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Clinicopathological Characteristics of Nonfunctional Pancreatic Neuroendocrine Neoplasms and the Effect of Surgical Treatment on the Prognosis of Patients with Liver Metastases: A Study Based on the SEER Database

Background The incidence of nonfunctional pancreatic neuroendocrine neoplasms (NF-pNENs) has been increasing annually. This study is aimed at investigating the clinicopathological characteristics and high-risk factors of NF-pNENs and the influence of surgical treatment on the prognosis of NF-pNEN patients with liver metastases. Methods pNEN patients in this study were obtained from the Surveillance, Epidemiology, and End Results (SEER) database. This study analyzed patients diagnosed with NF-pNENs from 2000 to 2017 who met the inclusion criteria. A retrospective analysis of the clinicopathological characteristics of NF-pNEN patients was conducted. Kaplan-Meier method was used to calculate the survival time. A multivariate Cox regression model was used to analyze the survival outcomes and risk factors. Results From 2000 to 2017, the SEER database registered 10576 patients with pNENs and 1774 patients with liver metastases. Cox analysis revealed that age, sex, primary site, grade, tumor stage, surgery, tumor size, and liver metastasis were risk factors of prognosis, with grade being the most influential index. Patients with NF-pNENs with liver metastasis and no metastasis had different primary site, grade, and tumor size. In general, a higher grade was associated with a larger tumor and a greater risk of liver metastasis. Meanwhile, patients with liver metastasis showed that those with tumors originated from the tail of the pancreas had better prognoses than those with tumors originated from other parts. Surgical treatment can improve the prognosis of patients with liver metastases, despite the tumor grade. Conclusions The incidence of pNENs has been increasing annually, and the liver has been the most common site of metastasis. Liver metastasis in patients with NF-pNENs, related to tumor size and grade, affected their long-term survival. Surgery significantly improved the prognosis of patients with liver metastases secondary to NF-pNENs with different grades.


Introduction
Pancreatic neuroendocrine neoplasms (pNENs) are a variety of heterogeneous tumors originated from pancreatic neuroendocrine cells, accounting for 8.7% of gastrointestinal pancreatic neuroendocrine tumors and 1%-2% of all pancreatic tumors [1]. According to clinical manifestations, pNENs can be divided into functional pNENs and nonfunctional pNENs (NF-pNENs). Functional pNENs are characterized by tumor tissues that can secrete insulin, gastrin, glucagon, somatostatin, and vasoactive intestinal peptide, producing corresponding clinical symptoms. The most common types include gastrinomas and insulinomas. NF-pNENs usually manifest with nonspecific symptoms, such as abdominal    Computational and Mathematical Methods in Medicine pain, bloating, or weight loss. Most pNENs are NF-pNENs [2]. PNENs encompass tumors with a wide spectrum of clinical behaviors. Consequently, these tumors are often diagnosed at an advanced stage [3]. At the time of diagnosis, approximately 40%-93% of patients with NF-pNENs have metastases, such as the lung and bone metastases, with the liver being the most common site of metastasis [4]. Besides, the potential survival benefits of surgery need to outweigh the morbidity associated with pancreatic surgery which explains the current controversy regarding small asymptomatic NF-pNENs [5][6][7]. Some of which advocated surgical treatment while the others recommend conservative treatment. Therefore, studying the clinicopathological characteristics and high-risk factors of patients with NF-pNENs are important for improving patient prognosis. The Surveillance, Epidemiology, and End Results (SEER) database is a publicly available cancer reporting system funded by the federal government and represents a collaboration between the CENTERS for Disease Control and Prevention, the National Cancer Institute, and regional and state cancer registries [8]. Our motivation is to use SEER data to analyze the epidemiology and survival of patients with pancreatic neuroendocrine neoplasms; furthermore, we also aimed to confirm the potential prognostic factors that might provide more robust evidence for surgeons to make determinations for selecting operable patients.

Data and Methods
2.1. Data Source. This study used data from the Surveillance, Epidemiology, and End Results (SEER) database (https:// seer.cancer.gov/data/) sponsored by the National Cancer Institute, including data on age at diagnosis, sex, race, pathological type, grade, tumor stage, primary site, surgery, tumor size, liver metastasis, survival time, and survival status.

