The WHO classification system of gastroenteropancreatic neuroendocrine tumors was adopted in previous studies [3, 7, 8, 10–12, 14]. Some of these studies only focused on particular types of GEP-NENs such as well-differentiated endocrine tumors, poorly differentiated endocrine carcinomas or a single site of tumors (pancreas, colon or rectum). Our study investigated the pathologic features of GEP-NENs by using the latest histopathologic diagnosis consensus for the first time. It also analyzed any possible tumor site of digestive system including pancreas, biliary and peritoneal cavity. This study should contribute to establishing a database of the epidemiology, clinical pathological features, treatment and prognosis of GEP-NENs in China.
It is confirmed in our study that GEP-NENs comprise a heterogeneous group in relation to their primary locations. Previous researches indicated that the small intestine and appendix were the most predominant NENs locations [2, 15–17]. But according to our study, pancreas is the principal site of GEP-NENs. The rectum is the most frequent sites of gastrointestinal tract, followed by the stomach and duodenum, whereas the jejunum/ileum accounts for no more than 2% tumor cases. A similar distribution of NENs was also found from a Korean study , which observed that rectum was the most common primary site of tumor in 470 available cases, followed by the pancreas, stomach and duodenum. Results from another three registries including SEER, National Cancer Registry for Gastroenteropancreatic Neuroendocrine Tumors (RGETNE, http://www.retegep.net) and Norwegian Registry of Cancer (NRC) significantly differed from that in our series: Rectum and jejunum/ileum were the most common sites for NENs in the SEER Program tumor registry, pancreas NENs were only the third most common NENs; The pancreas and jejunum/ileum were the most frequent positions in RGETNE; whereas the small intestine was the most frequent sites of origin, followed by the colon and rectum in NRC. These inconsistencies may be due to the racial disparities, as well as the selection bias among population based data and hospital series. So a larger patient population is required to carry on further investigation.
NENs can be classified into functional and nonfunctional tumors according to the presence or absence of symptoms associated with hormones overproduction . The current study demonstrated that the majority of nonfunctional NENs usually presented with non-specific symptoms, which may give rise to misdiagnosis of the tumors as irritable bowel syndrome or digestive adenocarcinomas. Our study also showed that insulinomas were the most frequently encountered functional tumors in the pancreas, accounting for 93.1% of pancreatic NENs. No case, however, presented with carcinoid syndrome in this study. Interestingly, the incidence of carcinoid syndrome (10–32%) in the Western population [8, 17, 19–21] is significantly different from our report, with the fact that ileal tumors account for the vast majority.
Assessments of the locations and extents of GEP-NENs were crucial for management. The present study analyzed imaging methods, which is commonly used in current clinical practice, in this patient population. Conventional imaging procedures include endoscopy, ultrasound, EUS, CT scan, MRI and PET-CT, with detection rates ranging from 77.8 to 98.7%. CT scan was one of the most widely used imaging modalities (123/178) whereas endoscopy promised the highest yields of tumor detection (98.7%). The introduction of EUS provides unique advantages in evaluating the pancreatic biliary system, especially in tumors <1.0 cm in diameter and micrometastasis. The typical EUS patterns of NENs includes rounded, homogeneous, hypoechoic, well defined and vascularized masses, with the detection rate of 91.9% in our study, rather comparable to the results achieved in other series [22–24]. Small tumors and liver metastasis (i.e., tumors <0.5 cm in diameter) may be missed, resulting in underestimate of the exact disease extent. No single technique is 100% sensitive and accurate. Therefore, multiple imaging modalities should be combined to detect small, biochemically diagnosed tumors.
Despite the advances in both morphology and biology, the classification of NENs is still under debate. The lack of a uniform classification system for NENs hampers evaluation of therapy and comparison between clinical trials . European Neuroendocrine Tumor Society (ENETS) and the North American Neuroendocrine Tumor Society (NANETS) have published diagnosis standard and pathology reports of NENs in 2009 and 2010 [18, 26], respectively. Furthermore, the WHO revised the nomenclature and classification of GEP-NENs in 2010, version 4 . In 2011, China established her own classification system for NENs . Chinese Pathologic Consensus Group suggested the term “Neuroendocrine neoplasm (NEN)” instead of “Neuroendocrine Tumor (NET)” and formulated the classification criteria by the use of Ki-67 index/mitotic rate and histology. The pathologic features of NENs in our hospital were reviewed according to this diagnosis consensus in the current analysis, which to our knowledge, is the first study using the newest consensus. Overall, G1 tumors accounted for 51.5% of 169 available cases, followed by G3 (30.2%) and G2 (18.3%). The occurence of NET, NEC and MANEC were 69.8%, 27.2% and 3.0%, respectively. The availability of this uniform system for NETs greatly facilitates classification of the tumors, evaluation of treatment, and comparison of clinical trials.
