In this study, we generated a nomogram to predict metastatic LNs based on nodal size and elasticity values such as mean stiffness and elasticity ratio of harvested LNs on B-mode ultrasound with ex vivo SWE using data from our previous study. Thereafter, we tested the predictive ability of the nomogram in an independent set consisting of sentinel LNs. The performance of constructed nomogram to predict sentinel LNs metastasis was high in terms of good discrimination and calibration.
Cancer researchers, clinicians, and the public are becoming increasingly interested in statistical models designed to predict the occurrence or the outcome of cancer, along with the efficacy of treatments [25,26,27]. Among several prediction models, nomograms have been shown to provide personalized reasonable risk estimates that facilitate management-related decisions [27]. Indeed, our nomogram can be used to calculate the probability of each axillary LN metastasis easily and rapidly. When the total point calculated using the nodal size, mean stiffness, and elasticity ratio is over 35 points, the probability of LN metastasis exceeds 90%. If the total point is over 48 points, the probability of LN metastasis exceeds 99%. Indeed, the mean total point in the nomogram was significantly different between the non-metastatic and metastatic sentinel LNs (Supplementary Table 3). In the metastatic sentinel LNs, the probability of LN metastasis was approximately 70%, as the mean total point was 28 points; in the non-metastatic sentinel LNs, the probability of LN metastasis was less than 10%, as the mean total point was 8 points.
The discriminative power of the nomogram was quantified using the AUC exhibiting the accuracy of the test, with an AUC of 0.5 being defined as non-informative; 0.5–0.7, fair; 0.7–0.9, good; and > 0.9, excellent. Our nomogram revealed good discriminative power with the AUC of 0.856 in the development cohort and 0.791 in the validation cohort. In addition, the calibration plot showed good agreement between the observed and predicted probabilities in both the development and validation cohorts. Therefore, our constructed nomogram was suitable in predicting the probability of metastasis among the harvested sentinel LNs.
In the validation cohort, the ultrasound and SWE characteristics of sentinel LNs were not different between histologic types of breast cancer in line with previous report [28]. The mean stiffness and elasticity ratio of the metastatic sentinel LNs were significantly higher than those of the non-metastatic LNs. These findings are similar to those of the development cohort in our previous study. The nodal size tended to be larger in the metastatic LNs than in the non-metastatic LNs, although the trend was not significant. It was presumed that the proportion of the metastatic LNs was higher in the development cohort than in the validation cohort (Table 2). Further research with a larger number of axillary LNs is needed to investigate the accurate relationship between the nodal size and LN metastasis.
Recently, the American College of Surgeons Oncology Group Z0011 prospective, randomized clinical trial changed the standard approach to axillary surgery, showing that the omission of ALND was possible in early breast cancer patients who underwent breast conserving surgery and adjuvant systemic therapy even with 1–2 metastatic sentinel LNs [29, 30]. Several previous trials also reported consistent findings with those of the Z0011 trial [2, 31]. Since these trials presented that ALND could be omitted even in patients with positive sentinel LNs, the need for intraoperative frozen section analysis of sentinel LNs is questionable. In fact, not only ALND but also intraoperative frozen section analysis of sentinel LNs in patients who underwent breast conservative surgery was declined after the Z0011 trial was published [32, 33]. Furthermore, Noordaa et al suggested that omitting intraoperative pathologic assessment of LNs was a reasonable option in patients with a low nodal burden, such as clinically node-negative breast cancer, who were treated with upfront surgery [34].
However, one of the major disadvantages of omitting intraoperative frozen section analysis is secondary surgery for ALND. To date, the exact rate of secondary surgery for ALND after skipping intraoperative frozen section analysis has not been well researched. Nevertheless, concerns of secondary operation for ALND can be reduced if intraoperative pathologic examination was performed for sentinel LNs in suspicion of metastasis, as identified by our nomogram.
Our study has some limitations. First, the cutoff value of nomogram to determine the intraoperative pathologic examination was not clarified in this analysis. Further prospective studies are needed to verify this issue. Second, the rate of metastatic LNs in the validation cohort was lower than that in the development cohort. It was probably because only sentinel LNs were included in the validation set. The number of metastatic LNs was 15 in the validation cohort of our study; nevertheless, these events were statistically sufficient to perform calibration and discrimination to warrant the use of our nomogram. Finally, concerns about the reproducibility of SWE still remained because interobserver or intraobserver agreements for the elasticity values were not assessed in this study. However, the other previous study showed reliable intraobserver and interobserver reproducibilities of SWE [35, 36]. In addition, the reproducibility of SWE was expected to be higher in our study because specialized radiologists examined the LNs by ex vivo SWE that was less likely to interfere with surrounding tissue compared with in vivo SWE. Hence, it might be expected to precisely perform intraoperative ex vivo SWE of excised LNs within a short time.
In conclusion, our well-validated nomogram can be applied to predict nodal metastasis during SLNB. The clinical application of nomogram, which is mainly based on ex vivo SWE values, may help in reducing unnecessary intraoperative frozen section analysis as well as secondary operation rate in breast cancer patients who underwent SLNB.