**4. Etiopathogenesis**

## **4.1 Cellular biology of net**

As such pNETs were classically thought to arise from pancreatic islet cells or the islets of Langerhans, hence the term islet cell tumors was coined [22]. Islet cells are the endocrine cells of the pancreas and they constitute 1–2% of total pancreatic mass. They are therefore distinct from the exocrine cells, from which pancreatic ductal adenocarcinomas arise. They are composed of various cell types and responsible for secretion of hormones like beta cells (insulin), alpha cells (glucagon), delta cells (somatostatin), and PP cells (pancreatic polypeptide) [23]. However, current theory says that pNETs in fact arise from the APUD (amine precursor uptake and decarboxylation) cells [24]. The presence of neurosecretory granules is the characteristic feature of APUD cells and these neurosecretory granules have autocrine, paracrine and neuromodulatory functions, in addition to the endocrine property. These cells are thought to originate in the embryologic neural crest, but more recent research suggests that they originate in the embryologic endoderm [25].

The most common genes involved in pancreatic neuroendocrine tumors are mentioned in **Table 4**.

Other specific genes suggested to be implicated in the etiopathogenesis of NETs include BIN1, Serpine 10, BST2, IGFBP3, LCK, MET, fibronectin, PDGF, IGF- 1, fibroblast growth factor, TGF-alpha and–beta, EGFR, and stem cell factor receptor [27].

Multiple studies have elucidated the underlying genetic mechanism regarding molecular development and progression of these tumors but still much remains unexplored in this area. Loss of chromosomes 3q, 6pq, and 10 pq, and gains of 5q, 12a, 18q, and 20q is associated with malignant behavior in these tumors [28]. In tumors less than 2 cm in size, it has been observed that Chromosome 1 and 11q loss with gain of 9q is associated with genetic instability [29].

#### **4.2 Olecular pathology of PNET and its role in prognosis**

Most recent advancements in assessment of pancreatic NET is the development of microRNA profiling which corresponds to various proliferation indices and also propensity of tumor to cause local spread and distant metastasis [30]. MicroRNA are non-coding RNA sequences having length of 21–25 nucleotides. They regulate genes at post translational level [31]. They can act as oncogenes or tumor suppressor genes and play a significant role in proliferation of tumors or their dissemination [32]. They can act as diagnostic as well as a prognostic marker.

There is very limited data available regarding microRNA profiling of pNET. In one large study done on pancreatic NET, 28 different miRs have been shown to


#### **Table 4.**

*Common genes in pancreatic neuroendocrine tumors vs. pancreatic adenocarcinoma [26, 27].*

*An Overview of Pancreatic Neuroendocrine Tumors DOI: http://dx.doi.org/10.5772/intechopen.96259*

be differentially expressed with 18 of them being higher expressed and 10 lower expressed as compared to healthy pancreatic tissue [33]. There is a higher expression of miR-103, miR-107 and miR-193b and lower expression of let-7 miR and miR-155 in pancreatic neuroendocrine neoplasias [34]. Tumor proliferation is denoted by expression levels of miR-196a, miR-21 and miR-642 while miR-210 and miR-21 seem to correlate with metastatic disease and tumor recurrence is predicted by expression of both miR-196a and miR-27b [35, 36].

Circulating tumor cell count also plays an important role in delineating the prognostic value of pNETs, especially before and during the treatment. Liquid biopsy is emerging as a newer and more profound biomarker test which provides valid cytochemical, morphological, pathological and molecular information regarding response of anti tumor therapy for pNET [37]. Circulating tumor cells (CTC) are shed from the primary or metastatic component of the tumor and they are evaluated by liquid biopsy [38]. CTC are considered as prognosticators in many solid malignancies but their role in neuroendocrine tumors was highlighted first by Khan et al. in 2011 [39] patients with advanced NETs who were starting either systemic or local therapy were enrolled. It was found that patients with one or more circulating tumor cells (CTC) were more likely to have worse progression free and overall survival.

Further placental growth factor (PIGF) is also evaluated as a prospective biomarker in NET. pIGF is a derivative of VEGF, which shows increased expression in NETs. It was found that PlGF levels were elevated in pNET samples and serum as compared to control pancreatic tissue and control serum. It was concluded that elevated PIGF levels are seen in pNET and it has also been projected that increase PIGF levels correlate with shorter time to progression [40].

#### **5. Clinical presentation**

Since non functional pNET represent up to 90% of PanNETs, they present with high hormone levels without symptoms. However, upto 60% of these patients have a metastatic disease at diagnosis, while 21% present with a locally advanced disease [41].

Non specific symptoms of pNET include abdominal pain, weight loss, or mass effect related to the pancreatic tumor or to the distant spread. Less frequently it is associated with complaints of jaundice, hemorrhage from tumors, and a palpable mass. Symptoms often do not appear until metastases develop [42].

Usually endocrine tumors of the pancreas present with typical symptoms due to hormonal hypersecretion, such as insulinoma, gastrinoma, VIP-oma, glucagonoma and somatostatinoma. In upto 40%-50, cases may present as non-functioning tumors or secrete pancreatic polypeptide (PP) and neurotensin [43]. The various pancreatic NET subtypes with their incidence, clinical presentation and survival are mentioned below (**Table 5**).

