Cyto-histology in NET: what is necessary today and what is the future?

Insulinoma-associated protein 1 (INSM1) immunohistochemical expression in normal, hyperplastic, and neoplastic canine neuroendocrine tissues

Insulinoma-associated protein 1 (INSM1), a recently identified neuroendocrine marker, is a transcriptional regulator with highly conserved INSM1 homologues in various species. This study investigated the immunohistochemical reactivity of the INSM1 antibody in 20 normal canine neuroendocrine tissues from various anatomical locations, 87 hyperplastic or neoplastic tissues of neuroendocrine origin, and 62 non-neuroendocrine neoplasms and compared the results with those of chromogranin A and synaptophysin in neuroendocrine neoplasms. Western blot was performed on fresh canine pituitary glands and canine parathyroid glands to confirm the specificity of the anti-INSM1 antibody. The results showed that the anti-INSM1 antibody could detect nuclear expression in normal canine neuroendocrine tissues, except for the parathyroid glands. INSM1 was detectable in 79/87 (91%) of the hyperplastic and neoplastic neuroendocrine lesions, but all parathyroid carcinomas and parathyroid adenomas (three samples each) were negative for INSM1. In contrast, INSM1 was detected in only one of 62 (2%) non-neuroendocrine neoplasms. The overall percentage of neuroendocrine neoplasms that immunolabeled positively for all three markers was 89%. In addition, the nuclear expression of INSM1 was easier to interpret than that of chromogranin A or synaptophysin. These findings confirm that INSM1 is a useful immunohistochemical marker for diagnosing canine neuroendocrine neoplasms, except for parathyroid neoplasms, and should be considered as part of immunohistochemistry panels to improve diagnostic capability.

Overview of the 2022 WHO Classification of Neuroendocrine Neoplasms

In this review, we detail the changes and the relevant features that are applied to neuroendocrine neoplasms (NENs) in the 2022 WHO Classification of Endocrine and Neuroendocrine Tumors. Using a question-and-answer approach, we discuss the consolidation of the nomenclature that distinguishes neuronal paragangliomas from epithelial neoplasms, which are divided into well-differentiated neuroendocrine tumors (NETs) and poorly differentiated neuroendocrine carcinomas (NECs). The criteria for these distinctions based on differentiation are outlined. NETs are generally (but not always) graded as G1, G2, and G3 based on proliferation, whereas NECs are by definition high grade; the importance of Ki67 as a tool for classification and grading is emphasized. The clinical relevance of proper classification is explained, and the importance of hormonal function is examined, including eutopic and ectopic hormone production. The tools available to pathologists for accurate classification include the conventional biomarkers of neuroendocrine lineage and differentiation, INSM1, synaptophysin, chromogranins, and somatostatin receptors (SSTRs), but also include transcription factors that can identify the site of origin of a metastatic lesion of unknown primary site, as well as hormones, enzymes, and keratins that play a role in functional and structural correlation. The recognition of highly proliferative, well-differentiated NETs has resulted in the need for biomarkers that can distinguish these G3 NETs from NECs, including stains to determine expression of SSTRs and those that can indicate the unique molecular pathogenetic alterations that underlie the distinction, for example, global loss of RB and aberrant p53 in pancreatic NECs compared with loss of ATRX, DAXX, and menin in pancreatic NETs. Other differential diagnoses are discussed with recommendations for biomarkers that can assist in correct classification, including the distinctions between epithelial and non-epithelial NENs that have allowed reclassification of epithelial NETs in the spine, in the duodenum, and in the middle ear; the first two may be composite tumors with neuronal and glial elements, and as this feature is integral to the duodenal lesion, it is now classified as composite gangliocytoma/neuroma and neuroendocrine tumor (CoGNET). The many other aspects of differential diagnosis are detailed with recommendations for biomarkers that can distinguish NENs from non-neuroendocrine lesions that can mimic their morphology. The concepts of mixed neuroendocrine and non-neuroendocrine (MiNEN) and amphicrine tumors are clarified with information about how to approach such lesions in routine practice. Theranostic biomarkers that assist patient management are reviewed. Given the significant proportion of NENs that are associated with germline mutations that predispose to this disease, we explain the role of the pathologist in identifying precursor lesions and applying molecular immunohistochemistry to guide genetic testing.

SATB2 is expressed in neuroendocrine carcinoma of the uterine cervix

Neuroendocrine carcinoma (NEC) of the uterine cervix is less characterized than neuroendocrine neoplasms of other sites such as of the digestive system and the lung. Special AT-rich sequence-binding protein 2 (SATB2) recently emerged as a marker of well-differentiated neuroendocrine tumors of the lower gastrointestinal (GI) tract. Among NECs, SATB2 is more frequently expressed in cutaneous Merkel cell carcinoma than in NEC of other anatomical sites. In our study, we performed an immunohistochemical study of SATB2 in 16 NECs of the uterine cervix, where the expression of these markers is still undefined. SATB2 was expressed in 12/16 cervical NECs (75%), with 7/16 cases (44%) showing SATB2 positivity in ≥ 50% of cells. In 7 cervical NECs associated with a non-neuroendocrine component, the expression of SATB2 was restricted to the neuroendocrine component. SATB2 was positive in all cases that expressed CDX2 (n = 7) and TTF1 (n = 5), with no evident association with p16 and p53. Our study demonstrated that SATB2 is often expressed in NECs of the uterine cervix. This information should be taken into account when assessing the origin of a NEC.

