Lymph Node Positive Rate: An Ignored Factor Affecting the Prognosis of Medullary Thyroid Cancer

To investigate the relationship between the positive rate of examined lymph nodes and the prognosis of patients with medullary thyroid cancer (MTC). Method Information on demographic and clinicopathological characteristics of patients with medullary thyroid cancer was extracted from Surveillance, Epidemiology and End Results (SEER) database of the National Institutes of Health (NIH) between 1998 and 2015. Lymph Node Positive Rate (LNPR) is dened as the number of positive lymph nodes divided by the total number of lymph nodes removed. Eligible MTC patients were divided into four LNPR groups (0-0.25, 0.26-0.50, 0.51-0.75 and 0.76 -1.00). Compare the overall survival rate (OS) and cancer-specic survival rate (CSS) calculated by the Kaplan-Meier method between the four MTC patient groups and Perform univariate and multivariate analyzes to assess the relationship between Lymph Node Positive Rate and prognosis of the MTC patients. CSS and OS rates [(CSS: HR 1.77, 95% - CI 0.99 3.19, P = 0.045); (OS: HR 1.69, 95% CI 1.03-2.79, P = 0.038)] and group (LNPR: 0.76-1.00) correlated signicantly with poorer CSS and OS rates [(CSS: HR 2.09.95% CI 1.12-3.92, P = 0.021); (OS: HR 2.23, 95% CI 1.32 -3.76, P = 0.003)].


Introduction
Medullary thyroid cancer(MTC) is a rare and aggressive kind of thyroid cancer that is derived from parafollicular C cells of the thyroid, accounting for 2-4% of the total frequency of thyroid cancer [1,2]. In recent years, the prevalence of MTC has gradually increased. According to reports, the average annual rate of change in the incidence of MTC is as high as 1.87% in the United States [3,4].Almost 25% of the cases are familial and 75% are considered sporadic. Familial cases are relevant for germline RET mutations; 43% -65% of sporadic cases relate to somatic events in the gene [5].In general, the early detection of MTC is extremely di cult. Although ultrasound examinations, tumor markers, ne needle aspiration, and genetic testing are currently used clinically for early detection, Lymph node metastases or even distant metastases have occurred when most patients with MTC are initially diagnosed [6,7]. Therefore, in addition to the poor prognosis of most patients with thyroid cancer due to the aggressive tumor, the untimely detection is also an important motivation for the poor prognosis [8]. A retrospective analysis of the data collected demonstrated that age, tumor size, tumor invasion and lymph node metastasis are all independent factors affecting the prognosis of MTC [9].In the prognostic evaluation of patients with MTC, however, the lymph node-positive rate is ignored. The lymph node positive rate is described as the number of positive regional nodes divided by the number of Regional nodes examined. Previous studies focused on the patterns of cervical lymph node metastasis of medullary thyroid carcinoma and believed that the occurrence of cervical lymph node metastasis is higher in patients with MTC [10]. So far, few studies have described the in uence of a positive lymph node rate on the prognosis of patients with MTC. The important clinical value of the lymph node positive rate has been ignored. The aim of this study is to investigate the in uence of the positive lymph node rate on the prognosis of patients with MTC after the operation and the guiding importance of the positive lymph node rate for further treatment in the later period using the information gathered from the SEER database, which is a popular database in the United States that collects basic clinical information from a large number of patients and information and some rare cases [11].

Data collection
This is a clinical retrospective investigation using information from the SEER database. The inclusion criteria for the study cohort are as follows: The histological type only needs to be determined MTC; The histological type should correspond to the International Classi cation of Disease for Oncology, Third Edition (ICD-O-3; coded as 8510/3); Year of diagnosis is from 1998 to 2015 The exclusion criteria for the study cohort are as follows: tumor size unclear, not assessable and not documented in the patient le; The extent of the tumor is not speci ed and cannot be assessed; reginal lymph node is not involved in the operation; number of examined lymph node less than ve in order to avoid calculation bias; dead reason is unknown or missing; and MTC is not the rst tumor. SEER*Stat (version 8.3.5) software is used for data collection.
The lymph nodes positive rate (LNPR) is calculated by dividing the number of positive lymph nodes by the total number of examined lymph nodes.
The dependent variables were overall survival (OS), which is described as the time from diagnosis to death from all causes, and cancer-speci c survival (CSS), which is described as the time from diagnosis to MTC-related death.

