This study confirmed the value of utilizing radiomics characteristics of PCTs extracted from CT images as well as clinical characteristics to accurately predict the CLNM among PCT patients.
The clinical-radiomics nomogram developed in this study will empower frontline medical practitioners to assess PCT with CLNM and formulate individualized treatment plans without the need for extensive prior experience in diagnostic imaging.
Studies have demonstrated that prophylactic central neck dissection (PCND) does not confer any significant benefits with regards to preventing local recurrence of PTC, and poses a higher risk of surgical complications [14]. The extent of surgery is primarily determined by the preoperative assessment of LNs, which is also a crucial factor that affects postoperative recurrence and overall prognosis. Performing PCND in cN0 patients is still a topic of debate, due to the increased risk of hypoparathyroidism together with RLNI associated with this surgical procedure [15]. Patients with preoperative evidence of lateral LNM generally necessitate a more aggressive regimen that includes lateral compartment LN dissection as well as high-dose radioactive iodine (RAI) therapy[16]. In the management of LNM, either performing therapeutic neck dissection or administering RAI ablation may pose a significant risk for morbidity and degrade patients' quality of life. In this context, minimizing the incidence of complications should be achieved through the judicious and selective use of surgical intervention on the CLNs to reduce the number of unnecessary procedures required. Henceforth, an accurate evaluation of CLNM prior to surgery is of paramount importance in selecting appropriate therapeutic interventions for PTC patients. Currently, there has been insufficient judgement on whether there is a spread of LNs in the neck, which makes it difficult to determine the appropriate surgical approach early on. Therefore, how to apply existing technologies to improve the accuracy of predicting CLNM has become a hot issue in clinical practice.
Presently, most medical practitioners utilize clinical characteristics for the prediction of the likelihood of CLNM among PTC patients. Thyroglobulin (TG) is a large glycoprotein that serves as a substrate in thyroid hormone synthesis [17]. It is secreted by thyroid follicular epithelial cells, both in their normal state as well as in well-differentiated malignant thyroid tumor cells[18].
Metastatic LNs often display observed traits of calcification, cystic necrosis, hyperechogenicity, a round shape, peripheral or mixed vascularity and absence of an echogenic hilum. US is the primary noninvasive imaging tool for evaluation of CLN status prior to the surgery. Neck US examination exerts a crucial effect on the staging of PTC. Numerous previous studies have extensively investigated the correlation between CLNM and US features of tumors. For instance, Zhan et al. identified that either a talle and wide shape or tumor size was predictive factors for LNM. In addition, the existence of calcification together with a smaller distance between the mass and the capsule have been suggested as potential indicators for LNM as well[19]. Nonetheless, the clinical utility of US in evaluating CLNM in cases of papillary Thyroid Microcarcinoma remains a topic of ongoing deliberation and debate among medical researchers and practitioners [1]. The effectiveness of preoperative US in detecting CLNM is constrained by the presence of the overlying thyroid gland, and such LNs often exhibit little or no abnormality on preoperative imaging or upon direct inspection during surgery. The detection rate of CLNM is suboptimal, largely attributed to the the limitations of US that prevent it from consistently visualizing the deep anatomic structures / areas that may be subject to acoustic shadowing from bone or air[20]. Furthermore, the accuracy of US examination in diagnosing lateral LNM is prone to interobserver variability and subjectivity since it is highly dependent on the skill and the expertise of the operator [16]. Moreover, US is operator-dependent, and it has been observed that nodes < 1 cm and located in proximity to the mandible may be missed by US[22]. Hence, the predictive accuracy of US in identifying CLNM before surgery may be limited. Although US has been reported to possess high specificity (80.5%-97.4%) in many studies, its sensitivity is relatively low, varying between 36.7% and 61.0%[21][22]. Several studies have attempted to address the significant variability in the sensitivity (10.9–94%) and specificity (69–90%) of US when detecting CLNM[23].