The Role of Conventionally Fractionated Radiotherapy and Stereotactic Radiotherapy in the Treatment of Carcinoid Tumors and Large-Cell Neuroendocrine Cancer of the Lung

The occurrence of neuroendocrine tumors among the diagnosed neoplasms is extremely rare and is associated with difficulties in undertaking effective therapy due to the histopathological differentiation of individual subtypes and the scarce clinical data and recommendations found in the literature. The choice of treatment largely depends not only on its type, but also on the location and production of excess hormones by the tumor itself. Common therapeutic approaches include surgical removal of the tumor, the use of chemotherapy, targeted drug therapy, peptide receptor radionuclide therapy, and the use of radiation therapy. This article reviews the current knowledge on the classification and application of radiotherapy in the treatment of lung NETs. Case reports were presented in which treatment with conventional radiotherapy, radical and palliative radiochemotherapy, as well as stereotactic fractionated radiotherapy in the treatment of typical (TC) and atypical (AT) lung carcinoids and large cell neuroendocrine carcinoma (LCNC) were used. We hope that the solutions presented in the literature will allow many radiation oncologists to make the best, often personalized decisions about the therapeutic qualifications of patients.

Chronotype: what role in the context of gastroenteropancreatic neuroendocrine tumors?

BACKGROUND

Chronotype is defined as a trait determining the subject circadian preference in behavioral and biological rhythms relative to external light-dark cycle. Although individual differences in chronotype have been associated with an increased risk of developing some types of cancer, no studies have been carried out in gastroenteropancreatic neuroendocrine tumors (GEP-NET).

MATERIALS

We investigate the differences in chronotype between 109 GEP-NET and 109 healthy subjects, gender-, age-, and BMI-matched; and its correlation with tumor aggressiveness.

RESULTS

GEP-NET patients have a lower chronotype score (p = 0.035) and a higher percentage of evening chronotype (p = 0.003) than controls. GEP-NET patients with morning chronotype had lower BMI, waist circumference, and higher percentage of MetS (p < 0.001) than evening type. Interestingly, considering the clinical pathological characteristics, patients with the presence of metastasis, grading G2, and in progressive disease presented the lower chronotype score (p = 0.004, p < 0.001, and p = 0.002; respectively) compared to other categories. Chronotype score was negatively associated with anthropometric measurements, metabolic profile, percentage of MetS, and Ki67 index (p < 0.001 for all).

CONCLUSIONS

GEP-NET patients have an unhealthy metabolic profile and present more commonly an evening chronotype. These results support the importance of including the assessment of chronotype in an adjunctive tool for the prevention of metabolic alterations and tumor aggressiveness of GEP-NET.

Neuroendocrine tumor (NET) of the vagina in the light of WHO 2020 2-tiered grading system: clinicopathological report of the first described case

Up to now, a vaginal well-differentiated neuroendocrine tumor (NET) has never been well described in the literature. A 2-cm vaginal nodule morphologically revealed a proliferation of mild to moderately atypical eosinophilic epithelioid cells, without tumor cell necrosis. Immunohistochemistry (IHC) showed positivity for CK (AE1/AE3), chromogranin A, and synaptophysin, focal positivity for CDX2, and negativity for PAX8 and TTF1. Metastatic origin was excluded by imaging and the morphological context with benign mucinous glands as present in the surgical resection specimen. Considering mitotic index and Mib1/Ki67 (8 mitoses/2 mm²; >20%), the case was diagnosed as vaginal NET G2 in the light of the WHO 2020 grading system for the gynecologic neuroendocrine neoplasms (NENs). Ranges of Mib1-Ki67 are not yet standardized. Currently, mitotic index and tumor cell necrosis were taken into consideration for the grading system. Gynecologic NENs still represent a diagnostic challenge. A clinico-radiologic workup and an appropriate diagnostic path ruling out the metastatic nature are mandatory to achieve the diagnosis.

A Retrospective Cohort Study of Histology-Proven Neuroendocrine Neoplasms in the Italian Area of Udine

AIMS

The aim of this study was to investigate the epidemiology of histology-proven Neuroendocrine neoplasms (NENs) in an Italian area.

BACKGROUND

NENs are a rare and poorly known disease and the global incidence and prevalence appear to be increasing over the past decades.

OBJECTIVE

The objectives of this study were to estimate the incidence and trends of NENs in a 250,000-inhabitant area in the North-East of Italy in the 1998-2018 period and to compare them with international data.

