Feasibility and diagnostic performance of dual-tracer-guided sentinel lymph node biopsy in cT1-2N0M0 gastric cancer: a systematic review and meta-analysis of diagnostic studies

To the best of our knowledge, this is the first meta-analysis on the detection rate and diagnostic performance of SLN biopsy guided by dual tracers in cT1-2N0M0 gastric cancer.

Our meta-analysis showed that the use of dual tracer in SLN mapping got a high pooled identification rate and sensitivity (97.0 and 89.0%). As reported by previous meta-analysis, radioactive isotope alone got a pooled identification rate and sensitivity of 92.1 and 76.4% while dye alone were 92.1 and 72.7% [32]. So, it seems that the diagnostic performance of dual tracer is the most excellent. The dye-guided method has the merits of safety, cheapness, and convenient injecting procedure and the ability of detecting not only lymph nodes but also lymphatic vessels. However, it is associated with a lower identification rate due to the rapid transit of the dye after injection and the blind sites in dense tissue [33]. On the other hand, the radiopharmaceutical-guided method has several advantages such as the objectivity of detecting SLN by quantitative evaluation of sentinel node radioactivity even in thick adipose tissue and suitability for laparoscopic surgery because of its longer deposition in lymph node than dye. But it also has several disadvantages such as high radioactivity interference at the primary injection site when detecting nearby SLNs and disability of visualizing the lymphatic vessels [34]. Thus, the combination of two kinds of tracer may be more reliable.

We performed subgroup analysis in terms of preoperative T stage. However, our results showed that the sensitivity of the cT1 subgroup was lower than that of the cT2 subgroup. This was different with the results of previous studies [32]. We could not find any explanations for this result: we reviewed the studies of cT1 subgroup and found most of the false-negative cases invaded muscularis propria according to the results of pathology detection. For instance, in the study by Niihara et al. which was classified as the cT1 subgroup, although there were three false-negative cases, the result of pathologic detection showed that two of them were pathologically T2 stage [15]. And in another cT1 study by Lee which was published in the year 2006, although there were five false-negative cases, the result of pathologic detection showed that four of them were pathologically T2 stage [27]. When tumors invade muscularis propria, the lymphatic drainage may be obstructed or altered and novel lymphangiogenesis makes lymphatic drainage more complicated. This may explain the reason why the sensitivity of SLN biopsy becomes lower in pT2 stage. Our results demonstrate that the accurate T stage before operation is important to optimize SLN biopsy and the clinical application of SLN biopsy should be limited to the patients of cT1N0M0 gastric cancer.

Because there were no uniform types of dual tracers in the included studies, we tried to figure out the effect of different types of dual tracers on the sensitivity and negative LR. The subgroup analysis showed that the combination of ^99mTc-human serum albumin and ICG and the combination of ^99mTc-antimony sulfur colloid and ICG had the highest sensitivity and satisfactory identification rate. The following points may be the reasons: firstly, both radioisotope ^99mTc-human serum albumin and ^99mTc-antimony sulfur colloid have a small particle size (≦200 nm) and have the advantages of migrating faster than ^99mTc tin colloid, being easy in predicting the proper time to probe, being able to be injected simultaneously with the dye, and being very stable. In addition, the tracer ^99mTc-human serum albumin has the merits of being biodegradable and reduced potential for allergic reactions [35, 36]. Secondly, the dye ICG was also reported to have a high detection rate and sensitivity. It has the merits of cheapness, convenience, being able to detect bright sentinel nodes in dense fat, signal stability, and clear visualization of lymph nodes and lymphatic canals. Besides, the recent technology such as HyperEye Medical System can simultaneously detect color and near-infrared rays of ICG and can be used under room light [37, 38]. However, the strength of this subgroup analysis is limited by its small sample size, and we hope that more relevant studies will be carried out in the future for a more precise evaluation.

We found that performing SLN biopsy ≥15 min after dye injection had a much higher identification rate and sensitivity than those of <15 min group. This result suggests that it is very important to control the time performing SLN biopsy because there will not be sufficient visualization of SLNs and lymphatic basins if researchers do biopsy in a hurry. However, the dye may diffuse and saturate the tissues which will make mapping difficult if it takes more than 20 min after dye injection [39, 40]. As for radioactive tracer, ^99mTc-tin colloid has been reported to be injected ranging from 2 h to the day before surgery while ^99mTc-human serum albumin is usually injected less than 30 min before surgery due to its small particle size [20].

In the meta-analysis by Ryu et al., sensitivity was found to depend significantly on the number of SLNs that were harvested [41]. Our result is consistent with the conclusion of this previous meta-analysis, as the pooled sensitivity for SLN number ≥5 group showed a significantly higher sensitivity than did the SLN number <5 group. These results can be explained by the complicated lymphatic drainage system of the stomach. Thus, as many SLNs as possible are necessary in the clinical application of SLN biopsy in gastric cancer.

