Thyroid carcinoma is a common endocrine malignant carcinoma, and PTC has the highest incidence of all thyroid carcinomas. PTC shows low malignant growth with a low incidence of distant metastasis and a low mortality rate; however, CLNM tends to occur at the early stage . Lymph node metastasis in PTC is related to the diameter, location, number, and invasive growth of the primary tumor [14–15]. There is no uniform conclusion on the correlation between gender, age, pathological type and lymph node metastasis [16–17]. Ultrasonography is the primary examination method for the thyroid gland, but the detection of cervical lymph nodes by US is 18.8%-31%, and is limited due to interference from the trachea, esophagus, osseous tissue, underlying thyroid diseases and the examiner’s experience [18–19]. Therefore, evaluation of the correlative factors of CLNM in PTC has great clinical value. Previous similar studies have not provided consistent conclusions, as some included fewer cases, some included multifocality, and some included complex parameter characteristics. This study only enrolled unifocal PTC and analyzed the clinical information and US measurements. A stratified study is necessary, with additional subsequent and multiple ultrasonic signs, TI-RADS (Thyroid Imaging, Reporting and Data System), multifocality, pathological types and so on, in order to obtain more complete and systematic research results.
PTC is more common in female patients, with a male to female ratio of approximately 1: 3, and the ratio in this study was 1: 3.46. Mao et al.  and Heaton et al.  reported that women and elderly patients were at risk of PTC, while men and younger patients were at risk of CLNM. Sun et al.  confirmed that men had an increased risk of central neck lymph node metastases. In this study, CLNM occurred in 42.5% (127/299) of male patients, and occurred in only 29.4% (305/1036) of female patients, which also suggests that male patients have a higher risk of lymph node metastasis. This may be related to high hormone levels in male patients, which is consistent with results reported in the literature.
PTC occurs in all age ranges, with a high incidence between the age of 30 and 60 years. The mean age in the positive group included in this study was younger than that in the negative group, and the difference was statistically significant (P < 0.001), which was also consistent with previous reports where age was an independent risk factor for CLNM in PTC [23–24]. We used 55 years as the threshold in this study according to the TNM staging system for thyroid cancer , and 35.4% (371/1048) of patients who were younger than 55 years had CLNM, and among those who were 55 years or older, lymph node metastases occurred in only 21.2% (61/287) of patients in this study. This was consistent with the study reported by Zhou et al. , where age was an independent risk factor for CLNM, and the risk of CLNM in patients who were younger than 55 years was 2.6 times that of patients who were 55 years or older.
The maximum tumor diameter is an important reference index for PTC treatment protocols and the range of surgery . The diameter of the lesion was closely related to invasion of the tumor. The growth of a malignant tumor is a process of self-proliferation and external invasion, the range of invasion continues to expand, and the contact area between the cancer focus and the capsule, blood vessels, and lymphatic vessels of the thyroid also increases , thereby the incidence of lymph node metastasis also increases . A retrospective analysis based on large samples has shown that CLNM tended to occur in patients with a maximum tumor diameter of 10 mm or larger [26–27]. The results demonstrated that the maximum tumor diameter in the positive group was about 1.7 times that in the negative group. The tumor seemed to be ellipsoid, and the volume as the evaluation parameter made the result more objective and scientific. The tumor volume in the positive group was about 3.5 times that in the negative group. Tumor volume was significantly different between the only central metastasis group and the central and lateral metastasis group (P < 0.001). For larger tumors, cervical lymph nodes should be examined in order to improve the detection rate of CLNM. In particular, in patients with large volume tumors, central or both central and lateral lymph node metastasis should be determined in advance. If both of these examinations were added to routine thyroid and cervical lymph node ultrasonic screening, it would provide a guide for performing FNA for the detection of CLNM before surgery.
An aspect ratio ≥ 1 is a highly specific index for the diagnosis of malignant thyroid nodules [28–29]. Zhan et al.  showed that the sensitivity of the aspect ratio in the differential diagnosis of benign and malignant thyroid nodules decreased gradually as the volume of thyroid nodules increased, and an aspect ratio ≥ 1 was more significant for the diagnosis of PTC with a smaller volume. Nam et al.  divided 488 cases of PTC into two groups with one group having malignant ultrasonic signs, including an aspect ratio > 1, solid mass with low echo, microcalcification, and a blurred boundary, while the other group had no malignant ultrasonic signs. A comparison between these two groups showed that patients with PTC and malignant signs were more prone to CLNM. Studies by Zhou et al.  showed that an aspect ratio > 1 in 1174 cases of unifocal PTC was a risk factor for CLNM. Deng et al.  reported that no statistical significance was seen in 908 PTC patients with an aspect ratio ≥ 1 in predicting cervical lateral lymph node metastasis. Combined with previous literature, on the one hand, there was no distinction between the transverse or longitudinal aspect ratio; on the other hand, the correlation between the aspect ratio and PTC CLNM was inconsistent.
According to the morphology of the tumor, univariate analysis demonstrated that the cross-sectional aspect ratio and longitudinal-section aspect ratio were both statistically significant (P < 0.001) in this study, while logistic regression analysis showed that there was no statistically significant difference between the two groups. the main reason was that PTMC patients were 65.5% (874/1335) in the study, just 20.7% (181/874) were in the positive group. We compared cross-sectional and longitudinal section aspect ratio for PTMC and PTC patients excluding PTMC, it could be concluded that cross-sectional aspect ratio had a better predictive value for CLNM in PTC excluding PTMC, compared with the longitudinal aspect ratio. There was less relevant literature with the association between cross-sectional and longitudinal section aspect ratios in predicting cervical LN metastasis, especially for PTC patients excluding PTMC. We will collect more data for further research in the future.
The limitations of this study were as follows: 1. This was a single-center retrospective study including unifocal PTC and lymph node dissection performed in the central area,which may have introduced subjective bias. 2. Cases with metastases in lateral location were not adequate, large samples are required to study the cervical metastases in different parts. For skip lateral lymph node metastases, more effective preoperative assessment should be adopted. 3. This is the preliminary study for large sample size of PTC patients, we will add detailed TI-RADS, clinical and pathological staging, subdivided pathological types, machine learning models for further research.