Inclusion and Exclusion
Criteria. This study used the data from the SEER database updated in September 2020 and selected data from 2000 to 2017 to analyze the prevalence of pNENs, focusing on the clinicopathological characteristics, presence of liver metastasis, and the effect of surgical treatment on prognosis. The screening process is shown in Figure 1. The specific inclusion criteria were as follows: (1) patients diagnosed before 2015 to ensure a followup time more than 5 years; (2) the International Classification of Disease for Oncology (3rd Edition) was limited to the following pathological types: pancreatic endocrine tumors (8150), carcinoid tumors (8240), enterochromocytoid tumors (8242), cup shape cell carcinoids (8243), mixed glandular neuroendocrine tumors (8244), adenocarcinoids (8245), neuroendocrine carcinomas (8246), and atypical carcinoids (8249); (3) the location of ICD-O-3 was limited to the pancreas; and (4) tumors confirmed by histology. The exclusion criteria were as follows: (1) patients with unclear surgical history and (2) patients with metastases in other parts rather than liver metastasis.

Statistical Analysis.
The categorical data were presented as n (%). Univariate analysis was performed to analyze the demographic and tumor characteristics, while the Kaplan-

Computational and Mathematical Methods in Medicine
Meier survival curve method was performed to analyze patient survival. Multivariate analysis included statistically significant univariate analysis results. A Cox proportional hazard model was then established, and adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated to assess the strength of a single variable. All statistical analyses were performed using SPSS version 22.0 (IBM, New York, USA), and all figures were made using GraphPad Prism7.0 (GraphPad, CA, USA). Statistical significance was set at P < 0:05.

Prevalence of pNENs and Clinicopathological
Characteristics of NF-pNENs. From 2000 to 2017, 10594 patients with pNENs were registered in the SEER database. The prevalence of pNENs has increased significantly, leading to a concomitant rise in the number of registered NF-pNEN cases (Figure 2(a)). Among them, 2163 (20.4%) patients had distant metastases, and 1774 (16.7%) had liver metastases (Figure 2(b)), which was associated with a poor prognosis (Figure 2(c)). This study included 2944 patients with NF-pNENs, accounting for 27.8% (2944/10574) of the total number of registered patients with pNENs. Table 1 shows the clinicopathological characteristics of patients with NF-pNENs. The median age was 61 years (range, 11-85 years), and the average age at onset was 59:9 ± 13:3 years. There were more men than women (male : female = 1 : 23), and 78.9% were white. The top three tumor sites were the pancreatic tail (37.9%), head (29.3%), and body (15.4%).

Clinical Characteristics of NF-pNEN Patients with Liver
Metastasis. Survival analysis showed that the occurrence of liver metastasis indicated a poor prognosis (Figure 2(c)). The clinical characteristics of major liver metastasis and no metastasis NF-pNENs were compared to clarify the pathological characteristics of patients with liver metastasis secondary to NF-pNENs. Of the 2944 patients, 529 patients had liver metastasis. The demographic, clinical, and pathological characteristics are summarized in Table 3. Patients with NF-pNENs with metastasis and no metastasis had different primary site, grade, and tumor size. In general, the higher the grade and the larger the tumor, the higher the proportion of liver metastasis (P < 0:001, Figure 3).

Factors Affecting the Survival of NF-pNEN Patients with
Liver Metastasis.  (Table 4).

Impact of Surgery on the Survival of Patients with Liver
Metastasis from Different Grades of NF-pNENs. Liver metastasis secondary to NF-pNENs affected the long-term survival of the 529 patients diagnosed with metastasis; of them, 212 (40.1%) patients received surgical treatment. Survival analysis showed that the survival of the surgical treatment group improved compared with the nonsurgery group. Since the prognosis of metastatic NF-pNENs varied depending on the grade, we merged the two highly differentiated tumors, G1 and G2 grades, and the two poorly differentiated and undifferentiated tumors, G3 and G4, to analyze survival. The analysis showed that the prognosis of patients with moderately and highly differentiated tumors was significantly better than those with poorly differentiated and undifferentiated tumors. In tumors of different grades, surgical treatment can improve the prognosis of patients with liver metastases (Figure 4).

Discussion
The number of NF-pNEN cases registered in the SEER database increased significantly from 2000 to 2017. Because NF-pNENs lack typical symptoms in the early stage and the patient's survival time is affected by liver metastasis, it is necessary to identify related risk factors and formulate the best treatment strategy [9].

