To date, the highest quality of evidence on the effect of NIRAF comes from a multicenter randomized controlled study published by Benmiloud et al[4], which showed that the rate of postoperative low calcium was significantly lower than that of the control group (from 26–11%). The rate of postoperative hypocalcemia after using NIRAF in our study was 20.6%. Our study included a higher proportion of patients with malignant tumors (83.2% vs 25.6%), and lymph node dissection was performed in most patients (186 of 238 patients, 78%), which led to a higher rate fo postoperative hypocalcemia. However, the mean number of PGs found on each side of the thyroid in their study was 1.61 (390 PGs of 242 patients), compared to 1.80 (595 PGs of 331 patients) in our study. We found more PGs benefiting from the use of nanocarbon but the incidence of postoperative hypocalcemia increased, which indicated that the scope of the operation and protection of the parathyroid blood supply also had a crucial impact[13, 14]. The effect of NIRAF is limited to the identification of PGs. Therefore, in evaluating the effect of this technique, the number of identified PGs during surgery seems to be more appropriate than the incidence of postoperative hypocalcemia.
The innovation of this study is the introduction of aIOPTH as a confirmatory method for PGs. The application of aIOPTH may provide an alternative to frozen section examination, dodging the injurious effect of the bioptic process and achieving similar or even more accurate results[15–17]. However, the application of aIOPTH is not widespread, many endocrine surgeons are not familiar with this technology, and traditional PTH detection methods take much time[18]. The rapid PTH detection method used in this study only takes approximately 10 minutes. In actual use, the test strip can turn red in an even shorter time with a positive result. When the cut-off value is 130 pg/mL, this method has proven to be highly consistent with pathological results in identifying PGs[19, 20]. It has already been widely used in China and written in the 2018 perioperative parathyroid function protection guidelines for thyroid surgery by the Chinese Thyroid Association. Comparing the pathological results, aIOPTH achieved 100% specificity in our study.
To date, most studies have used visual recognition to confirm PGs due to ethical limitations. The results showed that NIRAF had extremely high sensitivity in identifying PGs (from 76–100%)[5, 8, 21]. Most studies found that NIRAF could detect PGs before vision in nearly two-thirds of cases[4, 8, 22]. Our study concluded that the sensitivity of NIRAF for identifying PGs was 95% by combining aIOPTH results and pathological results, and in the inconsistent cases between vision and NIRAF, the accuracy of NIRAF was 90.84%, while the accuracy of vision was only 67%, indicating that NIRAF was more trustworthy. Among the coconfirmed PGs, 88 PGs were first detected by NIRAF, and 78 PGs were found only by NIRAF. Therefore, of the 596 PGs found in the trial, the surgeon benefited from NIRAF in identifying 166 (28%) PGs.
By comparing the fluorescence intensity of different tissues, we found that the autofluorescence intensity of the thyroid gland was closest to that of the PG (74.19 ± 17.82 vs 63.24 ± 17.53). This is concordant with a study by Falco et al[21]. The phenomenon that the autofluorescence intensity of the thyroid gland occasionally approaches or even exceeds the PG during surgery often affects the detection of PGs, and we suppressed the autofluorescence of the thyroid by injecting nanocarbon into the thyroid gland, thereby improving the fluorescence contrast of the PGs.
Of note, there was a significant difference in the autofluorescence intensity of the parathyroid and lymph nodes (74.19 ± 17.82 vs 33.97 ± 10.64), which is consistent with the result from Shinden et al[23]. Therefore, no lymph nodes were found in the 50 NIRAF false-positive tissues. Furthermore, we used NIRAF to sort out the tissues with no obvious fluorescence in lateral neck lymph node dissection, in which 536 lymph nodes were found pathologically. Meanwhile, no lymph nodes were found in the tissues with strong fluorescence intensity. We conclude from this result that the fluoresence intensity of lymph nodes is pretty weak compare to that of PGs. Lymph nodes are often misidentified as PGs during thyroid surgery. Of the 196 visual false-positive tissues in this study, 167 were lymph nodes. The surgical strategy of retaining all tissues with strong fluorescence intensity will help ensure a thorough dissection of lymph nodes, thereby reducing the tumor residue caused by misidentifying lymph nodes as PGs.
A total of 351 fine-needle aspirations were performed in 238 patients. According to the final result, the tissues with strong fluorescence intensity are mostly parathyroid and adipose tissues, thyroid tissues appear only in rare cases, which can be retained without further testing. During the trial, even if we identified a PG in a certain area, we still tried to find other suspicious parathyroid tissues. It is not necessary to do so in clinical applications, so the frequency of aIOPTH can be largely reduced, thereby shortening the operation time. The use of NIRAF requires only a small amount of time to remove the surgical lamp. Therefore, the use of NIRAF and aIOPTH technology in thyroid surgery will not significantly increase the duration of surgery.
To date, NIRAF has satisfied but not perfect sensitivity and specificity in published studies[24, 25]. However the strong fluorescence of thyroid sometimes may cover the fluorescence of PGs and thus decrease the sensitivity of NIRAF. The use of nanocarbon can solve the problem. Meanwhile aIOPTH can separate false positive tissues with high fluorescence from PGs and enhance the specificity. All these measures are time saving and easy to implement. These methods along with NIRAF form a perfect complement in thyroid surgery, which can effectively find atypical PGs and prevent them from being inadvertently resected.
This case series study, despite its careful experimental design, strict quality control, and large number of patients, is still not enough to show that this new surgical strategy can improve the number of identified PGs. Therefore, we plan to conduct a further randomized controlled study to illustrate this issue. Another drawback is that we did not puncture the PGs identified by both vision and NIRAF, which is mainly based on the purpose of reducing parathyroid damage. Although fine-needle biopsy poses minimal damage to the tissue, there is still a report that such puncture may cause secondary lesions of the PGs[26].