In this study, the research team successfully established an AI prediction model using the RepVGG-CBAM model to determine SLN metastasis in breast cancer patients. The results on the test set were satisfactory, with the diagnostic performance exceeding the expert level. For SLN metastasis, the higher sensitivity were of greater clinical importance. Compared with experts diagnosis, in TDUS, CEUS and CBUS, the AI prediction models established in this study showed higher sensitivity. To our knowledge, this was the first study to use deep learning algorithms to directly learn ultrasound images of SLN in breast cancer to determine SLN metastasis.
Zhou et al [21] conducted a deep learning study on TDUS features of primary breast cancer lesions to indirectly predict ALN metastasis, and achieved good diagnostic performance with an AUC of 0.89. Zha et al [24] predicted SLN metastasis in breast cancer by combining various clinical and pathological factors of the primary lesion, and achieved an AUC of 0.833 on the test set. However, the aforementioned studies did not learn the imaging characteristics of the lymph nodes themselves. In clinical practice, analysis of lymph node imaging, especially SLN, directly and accurately determines the presence or absence of metastasis. Our study precisely addresses this issue. Zheng et al [19] used AI to directly learn multimodal ultrasound images of ALN to predict metastasis, further improving diagnostic efficiency. However, ALN includes non-SLN, which may increase the false negative rate for predicting ALN metastasis, and the study did not include contrast-enhanced imaging, which can improve the diagnosis of SLN metastasis [15]. In this study, the research team obtained TDUS and CEUS of SLN after lymph node enhancement and established an AI model to directly learn and diagnose SLN metastasis. The study simulated a real clinical environment.
SLNB has ushered in a minimally invasive era compared to traditional axillary lymph node dissection (ALND). However, SLNB required intraoperative waiting for frozen pathology results, which increased operative time and medical costs. In addition, frozen pathology results may not fully reflect the true presence or absence of SLN metastasis, thus increasing the risk of secondary surgery [25, 26]. Studies have shown that 15–20% of patients have SLN metastasis [7]. In this study, the SLN metastasis rate was 18.31% (178/972), which is similar to previous research reports [27]. The use of the model developed by the research team can further reduce the false negative rate in determining SLNs metastases and improve the accuracy of preoperative SLNs assessment. For patients identified by the model as having SLNs metastases, it serves as a reminder for clinicians to perform meticulous operations during surgery to avoid missing suspicious SLNs.
For the images identified as false negative by RepVGG-CBAM in the test set, including 4 patients (1 form CEUS, 3 form CBUS ). All the patients underwent ALND after SLNB. Postoperative pathology confirmed that the total number of axillary lymph node metastases on the affected side were 1 to 2, including 2 patients with only 1 micrometastasis. Studies have shown that for patients with 1 to 2 axillary lymph node metastases, axillary radiotherapy can achieve the same long-term prognosis as surgical resection [26]. Therefore, for patients planning to undergo postoperative axillary radiotherapy, SLN diagnosis using the model is expected to prevent some patients from undergoing invasive SLNB in the future, which deserves further study.
SLN metastasis can be divided into isolated tumor cell clusters (ITC), micrometastasis, and macrometastasis. Because ultrasound images cannot observe the microvascular perfusion of SLNs, the ultrasound features of ITC and micrometastasis often appeared normal. In actual clinical work, experts cannot accurately determine micrometastasis or even ITC, as well as other non-morphological features. The model in this study also misdiagnosed for two cases of micrometastasis. Due to the extremely limited number of such cases there are certain limitations. However, with the advancement of clinical diagnostic techniques, updates in AI algorithms, and an increase in the number of cases, the combination of molecular biology and advanced AI models holds promise for overcoming the current research limitations in the future.
AI has achieved practical results in the diagnosis of breast cancer and lymph node metastasis in its drainage area, several studies [19, 20, 28–30] have shown that AI has achieved the same or higher diagnostic efficiency than clinical experts. Compared to commonly used models such as ResNet [31], the RepVGG model was chosen as the foundation for this study. Its uniqueness lies in the use of the reparameterization concept, which converts the three-branch network structure during the training phase into an equivalent single-branch network during the inference phase. This approach significantly improved inference speed without compromising accuracy. In addition, considering the challenges of the ultrasound images used in this study, such as high noise and low image quality, the researchers incorporated the CBAM attention mechanism into the network structure of RepVGG during model design. This effectively enhanced the feature extraction capability of the neural network, thereby improving classification accuracy. In the context of image classification, the attention mechanism mimics human visual attention by further extracting and processing important "eye-catching" feature information. This helped the neural network focus on more important features, resulting in better performance.
The main reason for the significant increase in the number of training data in the CEUS dataset compared to the TDUS dataset was that the storage time of CEUS image videos was longer than that of TDUS videos, which was necessary to meet the requirements of actual clinical work. Therefore, more CEUS images were obtained through video segmentation. In this study, the negative data volume in the CEUS dataset was much larger than the positive data volume. This was because the study only included patients with cN0 status which reflected the clinical reality. To prevent overfitting of the model, the described data augmentation methods were applied to the positive data in this study, resulting in a relatively balanced distribution of the final training dataset.
In binary classification tasks, the output of a neural network was a probability value. In this study, the output of the model represented the probability of SLN positivity. Therefore, a threshold was needed, where if the output probability was below the threshold, it was classified as negative, and if it was above the threshold, it was classified as positive. The optimal range for the threshold can be determined based on the AUC. Considering the goal of the study to identify all true positives and minimize false negatives, a binary classification threshold of 0.2 was chosen after conducting comparative experiments. This means that if the model's output probability of SLN positivity for a particular image was less than 0.2, it was classified as negative, and if it was equal to or greater than 0.2, it was classified as positive.
This study also has several limitations. Firstly, this was a retrospective study, and the data analysis was limited to images obtained in the past. Secondly, this was a single-center study with a small sample size, and future multicenter studies are needed to fully validate and optimize the model. Furthermore, although all ultrasounds were performed by experienced experts using a standardized procedure, real-world clinical work depends on the different conditions of different patients, and the quality of images may vary somewhat.