In our series, distant disease at initial diagnosis occurred at the rate of 23.0%, which increased to 28.1% during follow up. Liver was the most frequent site tumor involved and the distribution of distant metastasis was wider than that either in SEER or in NRC (18–22%). In RGETNE, however, a significant proportion of patients (44%) with widespread disease were reported compared with our series. The frequency of primary tumor sites associated with distant disease varied in different series: in our cohort, the most common sites was cecum (100.0%), followed by jejunum/ileum (75.0%), gallbladder (50.0%), duodenum (38.5%), Vater’s ampulla (33.3%) and stomach (28.0%); in the SEER Registry, the most common site was pancreas (64%), followed by cecum/colon (44%/32%) and jejunum/ileum (30%); and in the RGETNE Registry, it was jejunum/ileum (65%), followed by colon (48%) and rectum (40%). Therefore, jejunum/ileum tumors appear to have a greater propensity for distant metastasis. However, the diversity should be taken into account.
Among the many therapeutic options for NENs, surgery is the treatment of choice. A variety of operations are available to reduce load of tumor and improve survival. The extent of surgical resection depends on the tumor size and origin and approximately 75.9% of patients have undergone a radical surgery. Radiofrequency ablation or TACE is usually adopted to treat liver involvement, accounting for 6.2% of the cases.
Besides surgery, other therapeutic options such as chemotherapy, biological therapy and targeted therapy can be used for NENs. According to the new WHO 2010 classification, well-differentiated NENs are classified as G1 and G2 neuroendocrine tumors (NETs) and poor-differentiated NENs are referred to as G3 neuroendocrine carcinomas (NECs). It has been reported that existing cytotoxic chemotherapy agents have been of limited value for the treatment of well-differentiated gastrointestinal NENs (with response rates 10% ~ 15%) [27–29], but has been the standard of care for well-differentiated metastatic pancreatic endocrine tumors (with response rates 40% ~ 70%) [30–32]. However, chemotherapy is generally considered active in poor-differentiated NENs (with response rates 50% ~ 70%) [33–35]. According to the published documents, several chemotherapeutic regimens are available, most of them are either platinum based or flurouracil based [29, 34, 36, 37]. For the GEP-NEC, platinum-based combination regimens with etoposide or paclitaxel [33, 34, 36] are recommended. In our cohort, chemotherapy was performed in 23 patients. The most frequently used chemo regimen was etoposide–platinum combination. During follow-up, 3 of them died of tumor progression. It has been noticed that biological therapy and targeted therapy promise some effect on NENs in recent years [38–43]. Somatostatin analogues are effective therapeutic option for functional neuroendocrine tumors because they reduce hormone-related symptoms [44–46]. They have also been shown to stabilize tumor growth over long periods, even to inhibit tumor growth in patients with well-differentiated metastatic neuroendocrine midgut tumors [40, 47, 48]. Although the treatment effect of somatostatin analogues on foregut and hindgut tumors remain to be confirmed, 16 patients including 2 patients with functional neuroendocrine tumors and 14 patients with well-differentiated metastatic GEP-NENs received long-term administration of octreotide LAR at a dose of 20–40 mg monthly in our study.
The prognosis of GEP-NENs is more favorable than that of the adenocarcinomas of the digestive system. The overall 5-year survival rate in our series was 54.5%, rather comparable to that of SEER or NRC registry [2, 3, 6] (50–59%), but it was lower than that in some European countries [7, 9] (75–79%). The inconsistencies of survival rates may be due to the racial and geographical disparities. We also proved that prognosis differed statistically according to functional status, pathological grading and classification. As the great majority of functional tumors were insulinomas which are benign in most cases in our study, that may lead to the conclusion that functionality may be a favorable prognostic marker. The result obtained above may be caused by small sample in this series. We also confirmed that metastasis represented a worse outcome with a mean survival of 5.0 years (P = 0.000). Multivariate analysis was not done due to the small size of our series. Therefore, further evaluation in a larger patient population is required to estimate the independent prognostic factors of GEP-NENs.
A broad range of this heterogeneous tumors was reviewed in the current study, which to our knowledge, is the first report using the latest pathological diagnosis consensus of these tumors. We also confirmed that GEP-NENs may originate from any part of the digestive system, and the majority of them are nonfunctional tumors with non-specific symptoms. Endoscopy and radiographic examination play an important role in tumor detection. However, final diagnosis should be based on pathological detection. The prognosis of these tumors was more favorable compared with gastrointestinal carcinomas. Nonetheless, the outcome was extremely poor for patients with high grading tumor and distant metastasis. Further understanding of the molecular mechanisms should facilitate management of the disease. Early diagnosis is crucial for radical resection before development of local invasion or distant disease, and interdisciplinary cooperation is the direction of future.