MTC accounts for an extremely small proportion of thyroid carcinomas, it is a moderately malignant carcinoma, and is derived from parafollicular cells. Its biological characteristics and degree of malignancy differ from other types of thyroid carcinoma derived from thyroid follicular cells. MTC is prone to recurrence after surgery. In this study, the recurrence rate after surgery was 23% (17/74), which was lower than the recurrence rate reported in previous studies (44.1%-47.3%) [4-5].
In terms of general information, Zhang Zaixing et al.  using univariate analysis found that gender (female) and age (≥45 years) were important factors affecting survival (recurrence, death, etc.) (P <0.05). Using multivariate analysis, Hassan et al.  found that gender (male) was a poor prognostic factor (recurrence, death, etc.) in MTC patients (P <0.05). In this study, the recurrence group and the non-recurrence group were more often male and young (≤ 52.5 years old), and there was no statistical significance between gender, age and recurrence (P> 0.05). The conclusions obtained in this study are partially consistent and partially contradictory with the conclusions of the above studies, which are related to the different definitions of the endpoints of these studies and the small number of cases.
As previously mentioned, MTC is derived from parafollicular cells that secrete calcitonin, and has the characteristic of elevated calcitonin. Some studies have found that patients with MTC who have elevated calcitonin after surgery were more prone to cervical lymph node metastasis and recurrence . Using multivariate analysis, Hassan et al.  found that postoperative calcitonin doubling time in MTC patients was less than two years and that the rate of increase in calcitonin level was greater than 0.05 pg/ml/month and indicated a poor prognosis (P <0.05). A meta-analysis showed that procalcitonin can be used as a marker to monitor recurrence during postoperative follow-up in MTC patients . In this study, single factor KM survival analysis showed that preoperative calcitonin level > 565.8 pg/ml, and postoperative calcitonin level > 45.0 pg/ml were risk factors for the recurrence of MTC (P <0.05, see Table 1). Only 60 cases had preoperative data on calcitonin level and only 70 cases had postoperative data on calcitonin level; thus, they were not included in the multivariate analysis. The conclusion that there was statistical significance between elevated calcitonin (or procalcitonin) and the recurrence of MTC patients is not new, and the specific values of calcitonin (or procalcitonin) that affect recurrence are not the same in each study. This is related to the limited number of cases enrolled in various studies, the large age span, and different test levels.
Starting from the ultrasonic features of MTC, research on the relationship between MTC and cervical lymph node metastasis is relatively extensive, while there are relatively few studies on the relationship between preoperative ultrasonic features and recurrence of MTC. Studies have shown that in sporadic MTC, single-factor chi-square analysis demonstrated that patients with tumors > 15 mm, irregular morphology, sharp edges, and masses located under the capsule have a higher risk of lateral neck lymph node metastasis (P <0.05) . The ultrasound characteristics of MTC include the size and number of tumors, echo, composition, boundary, morphology, invasion of the capsule, aspect ratio, calcification, blood flow, etc. In this study, single factor KM analysis showed that a tumor＞ 40.0 mm, capsular invasion, and metastatic cervical lymph nodes were risk factors affecting the recurrence of MTC (P <0.05). These three factors represent the size of the tumor, the relationship with the surrounding adjacent tissues, and the status of the lymph nodes, which can be further applied to the preoperative T and N stages of MTC patients. Some studies have shown that T4 is a poor prognostic factor in multivariate analysis , and studies have shown that TNM staging affects prognosis in single factor analysis . The results of the multivariate Cox regression analysis in this study showed that metastatic cervical lymph nodes were the only independent risk factor affecting the recurrence of MTC. In addition, this study found that in the recurrence group and the non-recurrence group, the tumors were mostly characterized by solidity, low echo, clear boundaries, and calcification. There were no statistically significant differences between the ultrasound features such as the composition, echo, boundary, calcification and recurrence of MTC.
In this study, the results of univariate KM analysis showed that: tumor size > 40.0 mm, capsular invasion, metastatic cervical lymph nodes, preoperative calcitonin > 565.8 pg/ml, and postoperative calcitonin > 45.0 pg/ml were risk factors for MTC recurrence (P <0.05). The results of multivariate Cox regression analysis showed that metastatic cervical lymph nodes (HR=5.368, 95%CI 1.063-27.104, P=0.042) were independent risk factors for the recurrence of MTC. MTC patients with metastatic cervical lymph nodes are more likely to relapse. Although calcitonin was not included in the multivariate analysis, preoperative calcitonin >565.8 pg/ml and postoperative calcitonin >45.0 pg/ml also indicated the likelihood of relapse.
In this study, a total of 74 patients with MTC, including 17 cases with recurrence during follow-up and 57 cases with non-recurrence were included. Two patients who died due to other diseases were not included in the study. The findings of this study indicate that MTC patients with metastatic lymph nodes, as shown by ultrasound, are prone to postoperative recurrence of MTC. In addition, MTC patients with tumor size > 40.0 mm, capsular invasion, preoperative calcitonin level > 565.8 pg/ml, and postoperative calcitonin level > 45.0 pg/ml are more likely to have postoperative recurrence of MTC.
However, no other ultrasound features such as the composition, echo, boundary, and calcification were found to be related to MTC recurrence. In addition, the number of cases with MTC recurrence in this study did not reach half of the total number of cases, and the median survival time could not be determined in the K-M survival analysis. A future follow-up study is planned.