Computational and Mathematical Methods in Medicine
Study has shown that in gastrointestinal-pancreatic neuroendocrine tumors, tumor size, high grade, and distant metastasis are associated with poor survival [10]. In 2017, the World Health Organization redefined the grade of neuroendocrine tumors. One of the main changes in the latest classification standard was the division of pNENs into well-differentiated tumors (NENs) and poorly differentiated tumors (NEC). Clinically, pNENs grow slowly and have a high survival rate, while pNECs grow rapidly and usually manifest as liver metastasis, active mitosis, and aggressive invasiveness [11,12]. A higher grade is an independent prognostic indicator of shorter overall survival. When the grade increases, the survival time is greatly reduced. Gao et al. [13] found that G stage, TMN stage, lymph node, metastasis, vascular invasion, and the necrosis could be prognostic factors for pNENs. Therefore, patients with higher grades should be closely followed up. Therefore, determining the grades of primary tumors and metastatic tumors is vital for determining the prognosis and deciding on further treatment [14,15].
Tumor size affects the possibility of metastasis [16]. According to the European Neuroendocrine Oncology Society standards, 2 and 4 cm were considered to be important thresholds affecting the prognosis in pNEN patient [17]. Some studies have indicated that tumor size is an independent prognostic factor associated with disease-free survival (DFS). If the tumor size is >30 mm after pancreatectomy, the risk of recurrence is higher [18]. Some studies have shown that NF-pNEN tumors measuring >20 mm are less malignant than tumors measuring <20 mm. For patients with sporadic, small NF-pNENs, developing a reasonable treatment strategy has always been controversial [19].
Surgery can significantly improve the symptoms and quality of life of patients with liver metastasis [20]. Zheng et al. [21] revealed that resection of the primary tumor improves survival in patients with gastro-entero-pancreatic neuroendocrine neoplasms with liver metastases. It should be adjusted according to tumor characteristics. Observation is usually recommended for asymptomatic patients with small pNENs, but for patients with F-pNENs and NF-pNENs measuring >20 mm, surgical resection should be performed [22]. In the era of transplantation oncology, liver transplantation has become a viable treatment option for patients with unresectable pNENs and liver metastases, improving the 5-year overall survival rate after liver transplantation to 47%-71%. For patients with unresectable or incurable tumors, liver transplantation should be the final choice after detailed discussion by a multidisciplinary team composed of all experts in the field. However, the high recurrence rate after liver transplantation is a clinical obstacle [23,24].
This study has some limitations. First, it was a retrospective study. Second, as a database based on a large sample of people, the SEER database may lack some important clinical information, especially laboratory test results and detailed surgical records. Third, the tumor grading system was based on the old standard, which does not correspond well to the new standard. With the advancement of diagnostic technology, more and more early asymptomatic NF-pNENs will be detected. After a multidisciplinary comprehensive evaluation, surgical treatment and targeted therapy can improve the long-term survival rate. However, the prognosis of patients with pNENs is not the same, the malignancy of tumors of different grades varies greatly, and there are still controversies about the treatment of pNENs of different sizes. Therefore, identifying the malignant degree of pNENs and exploring the clinicopathological characteristics of different types of tumors are of great significance for guiding treatment. It is hoped that the study of large populations in the SEER database can provide a basis for the treatment of pNENs.

Conclusions
The incidence of pNENs has been increasing year by year, and the liver is the most common site of metastasis. Patients with NF-pNENs were mostly men who were younger than 60 years, mostly at G1. The predominant site was the tail of the pancreas, and the tumors were mostly localized tumors measuring <20 mm. Liver metastasis associated with the tumor size and grade, which decreased the long-term survival of patients. Surgery significantly improved the prognosis of patients with liver metastases of NF-pNENs with different grades. Surgical treatment can significantly improve the 5-year survival rate in patients with liver metastases. Therefore, surgical treatment should be performed when surgical indications are met.

Data Availability
The labeled dataset used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare no competing interests. The impact of surgery on survival in patients with liver metastasis secondary to NF-pNENs with G1 and G2 grades. (d) The impact of surgery on survival in patients with liver metastasis secondary to NF-pNENs with G3 and G4 grades. P < 0:05 was considered statistically significant.