#### **6. Diagnosis**

#### **6.1 Biochemical**

Chromogranin A is a secretory glycoprotein present in neurosecretory granules of pancreatic NET. Majority of pNET show elevated chromogranin A levels. The sensitivity depends upon the tumor burden and the levels of chromogranin A are directly correlated with the prognosis of the patient. In insulinomas elevated


*Incidence, clinical presentation and survival of pancreatic NET subgroups [27–32, 44].*

*An Overview of Pancreatic Neuroendocrine Tumors DOI: http://dx.doi.org/10.5772/intechopen.96259*

chromogranin A levels are rare. Other serologic markers include neuronal serum enolase, human chorionic gonadotropin, and pancreatic polypeptide, which are elevated in 20–40% of PNETs. (See **Table 6**) [52].

When any NET is suspected then fasting gut hormones such as chromogranin B, pancreatic polypeptide and urinary 5HIAA (a breakdown product of serotonin) are also useful baseline tests. False positive chromogranin A levels are caused due to treatment with a proton pump inhibitor, Parkinson's disease, hypertension, glucocorticoids, renal failure and atrophic gastritis, while various dietary factors and drugs can cause an elevated urinary 5HIAA [53].

Additional blood tests for secreted peptides can be useful if a clinical syndrome is suspected and calcium, prolactin and parathyroid hormones should be tested in possible MEN1 cases. For Nonfunctioning pNETs, pancreatic polypeptide is a useful test. For insulinomas the gold standard diagnostic tool is supervised fasting with serial blood glucose analysis. Diagnosis requires the fulfillment of Whipple's triad of hypoglycemia, symptoms and correction of symptoms with glucose, in the presence of non-suppressed insulin levels. Factitious hypoglycemia due to administration of insulin or sulfonylureas must be ruled out [54].

### **6.2 Radiological**

Cross sectional imaging plays an important role in the workup of PNETs by characterizing the primary tumor and determining the extent of disease. Location of the tumor and its spread can be delineated by the use of multimodality imaging which includes computed tomography (CT), MRI and various nuclear medicine scans. Endoscopic ultrasound (EUS), digital subtraction angiography and venous sampling can also be used [55]. The sensitivity of CT and MRI is more than 80% for the detection of PNETs which is more sensitive than an octreotide-based scintigraphic scans [56].

EUS acts as an indispensable accompaniment to CT or MRI and has superior resolution. For tumors with size as small as 2 mm, EUS shows sensitivity of more than 90% and when combined with cross sectional imaging the sensitivity reaches upto 100%. Addition of EUS is recommended when cross-sectional imaging fails to define the pancreatic mass, when the location of primary cannot be delineated or biopsy is needed to confirm the diagnosis before commencing the treatment [57].


#### **Table 6.**

*Biochemical tests for pNET [33, 51].*


#### **Table 7.**

*Histopathological features of well and poorly differentiated tumors.*

Since NETs have high levels of somatostatin receptor 2 (SSTR2) expression, Functional imaging comes into play in these tumors. For tumors lacking SSTR2, like insulinomas and poorly differentiated tumors, it is less useful [58]. It is used to detect primary tumors or metastatic disease which is not readily seen on crosssectional imaging. Also, the uptake can predict response to octreotide analogs [59].

Indium-111 (111In) pentetreotide scan (Octreoscan) is a readily available nuclear scan that is effective at identifying nonfunctional PNETs, glucagonomas, and gastrinomas [60]. Although High-resolution positron emission tomography (PET) in combination with CT is superior in detecting small tumors and identifying occult metastases as compared to 111In pentetreotide. For identifying well-differentiated NETs, Octreoscan appears more sensitive than (18) FDG-PET, whereas (18) FDG-PET demonstrates superior sensitivity for poorly-differentiated NETs [61].

Somatostatin receptors are overexpressed in a proportion of NETs and Somatostatin receptor scintigraphy (SSRS) is useful in detecting these tumors. There are five subtypes of SSTR and 80% of pNETs, excluding insulinomas, express SSTR-2. Less than half of insulinomas express SSRT-2, therefore Single-photon emission computed tomography (SPECT) has sensitivity of 50% when combined with SSRS. In gastrinomas, VIPomas, glucagonomas and nonfunctional tumors SSRS combined with SPECT has a diagnostic sensitivity of 75% [20].

Currently both 18F-FDG PET/CT and 68Gallium (Ga)-labeled somatostatin analog PET/CTs such as 68Ga-DOTATOC or 68Ga-DOTATATE PET/CTs are used. FDGPET use is limited to poorly differentiated NETs, as well differentiated NETs are not FDG avid. It may also be used to demonstrate aggressive behavior or heterogeneity between lesions in a single patient. 68Ga-labeled somatostatin analog PETs have been shown to be superior to CT or SSRS in sensitivity and specificity, for detecting an unknown primary, staging at diagnosis, and for follow-up [62].

#### **6.3 Histopathology**

They can be classified as well differentiated and poorly differentiated NET. the major differences are elaborated further (**Table 7**).

### **7. DIfferential diagnosis**

• Acinar cell carcinoma: It can be differentiated from pNET as it has granular PAS positive cytoplasm, BCL10, trypsin, chymotrypsin positive, Synaptophysin and chromogranin positivity <25% while pNET is PAS negative, BCL10, trypsin, chymotrypsin negative and Synaptophysin or chromogranin positivity over 25% [65].


Insulinoma [27]: the differential diagnosis includes conditions with increased insulin levels in blood


Glucagonoma [66].

