4. Diagnostic value of SLNB
Among the 325 patients, 46 (12.13%) had 133 metastatic lymph nodes. A total of 44 patients had SLN metastases, thereby accounting for 95.65% of metastatic cases. Of the 325 patients in whom SLNs were successfully identified, 26 patients had SLN metastasis and PLN metastasis, 18 patients had SLN metastasis but no PLN metastasis (including two patients with micro-metastasis in SLNs), two patients had only PLN metastasis and no SLN metastasis, and 279 patients had no metastasis in the SLNs and PLNs. As a result, the sensitivity of SLNB when used to identify nodal metastatic disease was 92.86%. Among the 281 patients with negative SLN results, 279 had true-negative non-SLNs, which resulted in a negative predictive value (NPV) of 99.29%. Two patients had negative SLNs but positive non-SLNs, yielding a false-negative rate (FNR) of 7.14% (Table 2). The common characteristics of the two false-negative cases were tumour diameter larger than 20 mm and the presence of DSI and LVSI (Table S3, Figure 1D). We analysed the subgroup of patients with tumours smaller than 20 mm (267 cases) and obtained an ideal FNR of 0% and higher DR (97.75%), sensitivity (100%), and NPV (100%). Compared to the ≥20-mm-tumour-size subgroup, the specificity, NPV, and accuracy were markedly improved (Table S4). Furthermore, we found that lymph node metastatic status was significantly associated with tumour size (p = 0.000), depth of stromal invasion (p = 0.028), LVSI (p = 0.000) and stage (p = 0.000), and multivariate analysis showed that tumour size, stage, and LVSI are independent risk factors for lymph node metastasis (Tables S5 and S6).