## 4 Discussion
(Figure 5) The rotation and movement of the iCT causes a more widespread irradiated area, while the primary radiation bundle of the fluoroscopy device irradiates a more localized area. This also explains why the lateral PSD is higher during surgeries with biplanar C-arm fluoroscopy than with iCT. (Figure 7 (left)) The abdominal GafchromicTM film was only exposed to radiation during pedicle screw fixation, while the lateral GafchromicTM film was exposed during pedicle screw and cage placement. Therefore, the abdominal PSD was relatively low and did not differ between surgeries with iCT or C-arm fluoroscopy. (Figure 7 (right)) [PERSON] et al. reported a median anteroposterior skin dose of the patient of 59.5 (8.3-252) mGy and a lateral skin exposure of 78.8 mGy (6.3-269.5 mGy) during fluoroscopy-guided transforminal LIF procedures [27]. In comparison to our results, this is a slightly higher anteroposterior skin dose and a halved lateral skin dose. However, in their study, no graft cages were inserted with fluoroscopy guidance. Even though the ED of the patient was higher with iCT, a postoperative CT scan was not required anymore unless desired by the neurosurgeon. Moreover, it is stated that there is no need to consistently evaluate the placement of the pedicle screws with a second iCT scan since the accuracy is even higher with iCT navigation than with C-arm fluoroscopy [25,28]. Furthermore, there is a moderate positive correlation between the BMI and the ED of the patient during surgeries with iCT. (Figure 6) This results from a personalized scan protocol in which the radiation dose is dependent on age, height, weight, and surgery level of the patient. Furthermore, the surgeon can choose to use the full proposed dose, to halve the proposed radiation dose, or to minimize the radiation output to 25% of the proposed radiation dose [20]. This choice often depends on the body composition of the patient, since abdominal fat is negatively correlated with image quality [29,30]. In contrast, a C-arm fluoroscope only contains a low-, medium-, and high-dose mode which can be selected to adapt image quality. However, the radiation output of a C-arm fluoroscope is mainly dependent on the fluoroscopy time, which is controlled by the surgeon and might be biased by the surgeon's experience or the complexity of the case [22]. Surgeries performed with iCT lasted approximately 90 min longer on average than MI-PLIF procedures performed with C-arm fluoroscopy, including an average of 42 extra minutes for, patient positioning and the insertion of pedicle screws (Figure 8). Similarly, [PERSON] et al. reported 30-50 min additional time for instrumentation with iCT set-up and navigation, including 18-34 min extra surgical time with iCT navigation [25]. A meta-analysis, comparing computer-assisted and fluoroscopy-guided surgery confirmed a longer total operative time but reported a reduced thoracic pedicle screw insertion time, compared to fluoroscopy-guided surgeries [31]. Several factors associated with iCT can increase the duration of the surgery. First, iCT is a newly developed technique and is associated with a steep learning curve [25]. In contrast, surgeons are experienced in using C-arm fluoroscopy as the golden standard during image-guided surgeries. Although reports, investigating the learning curve of iCT are sparse, it has been hypothesized that surgeonswith image-guidance skills merely need five cases to obtain sufficient experience with iCT. Furthermore, routine use of iCT and spinal navigation is required to overcome the learning curve [23,25]. During this research, iCT was not routinely used by the three neurosurgeons. The installment of the iCT during surgery is more difficult than the installment of the C-arm, due to the weight and positioning of iCT. Other possible influencing factors are disruption or interference with the navigation signal caused by bloodstains on the reflective markers or lightning of the surgery lamp. Retightening and recalibration after loosening of the navigation markers also occurred.