Data analysis
According to the positive lymph node rate, the total patients are in the first group with LNPR (0-0.25), the second group with LNPR (0.26-0.50), and the third group with LNPR (0.51-0.75) and the fourth group with LNPR (0.76-1.00). The chi-square test is used to evaluate the significance of the difference in variables between the above 4 groups of patients. All tests with P values of 0.05 were considered significant. The Kaplan-Meier (KM) method was used for the univariate analysis, followed by a log-rank test to identify the differences.
Variables with P <0.05 in the univariate analysis were included in the multivariate Cox proportional hazard analysis. In the end, a stratified analysis according to age, gender, tumor size, chemotherapy status and radiotherapy were carried out on the patients using the log-rank test. R software 3.8.9(https://www.rproject.org/) is utilized for data analysis and made of survival curves.

Results
From 1998 to 2015, a total of 632 patients with MTC (ICD-0-3 coded as: 8510/3) were eligible. The mean ± SD number of lymph nodes examined in LNPR 0-0.25 is 39.10±24.40, which is much larger than that in LNPR 0.76-1.00. However, the mean ± SD number of positive lymph nodes in LNPR 0-0.25 is 5.09±4.30, which is much is much smaller than that in LNPR 0.76-1.00. The detailed screening process is depicted in figure 1.
There are up to 225 MTC patients with LNPR 0-0.25, The following is MTC patients with LLPR (0.26-0.5) up to 199 cases, and the number of MTC patients with LNPR (0.51-0.75) is 105, and there are only 99 MTC patients with LNPR (0.76-1.00).
The difference in sex, tumor invasion, chemotherapy, radiotherapy and mean±SD of lymph node examined and positive lymph node between the four techniques was statistically significant. Detail clinical characteristic of MTC patients in shown in Table1.

