The main objective of this study was to assess the radiation exposure to the surgical team and the patients undergoing pedicle screw placement surgery in a hybrid OR equipped with ARSN with intraoperative CBCT imaging.
Radiation Exposure to the Operating Staff
Occupational radiation exposure can be significantly reduced by increased shielding and distance. Shielding can be achieved with lead aprons or lead shields. In fluoroscopy-guided spinal surgery the standard shielding is lead aprons. Bohoun et al showed a decrease of almost 90% when measured outside versus inside the apron (54.1 vs. 5.7 μSv), while performing non-navigated spinal surgery using 2D x-ray and intraoperative 3D CBCT imaging only. Delgado-Lopez et al reported similar exposure rate reductions (33,400 vs. 4000 μSv/h) at a distance of 1 m during 3D examinations with the O-arm. When positioned 2 m away, the exposure rate was 1290 μSv/h without shielding, 910 μSv/h when wearing apron, and 33 μSv/h when behind a lead screen. Abdullah et al observed an operator radiation dose of 0.44 μSv when 4.5 m away from the imaging system, not wearing a lead apron.
In our study, the OR staff did not wear lead aprons during the procedures and instead stayed behind a lead shield wall situated about 3 m away from the robotic C-arm. The OR staff radiation exposure reported in this study was found to be substantially lower than the values reported in the literature (Table 3), and far below the annual occupational dose limit of 20,000 μSv recommended by the International Commission on Radiological Protection.[11,15,19–22]
Despite the staff being positioned behind a 2 mm lead shield wall in the corner of the OR (Figure 2), x-rays scattered from the patient during the rotational image acquisition were the main source (83.8%) of the occupational exposure. A 2-mm lead shield has an absorption of about 99% for a 120 kV x-ray beam compared with 90% for a 0.25 mm lead apron. The staff were found to be subjected to higher levels of radiation exposure during the first cases of the study. However, as previously shown, the use of real-time dosimeters allowed optimization of staff protective measures in the OR for the subsequent procedures. Initially, staff were often standing close to the edges of the lead wall but improved positioning behind the lead wall, away from the edges, resulted in reduced exposure levels (Figure 5). Therefore, we conclude that improper positioning too close to the edges of the lead shield were to blame for the initially higher levels. Correct use of the lead shield resulted in minimized radiation exposure, confirming that there was no need for lead aprons. This is of benefit especially to the surgeons who often have to wear them for several hours, which can be exhausting and may interfere with the surgeon's mobility and ergonomics. Nonetheless, the best option to completely avoid staff radiation exposure is the use of a separate (shielded) room during 3D acquisition.[20,22] This could, however, potentially increase infection rates and increase the time required for an urgent intervention by the staff. Thus, we find that the use of large, fixed lead shields within the OR is an effective compromise.
Radiation Exposure to the Patients
It could be argued that navigation with intraoperative 3D imaging reduces occupational radiation exposure at the expense of an increased patient radiation exposure. Even though the exposure associated with a 3D CBCT acquisition may increase the patient radiation dose compared with fluoroscopy, the dose is on average less than that from a single-spine CT examination. Furthermore, the use of intraoperative 3D CBCT imaging has the potential to prevent repeat surgeries and reduce the amount of follow-up imaging needed, thus reducing the cumulative patient radiation exposure. At our institution, postoperative CT within 24 hours is part of the standard care, but was not performed for the patients in this study as these examinations were replaced by intraoperative CBCT scans.[9,10]
Several studies have reported an improvement in pedicle screw placement accuracy when using navigation with intraoperative imaging compared with conventional free-hand surgery, especially in scoliosis cases, due to the challenging anatomy.[5,10,25–27] However, additional imaging increases the total radiation dose to the patients. This is of particular concern in regards to young adult scoliosis patients who are more radio-sensitive than older patient groups. Ronckers et al reported on following 5573 female scoliosis patients (diagnosed before the age of 20 yrs) for a median time of 47 years and found a cancer mortality 8% higher than the US female average.
In the present study, the patient radiation dose was reduced by using a low-dose protocol with fewer x-ray pulses per second and a larger field of view, thereby improving the ratio between number of imaged spinal levels and total dose. Other strategies of lowering the patient exposure are possible but were not explored in this study. For example, meticulously restricting the field-of-view to the surgical area of interest may significantly reduce the radiation exposure.[20,30]
Table 4 summarizes a list of studies that report nonpediatric patient ED, DAP, and AK from four different types of intraoperative imaging systems for spine surgery: mobile CT,[21,31,32] mobile CBCT,[20–22,30–36] floor-mounted robotic CBCT,[37,38] and ceiling-mounted robotic CBCT.[9,11] This study reports a patient-effective dose of 15.8 mSv, which is within the range of 3.5 to 19.1 mSv reported in the literature. However, the number of 3D acquisitions performed in this study is higher because the patient cohort mainly corresponded to long construct scoliosis (median of eight levels treated) requiring at least two image acquisitions to cover the spine from the uppermost to the lowermost instrumented vertebrate. Thus, even though the number of spinal levels treated was larger than in most of the other studies, our patient effective dose is comparable to reported values in studies treating on average only two to four levels (Table 4). Lange et al demonstrated that the patient radiation dose using an O-arm (Medtronic, Minneapolis, MN) depends on the number of scans and the number of instrumented levels, in line with our findings. Additionally, screw placement verification scans were performed in the present study, replacing postoperative CT, which effectively doubled the intraoperative patient radiation dose. For a given imaging system configuration, radiation exposure to the patient depends on tissue type and the size of the imaged body part and patient effective dose on the different organs imaged. Therefore, cervical imaging generally is associated with lower radiation doses compared to thoracic or lumbar imaging.
Spine. 2020;45(1):E45-E53. © 2020 Lippincott Williams & Wilkins