Neoadjuvant on the gastroenteropancreatic neuroendocrine tumors: A Systematic Review and Meta-analysis

Clinical value of neoadjuvant therapy has been proven in many malignancies. With a low prevalence of neuroendocrine tumors, the survival benets of neoadjuvant therapy in gastroenteropancreatic neuroendocrine tumors (GEP-NET) are still unclear based on the up-to-date literature. Aim To Articles were searched electrically on PubMed, Embase, Cochrane Central Register of Controlled Trials, Web of Science, ClinicalTrials.gov and China National Knowledge from inception to 30th June 2020. The survival benet was assessed by Hard ratio (HR) for overall survival (OS). Median OS, progression-free survival (PFS) and complications were also compared.


Background
Neuroendocrine tumors (NETs) are a family of heterogeneous tumors arising from neuroendocrine cells.
The incidence of neuroendocrine tumors is relatively low (6.98 per 100 000) [1]. Given the increased detection of early-stage disease and stage migration, it is not surprising that its incidence is gradually increasing [1,2]. Therapeutic options for neuroendocrine tumors are multitudinous, such as chemotherapy, chemoradiotherapy, surgery, somatostatin analogs, and peptide receptor radionuclide therapy (PRRT). To date, surgical resection remains as the only means to cure GEP-NET. A characteristic feature of neuroendocrine tumors is the presence of metastases, with the liver being the most the dominant site of metastases. Given its limited systemic therapy and relatively indolent nature, metastases were not an absolute surgical contraindication for NET. [3] Cytoreductive surgery has been proved to be associated with better survival outcomes [4] and was recommended for G1 / G2 NETs with resectable liver metastases [5]. Hence, it is crucial to get access to surgery in advanced GEP-NET.
At the same time, despite the clear bene t of surgical resection in relieving symptoms and prolonging survival, the recurrence rate remained high even in patients who obtained complete resection or a mirrornegative incision margin [6]. Neoadjuvant therapy is considered as a multimodal therapy which aims at improving survival and has been broadly investigated [7]. Neoadjuvant therapy prior to surgery can convert unresectable tumors into resectable tumors and reduce recurrence rates by shrinking the tumor size and destroying micrometastatic lesions. The bene ts of neoadjuvant therapy have been proven in different digestive system tumors such as gastric cancer, colorectal cancer, pancreatic cancer, and liver cancer [8][9][10]. But owing to its relative rarity, there is a lack of uniform knowledge about the effect of neoadjuvant therapy on GEP-NETs. Therefore, we performed this review to evaluate the effectiveness of neoadjuvant treatment in patients with GEP-NETs. The primary endpoint was the overall survival (OS) and the secondary endpoint was the rate of complications.

Methods
In accordance with the guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses and Meta-Analysis of Observational Studies in Epidemiology [11,12], we performed this systematic review and meta-analysis which has been registered at the International Prospective Register of Systematic Reviews(number CRD42020207903)

Search strategy and inclusion criteria
The search was formulated in PubMed, Embase, Cochrane Central Register of Controlled Trials, Web of Science, ClinicalTrials.gov, and China National Knowledge Infrastructure using the same subject headings and keywords. Search terms included (neuroendocrine tumor OR neuroendocrine neoplasm OR neuroendocrine carcinoma OR mixed neuroendocrine-non-neuroendocrine neoplasm OR mixed adenoneuroendocrine carcinoma OR carcinoid) AND (neoadjuvant OR preoperative) AND (gastrointestinal OR gastroenteropancreatic OR digestive OR gut OR pancreas).

Data extraction
Two authors (Chen and Zhu) independently extracted information from eligible studies using standardized forms. Baseline clinical characteristics (author, year, country, and study design, included number, age, genders), endpoint-related variables (HR for OS and PFS, 3-Year OS,5-Year OS, 3-Year PFS,5-Year PFS), complications, and pre-and post-operative TNM grades were extracted. All discrepancies between the reviewers were resolved by consulting a third reviewer.

Inclusion Criteria and Exclusion Criteria
The inclusion criteria for clinical articles were (1) being a de nitive pathologic diagnosis of GEP-NETs, (2) patients without metastases or with only liver metastases having undergone surgery and at least one preoperative neoadjuvant therapy (including chemotherapy, radiation therapy, chemoradiotherapy and PRRT therapy), (3) at least including one of HR or survival rate, and (4) having full text of studies available.