Discussion
Medullary thyroid carcinoma is a rare and highly aggressive thyroid tumor of the endocrine system that originates from parafollicular C cells of the thyroid [1].Mutations in the RET proto-oncogene are implicated in the pathogenesis of MTC, but several other alternative patterns exist [12]. One of the major reasons for the poor prognosis for patients with MTC is that near of the surgical methods of the combination of thyroidectomy, neck dissection and resection of regional sites is up to 80.4% [14].
However, the scope of lymph node resection in MTC patients is still uncertain. We hypothesized that that the wider extent of lymph node removal in patients with MTC might suggests that the greater the number of positive lymph nodes removed, the lower the likelihood of recurrence and the better the prognosis of patients. The lymph node positive rate is de ned as the number of positive lymph nodes divided by the total number of lymph nodes removed. Assuming the same number of positive lymph nodes, the following applies: the lower the positive lymph node rate, the greater the scope of the lymph node resection. Therefore, the current investigation is primarily to explore the impact of the LNPR on the prognosis of patients with MTC by extracting information of MTC patients from the SEER database. Compared to previous studies, we divided the lymph node positive rates into four groups and compared the groups to analyze their impact on the prognosis of patients with MTC. We found that the positive lymph node rate is not only an important characteristic of clinical patients, it is also closely related to the overall survival rate and the disease-speci c survival rate of the MTC patients. Our studies have shown that the higher the positive lymph node rate, the lower the number of patients with MTC.
Our study found that 39.3% of patients with MTC with a lymph node positive rate of less than 25%, while only 11.9% of patients with medullary thyroid death with a lymph node positive rate of greater than 75% state which indicates that higher the positive lymph node rate, the lower the number of patients with MTC. Moreover, the number lymph nodes removed in more patients with MTC by far exceeds the number of positive lymph nodes, which shows that the scope of the surgical resection is relatively su cient. In addition, we found that in the MTC patients with a positive lymph node rate of less than 25%, the tumor size larger than 40 mm was only 22.9%, while in the MTC patients with a positive lymph node rate of more than 75% ,the tumor size greater than 40mm accounts for 41.9%, suggesting that the larger the tumor size, the more likely it is to have lymph node metastasis and the extent of the lymph node resection is larger. showed that the extent of individual surgery should be based on tumor size and lymph node status, which is consistent with our study [18]. As expected, patients with a higher positive lymph node rate have a poorer prognosis. Our study not only showed that the prognosis of the group with LNPR 0-0.25 was better than that of the group with LNPR 0.75-1.00, but also that the risk of death for the group with LNPR 0.75-1, 00 was 2.23 times higher than the group with LNPR 0 -0.25 on OS and 2.09 times higher on CSS. We believe that the reason for the poor prognosis of patients with a higher positive rate of lymph nodes is that although patients have undergone neck lymph node resection, the scope of surgical lymph node resection is not enough, and some metastases lymph nodes have not been completely removed, which accelerates distant metastasis and leads to poor prognosis. Because in our study, we found that the average number of lymph nodes removed in MTC patients with high lymph node positive rates is 23.96, which is much lower than that of MTC patients with low lymph node positive rate. But the average number of positive lymph nodes is 20.81 with high positive lymph rate, which is much higher than that of MTC patients with low positive lymph nodes.
On the contrary, patients with a low positive lymph node rate underwent extensive scope resections of lymph node and most metastases lymph nodes are completely removed, which will slow down the spread of cancer cells and the possibility of distant metastasis. Mao, Weipu, et al. have shown that the number of lymph node dissection is the independent element in the prognosis of patients with lymph node positive penile cancer, and that more lymph node dissection enhances overall survival in patients with penile cancer [19].
Although, most patients with MTC undergo other treatments such as chemotherapy and radiotherapy after thyroidectomy, neck lymph node dissection and distant site dissection [20]. However, surgery method is still a more effective treatment plan for MTC [21]. Therefore, we believe that the scope of lymph node resection should be expanded and the positive rate of lymph nodes should be reduced to improve the prognosis of patients. In addition, the positive lymph node rate can also be used as a reference indicator for the further treatment of patients with MTC after surgery. The comprehensive treatment concept for MTC today mainly includes surgery, chemotherapy, radiation therapy and targeted molecular therapies [2]. In patients with MTC with a different lymph node positive rate, the different therapeutic optional for radiation or chemotherapy should be considered, which indicates that in addition to calcitonin, lymph node positive rate also is an important guide for further treatment for improving the patient's prognosis [2,[22][23][24][25].For the MTC patients with high LNPR, the doses of chemotherapy and targeted therapy drugs may need more, and the speci c doses need to be further studied.

Limitation
In this article, the SEER database is used to examine the in uence of the lymph node positive rate on the prognosis of patients with MTC. But there are still many shortcomings. First, the receipt is from the SEER database, which does not include some factors that have an important impact on the prognosis of patients with MTC, such as calcitonin. Then there will be fewer patient samples in this article and in the later period more multicenter prospective or retrospective studies are needed to further con rm the theory. Finally, the speci c diagnostic and treatment measures for patients in the later phase are not clear, such as the type of chemotherapy drugs used.

Conclusion
This article uses the information on clinicopathologic features of MTC in the SEER database to con rm that the lymph node positive rate is closely related to patient prognosis. Patients with a high positive lymph node rate have a poor prognosis. In addition, chemotherapy and radiation therapy in patients with a higher positive lymph node rate must be further reconsidered and optimized. However, the study requires further clinical con rmation. Declarations