The CT holds several advantages when it comes to assessing CLNM, as it can effectively visualize LNs and illustrate their spatial their relationship to peripheral vessels[1]. Neck CT, as a supportive tool, can aid in the planning of thyroidectomy plus LN dissection, but it also has some potential drawbacks to consider. One of the major trade-offs is the exposure to ionizing radiation. Additionally, the use of iodinated contrast material in neck CT can delay or alter the timing of iodine ablation treatment(s), which is an important consideration for postoperative management of PTC. [9]. Moreover, Neck CT has demonstrated suboptimal sensitivity and specificity for detecting LNM in the central neck compartments. Therefore, we propose to utilize Radiomics technology to investigate the potential of utilizing whole thyroid CT radiomics for predicting the likelihood of LNM in cases of PTC. However, the above-mentioned studies are based on the analysis of primary lesions, which is not suitable for CLNM evaluation in multiple-lesion PTC. Moreover, the manually drawn primary lesion often cannot reflect the full extent of the lesion. Therefore, the whole thyroid CT radiomics approach has demonstrated its advantages in addressing these limitations [24].
High-quality and standardized imaging is the foundation of radiomics, and compared to traditional CT, DECT has lower radiation doses and can provide better image quality[25]. In this study, we used the vein-phase (50s after contrast injection) CT images acquired at 80kV for the radiomics analysis. The CT images at 80kV have higher contrast and lower signal-to-noise ratio, and the vein-phase CT images can better reflect the blood flow in the thyroid gland and primary lesions. The 5 radiomics features selected in this study were all features obtained from wavelet transformation, with 1 being first order feature and the other 4 being texture features. These features mainly reflect the values and distribution of image intensities within region of interest (ROI). The correlation of these features with CLNM may be due to the following reasons: (1) Previous studies have shown that patients with multiple PTC lesions or larger primary lesions are more likely to experience CLNM[26, 27]. These two conditions can increase the ratio of lower-enhanced primary lesions to higher-enhanced thyroid gland, resulting in uneven distribution of gray values within ROI. (2) LNM in PTC patients is associated with the increase of vascular endothelial growth factor-D (VEGF-D), which can promote tumor and glandal angiogenesis. These new vessels can increase the lymphatic venous linkage of the tumor, leading to more micro-metastatic cell clusters entering the lymphatic system[28]. Vein-phase CT images can directly reflect the blood flow in the gland and primary lesions, highlighting the texture differences among different groups of ROI. (3) It has been suggested that chronic lymphocytic thyroiditis can cause extensive infiltration of immune cells in thyroid tissue, thereby exerting regulatory effects on the tumor microenvironment and enhancing anti-tumor immune response, thus limiting the occurrence of CLNM in PTC patients. Chronic lymphocytic thyroiditis on enhanced CT is manifested as decreased and uneven enhancement of the gland, which can also result in changes in ROI gray values.
Among the three radiomics models constructed based on the five radiomics features, LR model showed the optimal performance, revealing the AUC to be 0.656 in the training set and 0.631 in the testing set. In previous studies, Shenshasa et al. constructed an LR model based on wavelet texture features of CT vein-phase images of primary PTC lesions, which had a slightly lower AUC of 0.602 compared to the LR model in this study. Li et al. applied six classifiers to construct a radiomics model on the basis of CT images of primary PTC lesions to predict CLNM, and the optimal model had better performance (AUC = 0.709) than the model in this study[29]. Yang et al. also used six classifiers to construct a prediction model for LNM in PTC patients, achieving a much higher performance in the testing set with an AUC of 0.859 compared to the model in this study.
Compared to the aforementioned studies, the advantages of this study include: (1) using the entire thyroid gland instead of primary lesions as ROI, which reduces the subjectivity of manual delineation and can more objectively reflect the overall extent of the disease; (2) widening the inclusion criteria of cases, no longer limited to the study of single PTC cases, resulting in a model with stronger generalization ability. Taking the previous findings into account, we identified the risk factors with higher predictive potentials for CLNM in our analysis, and selected those factors to be included in constructing the nomogram model.
There were several limitations in this study. Due to the complexity of the omics features extracted from whole thyroid gland, it was challenging to select more specific features, but the predictive efficacy of the nomogram can be augmented. In future, our research team will attempt (1) to incorporate non-contrast CT images based on venous phase CT images to enrich the omics features; (2) to construct a CLNM prediction model using clinical data, CT and US data, dual-energy CT quantitative parameters, etc., and combine it with the omics model; (3) to utilize more and updated imaging omics classification models such as XGBoost; (4) to explore the feasibility of predicting CLNM using deep learning methods including convolutional neural network.