Quality assessment
Two reviewers independently assessed the eligibility and validity of each study via the Cochrane tool for assessing the risk of bias in randomized controlled trials (RCTs) and the Newcastle-Ottawa scale in observational studies. Newcastle-Ottawa scale measured quality in the three parameters of selection, comparability, and exposure/outcome as well as allocated a maximum of 4, 2, and 3 points respectively.
High-quality studies scored greater than 7, moderate-quality studies scored between 5 and 7, and lowquality studies scored under 5. We conducted research bias detection on the articles included. The Grading of Recommendations, Assessment, Development and Evaluation (GRADE) method serves as the guideline in the systematic review and meta-analysis. Each study was independently assessed by 2 authors, and any disagreement was to be discussed with a third reviewer and resolved in consensus.

Statistical Analysis
This study primarily focused on the evaluation of the overall survival time, meanwhile covering the analysis of complications, PFS, 5-year OS and 5-year PFS.
For articles that presented Kaplan-Meier curves without Hazard Ratio or related survival information, we used software Engauge Digitizer (version 12.0) and SPSS (version 21) to extract variables in the coordinates of the Kaplan-Meier curves. Cochrane I 2 and χ 2 statistics were applied to estimate statistical heterogeneity, P <0 .05 and I 2 > 50% being highly heterogeneous. Random-effects models were conducted in the case of I² >50%; otherwise, xed-effects models were chosen. Statistical signi cance was de ned at the level of 5% (P = 0.05). The Inverse variance method was employed in analyzing subgroups. Odds ratios (OR) were calculated for the analysis of complications. Egger linear regression was conducted to de ne publication bias. All statistical analysis was performed in the software STATA version 14.0 and RevMan (version 5.3).
We identi ed 889 records from six databases (Figure 1). After removing duplicates, 841 publications were screened based on the title and the abstract, and 115 of them were selected for full-text review. Finally, 8 articles involving 457 participants were included ( Table 1). All of them were retrospective studies. According to the overall Newcastle-Ottawa Scale, the overall research quality was moderate (range, 5-8, mean 6.5, Table 2). Among the 8 studies, ve studies adopted adjuvant chemotherapy [13][14][15][16][17], one study chose PRRT [18], and two studies had PRRT combined with chemoradiotherapy [19,20]. In the studies involving chemotherapy, platinum-based regimens were the most common, with etoposide/cisplatin regimen accounting for the highest proportion.
There were no signi cant differences between the two groups in the baseline characteristics: age(P=0.

Survival Analysis
There was a drastic improvement in overall survival in the neoadjuvant group against the surgery-alone group (5-year OS 65.25% vs. 46.79% P<0.001, Figure 2

Complications and postoperative pathology
There was no signi cant difference in postoperative complications between the two groups (30% vs 29.4%, P=0.52). The rate of R0 resection in the neoadjuvant group was slightly but not signi cantly higher than that of the surgery-alone group(P=0.28). This may be related to the fact that most articles do not mention the R0 resection rate. In view of pathological ndings, the maximum diameters of resected tumors were smaller in the neoadjuvant group than in the surgery-alone group (4.4±2.1 cm vs. 5.7±2.8 cm, P = 0.02). Furthermore, the rate of post-operative N0 was higher in the neoadjuvant group (68.1% vs. 49.09%, P=0.02). However, the difference in postoperative pathological TNM grading was not statistically signi cant(P=0.13).

