Diagnostic Significance of Single Center, Open and Prospective Evaluation of <Sup>18<Sup>F-FDG PET/CT Dynamic Imaging in Detecting Metastatic Central Lymph Nodes in Patients With Papillary Thyroid Cancer

Thyroid cancer is the most common malignant tumor in human endocrine system, accounting for 0.2% ~ 1.0% of malignant tumors .According to the global cancer report released by the world health organization (WHO) in 2014, 50% of the new cases of thyroid cancer are papillary thyroid cancer (PTC). In recent years, the incidence of thyroid cancer in China shows an increasing trend, especially the composition ratio of thyroid micro-papillary carcinoma with the focal diameter <1 cm increases rapidly.Although PTC has a good prognosis, according to a Meta analysis , PTC lymph node metastasis rate is as high as 28.0%.The central lymph node is the most common metastatic site of papillary thyroid carcinoma.In addition, it has been reported in literature that lymph nodes in the central region are the first stop of thyroid papillary carcinoma metastasis (i.e., sentinel lymph nodes), and lymph nodes metastatic to this point are prone to infringe the peripheral esophagus, trachea and recurrent laryngeal nerve at an early stage.A large number of clinical cases have confirmed that lymph node metastasis of thyroid cancer significantly increases the local recurrence rate after surgery.More studies have shown that regional lymph node metastasis increases mortality.Therefore, the treatment of lymph node metastasis should not be ignored in PTC patients.However, at present, preoperative imaging has certain limitations in the judgment of lymph node metastasis in the central region : ultrasound has advantages in the judgment of lymph node metastasis in the peripheral group, but poor display in the central group.Although MRI scan of head and neck can be multi-directional and multi-sequence imaging, and the resolution of soft tissue is high, it is not good at showing calcification.Conventional CT alone based on lymph node morphology to determine metastasis also has some limitations;Positron emission tomography (PET) sensitivity was < 40%.Although one study showed that the probability of non-missed diagnosis of lymph nodes in central PTC region by thin slice CT scan was 100%, the specificity was low and the study was retrospective.Therefore, b-ultrasound, MRI, CT, PET and other auxiliary examinations have not been able to effectively detect all central metastatic lymph nodes before surgery.For management of cervical lymph nodes ,2015 American Thyroid Association（ATA） guidelines show that thyroid+therapeutic Ⅵ area lymph node dissection should be undertaken if in central or lateral lymph nodes are involved.Chinese guidelines recommend that, in order to avoid operative complications as far as possible (i.e. effective retainment of the parathyroid gland and recurrent laryngeal nerve), regardless of the size of the primary thyroid lesion, lymph node dissection in the central area on the same side of the lesion should be performed at least.However, in 2017, the second edition of National Comprehensive Cancer Network(NCCN) thyroid tumor guidelines believed that if lymph nodes in the central region were negative, preventive central region lymph node dissection was not recommended.Above all，there is so much controversy over lymph node dissection in the central region. Therefore，it is of great significance to effectively evaluate the preoperative lymph nodes station in the central region of thyroid papillary carcinoma.

Early dynamic imaging examination of <Sup>18<Sup>F-FDG PET/CT can be defined as a imaging method to detect the status of radioactive tracer (<Sup>18<Sup>F-FDG) which travels with the blood flow and is continuously absorbed and excreted by organs (or diseased) tissues after being introduced into the body in the early stage.This development reflects the change of <Sup>18<Sup>F-FDG activity with time in the early stage of entering the body, and can provide information about the dynamic change of tracer metabolism and distribution in tissues with time, so that it can provide more metabolic and distribution details of tumor lesions and metastatic lesions than static scanning.At present, many studies have shown that <Sup>18<Sup>F-FDG PET/CT early dynamic imaging technology can effectively improve the sensitivity of hepatocellular carcinoma ,renal cell carcinoma and other tumors.More amazing，there may be more advantages in evaluating lymph node metastasis.The blood supply of lymph nodes metastatic of thyroid papillary carcinoma is abundant, which helps to locate the metastatic tumor lymph node.However，there is no relevant study.Now , investigators plan to observe the significance of the method to evaluate the central lymph node metastasis in thyroid papillary carcinoma as well as the intrinsic correlations between the significance and postoperative pathological results ,which may help to develop surgical strategy by preoperative evaluation and avoiding additional dissection.