Two methods to retrieve SLNs have been reported: the pick-up method and the basin dissection method. The gastric lymphatic basins were divided into five different directions along the main arteries. The dissected lymphatic basins contain lymph nodes and lymphatic vessels that are hot or stained with dye [20, 42]. Our subgroup analysis found that the basin subgroup had a much higher sensitivity than those of the pick-up group. This factor may be explained by the following reasons: when the lymphatic duct was dyed with invisible SLNs, basin dissection may solve this problem. And when SLNs are undetectable as the hot areas of them are close to the primary tumor, basin dissection may decrease false-negative rate. Moreover, a previous study also showed that more SLNs can be found using basin dissection than the pick-up method [25].

Our subgroup analysis showed that the sensitivity of the HE and IHC combined intraoperative pathologic detection method was lower than that of the HE alone group. This result is different from the result of a previous meta-analysis by Wang et al., who found the sensitivity of SLN biopsy by HE and IHC to be higher than that by HE alone (80.8 vs 73.7%) [32]. And the inverse result is explained by the following reasons: in the study by Kitagawa et al. which was classified as the HE subgroup, it was reported in the final result that only four false-negative cases were observed. However, it was also reported in the paper that this final result excluded nine patients who showed sentinel nodes negative in intraoperative examinations but positive in postoperative examinations of permanent tissue sections [20]. When it takes the sensitivity of intraoperative pathologic detection by frozen tissue sections into account, the pooled sensitivity of HE alone is 84.5%. And this is lower than the HE and IHC combined subgroup whose pooled sensitivity is 86.4%. Therefore, it is very important to improve the diagnostic accuracy of intraoperative pathologic examinations of frozen tissue sections. Except for combination of the HE and IHC method, multistep level sections, reverse transcription polymerase chain reaction (RT-PCR), and the one-step nucleic acid amplification assay (OSNA) have all been developed to improve the sensitivity of intraoperative pathologic diagnosis and detect SLN micrometastases. Moreover, many of these novel methods can be completed in 30 min which is convenient for intraoperative application [43].

Our subgroup analysis showed that the laparoscopic SLN biopsy got lower identification rate than the opening SLN biopsy. And this result may be due to the limited operation space and insufficient experience of surgeon when conducting laparoscopic SLN biopsy [44]. Despite these shortcomings, our subgroup analysis demonstrated that the sensitivity of the laparoscopic SLN biopsy was higher than the opening SLN biopsy. This may be explained by the more detailed observation with visual magnification during laparoscopic SLN biopsy [45]. Moreover, it was reported by previous study that laparoscopic surgery could decrease operative blood loss and hospital stay time as well as increased postoperative quality of life in gastric cancer patients compared with opening surgery [46]. We can anticipate that the laparoscopic SLN biopsy will have more and more advantages over opening SLN biopsy with the development of laparoscopic instruments as well as the improvement of experience of surgeons.

We assumed that the experience of surgeons and the application of dual-tracer-guided SLN biopsy would have improved with time. Thus, we set the year 2013 as the cut-off point to differentiate later and earlier studies. Improved identification rate and sensitivity were seen in the ≥2013 year subgroup. It was reported that sensitivity ranging from 90 to 95% was recommended for the feasibility of SLN biopsy clinical application in breast cancer [47]. As gastric cancer responds less to systemic adjuvant therapy or radiotherapy than breast cancer, some investigators argued that sensitivity of SLN biopsy should be higher for clinical application in gastric cancer than breast cancer. Therefore, although sensitivity of dual-tracer-guided SLN biopsy was significantly higher in the later studies than the earlier studies, the diagnostic performance of it in gastric cancer still need to be improved before finally putting into clinical application. Besides, lack of the approved area for injection of radioactive tracer and special instruments in the general hospitals is also an obstacle for the clinical application of this technique. Therefore, the single dye ICG-guided SLN biopsy has also been developed as a promising technique for gastric cancer recently. With the novel ICG fluorescence systems such as the D-light P system, the technique can be used in the room light and the laparoscopic surgery. It was reported in the small-scale studies that the identification rate and sensitivity of the technique could reach to 100% [37, 48, 49]. However, a large-scale multicenter clinical trial by the Japan Clinical Oncology Group reported that the detection rate of green nodes was 97%. However, the rate of false-negative was 46.4% which was surprisingly high [50]. Thus, further large-scale trials are needed to evaluate the clinical efficacy of this single dye method.

There were several limitations in our meta-analysis. First, the sample sizes of most of the included studies were small which may cause bias. Second, tumor size, pathologic type of tumor, and standard of preoperative clinical tumor stage between studies were not uniform, and all of which will affect the diagnostic performance of SLN biopsy. Moreover, all the involved studies are conducted in Japan and South Korea, and the performance of it in other countries is uncertain.