi-camera OTS. SD of the collected position, the number of optical markers of the pointer or the applicator actually seen and the number of cameras occluded would be taken into account in this figure of merit. On the other hand, as the skeleton phantom represents an idealised and simplified patient, the accuracy of the applicator pose achieved in our experiment may be considered to be a lower limit. The real treatment procedure will introduce several factors, such as retraction of structures, removal of affected tissues, internal organ displacement and placement of protections, which require further study. In addition, these issues could modify the geometry of the surgical site and increase the difference with the preoperative image used to plan the radiation treatment. Conventional preoperative CT imaging of the patient in a position resembling the surgical operation could decrease these dissimilarities. A better solution would be to use intraoperative imaging, which offers updated anatomical information [85]. To this end, a mobile C-arm that provides cone-beam CT (CBCT) imaging capability, but with a limited FOV, has been proposed [86, 87]. Such a solution would enable IOERT dosimetry planning to be more easily adapted to a real scenario after physical-to-image space registration. Regarding the protections, the tip of a sterilised pointer could be used to delimit the contour of the shielding plates in the surgical site in order to obtain its position in relation to the applicator pose, provided the handle of the pointer where the optical markers are placed is visible.