Discussion
There has been a gradual growth in the incidence of NETs in recent years but no signi cant change in the survival of patients with high-grade NET [16,21]. This situation may re ect a lack of awareness in the treatment. Given the biological similarities, the main treatment of NET was extrapolated from the experience in small-cell lung cancer (SCLC) currently [14,16,22]. Neoadjuvant therapy in SCLC has been proved effective in some studies [23][24][25].
In digestive system tumors such as gastric cancer, colorectal cancer, and pancreatic cancer, neoadjuvant chemotherapy can improve the OS or PFS and manifests its e cacy and safety [26]. Effective neoadjuvant therapy can improve the potential respectability by downstaging the disease and addressing the micrometastatic lesions through cytotoxic effects [27].
Previous studies had shown that the statistical cure for neuroendocrine liver metastasis tumors (NELM) with liver resection was possible [28]. Hence, we included neoadjuvant studies on patients with only liver metastases. However, due to limited studies, whether neoadjuvant therapy confers a survival bene t in GEP-NET still lacks su cient clinical data. Therefore, we performed this meta-analysis.
In the current study, we found that the pooled HR for OS in the neoadjuvant chemotherapy group was 0.52 [95% CI, 0.41-0.66], suggesting a nearly 48% reduction in the risk of death in the neoadjuvant chemotherapy group compared to the surgery-alone group. There was no publication bias among the included trials. In respect of OS, neoadjuvant therapy had a better 5-year overall survival rate or progression-free survival rate, with the 5-year survival time up to 63.83%. The reconstructed Kaplan-Meier on median OS showed a longer overall survival in the neoadjuvant without increasing postoperative complications, demonstrating that neoadjuvant therapy is a safe and effective treatment.
Platinum-based chemotherapy is recommended as the rst-line therapy for advanced NET [29]. In this meta-analysis, platinum-based chemotherapy (such as etoposide/cisplatin regimen, irinotecan/cisplatin regimen) was the most common preoperative regimen. Our study included not only neoadjuvant chemotherapy but also neoadjuvant PRRT, a targeted SSTR2 receptor-based regimen that has a de nite anti-tumor effect by causing deleterious DNA damage on tumor cells via radionuclides [30].Due to the small number of studies, we could not compare the bene ts of neoadjuvant chemotherapy and neoadjuvant PRRT therapy. This study con rmed the survival bene t of neoadjuvant therapy, possibly primarily through a reduction in tumor diameter and lymph node metastasis rate. Larger tumor size has been proven to be associated with a poor tumor prognosis. Our analysis identi ed a smaller diameter in the postoperative specimens of the neoadjuvant group, suggesting that neoadjuvant therapy may be clinically bene cial by downsizing tumor sizes. However, due to the missing data related to preoperative tumor diameter, further studies are expected to con rm this hypothesis. Lymph node metastasis was an independent risk factor for poor prognosis in NETs [31,32]. Our study showed that there was no difference in clinical N stage between the two groups before operation (P = 0.46). That the neoadjuvant group had a higher pathological N0 lymph node ratio after the operation suggests that neoadjuvant therapy achieves better survival rates by reducing metastasis of lymph nodes.
Traditionally, neoplasm metastasis is a relative contraindication to surgery, whereas the majority of patients with advanced GEP-NETs have liver metastases. Surgical resection is still an option in the absence of diffuse involvement, compromised liver function, or extrahepatic metastases [33][34][35] and the clinical value of hepatic resection in NETs with only liver metastases has been con rmed [36][37][38]. Our neoadjuvant study suggested that the pooled HR was 0.62 95% CI (0.41-0.94) for patients with NELM, illustrating that patients with NELM can achieve further clinical bene ts from neoadjuvant therapy. Molecularly targeted therapy such as Everolimus or inhibitors of VEGFR are also systemic treatment options for GEP-NETs, but the studies on their role in neoadjuvant therapy were few and more research is needed.
Our study also had some shortcomings. Firstly, the use of meta-analysis for observational studies is controversial and the heterogeneity of study design, disease characteristics and patient population may affect the aggregated results. However, because of the low overall prevalence, there are no high-quality large RCTs to study follow-up. Secondly, all included studies were retrospective, so there was a potential imbalance in patient characteristics, and some of the studies did not take neoadjuvant as the main observation point, leading to missing data in the preoperative tumor diameter, speci c preoperative chemotherapy regimens and frequency, types of NETs(functioning and non-functioning) and/or location of NETs, all of which expect more in-depth studies. Although our analysis indicated that survival was affected by neoadjuvant, we need more well-designed randomized controlled studies to control bias such as tumor grade, localization, and pathological classi cation. And these studies need to focus more on the reasonable number of neoadjuvant therapies and reasonable operation time.
However, this is the rst systematic analysis attempting to assess the most appropriate treatment for advanced NETs. With low overall heterogeneity and signi cant publication bias, this study improves the feasibility of the results and corroborates the clinical e cacy of neoadjuvant therapy in GEP-NET.  Tables   Table 1: baseline characteristics