METHODS

This retrospective cohort study was based on the analysis of anonymous health administrative databases, linked with each other at individual patient level through an anonymous stochastic key. NENs were identified from the anatomical pathology database. The standardized incidence rate (2010ESP and US2000) ± 95% CI per 100,000 were calculated, both annually and globally, for the whole period. Incidence was also calculated for specific anatomical sites and by gender. Trends for the considered periods and sites were summarized through the annual percent change (APC) and average increase (cases per 100,000 per year).

RESULTS

In the 1998-2018 period, the standardized incidence rate of NENs in the area of Udine was 2.49 (APC 3.33). A total of 162 cases were observed (51.2% males). Differences in incidence and trend were observed between sexes. The obtained results were consistent with those reported in other countries, confirming a significant and steady increase in NENs incidence in the last twenty years.

CONCLUSION

This study provides new epidemiological data on NENs in Italy. The observed sex differences deserve further investigations.

Cardio-Metabolic Indices and Metabolic Syndrome as Predictors of Clinical Severity of Gastroenteropancreatic Neuroendocrine Tumors

BACKGROUND

Obesity, mainly visceral obesity, and metabolic syndrome (MetS) are major risk factors for the development of type 2 diabetes, cardiovascular diseases, and cancer. Data analyzing the association of obesity and MetS with gastroenteropancreatic neuroendocrine neoplasms (GEP-NEN) are lacking. Fatty liver index (FLI) is a non-invasive tool for identifying individuals with non-alcoholic fatty liver disease (NAFLD). Visceral adiposity index (VAI) has been suggested as a gender-specific indicator of adipose dysfunction. Both indexes have been proposed as early predictors of MetS. This study aimed to investigate the association of FLI VAI as early predictors of MetS with gastroenteropancreatic neuroendocrine tumors (GEP-NETs).

METHODS

A cross-sectional, case-control, observational study was carried out at the ENETS Centers of Excellence Multidisciplinary Group for Neuroendocrine Tumors, University "Federico II". VAI and FLI were calculated.

RESULTS

We enrolled 109 patients with histologically confirmed G1/G2 GEP-NET (53 M; 57.06 ± 15.96 years), as well as 109 healthy subjects, age, sex- and body mass index-matched. Forty-four GEP-NET patients were G2, of which 21 were with progressive disease, and 27 patients had metastases. GEP-NET patients had a higher value of VAI (p < 0.001) and FLI (p = 0.049) and higher MetS presence (p < 0.001) compared with controls. VAI and FLI values and MetS presence were higher in G2 than in G1 patients (p < 0.001), in patients with progressive disease, and in metastatic vs non-metastatic patients (p < 0.001). In addition, higher values of VAI and FLI and higher MetS presence were significantly correlated with the worst clinical severity of NENs. The cut-off values for the FLI and MetS to predict high grading of GEP-NETs and the presence of metastasis were also provided.

CONCLUSIONS

This is the first study investigating an association between VAI and FLI as early predictors of MetS and GEP-NET. Our findings report that the worsening of clinicopathological characteristics in GEP-NET is associated with higher presence of MetS, NAFLD, evaluated by FLI, and visceral adiposity dysfunction, evaluated by VAI. Addressing the clinical evaluation of MetS presence, NAFLD, and visceral adiposity dysfunction might be of crucial relevance to establish targeted preventive and treatment interventions of NEN-related metabolic comorbidities.

A functional gene expression analysis in epithelial sinonasal cancer: Biology and clinical relevance behind three histological subtypes

Epithelial sinonasal cancers (SNCs) are rare diseases with overlapping morphological features and a dismal prognosis. We aimed to investigate the expression differences among the histological subtypes for discerning their molecular characteristics. We selected 47 SNCs: (i) 21 nonkeratinizing squamous cell carcinomas (NKSCCs), (ii) 13 sinonasal neuroendocrine cancers (SNECs), and (iii) 13 sinonasal undifferentiated cancers (SNUCs). Gene expression profiling was performed by DASL (cDNA-mediated annealing, selection, extension, and ligation) microarray analysis with internal validation by quantitative RT-PCR (RT-qPCR). Relevant molecular patterns were uncovered by sparse partial-least squares discriminant analysis (sPLS-DA), microenvironment cell type (xCell), CIBERSORT, and gene set enrichment (GSEA) analyses. The first two sPLS-DA components stratified samples by histological subtypes. xCell highlighted increased expression of immune components (CD8⁺ effector memory cells, in SNUC) and "other cells": keratinocytes and neurons in NKSCC and SNEC, respectively. Pathway enrichment was observed in NKSCC (six gene sets, proliferation related), SNEC (one gene set, pancreatic β-cells), and SNUC (twenty gene sets, some of them immune-system related). Major neuroendocrine involvement was observed in all the SNEC samples. Our high-throughput analysis revealed a good diagnostic ability to differentiate NKSCC, SNEC, and SNUC, but indicated that the neuroendocrine pathway, typical and pathognomonic of SNEC is also present at lower expression levels in the other two histological subtypes. The different and specific profiles may be exploited for elucidating their biology and could help to identify prognostic and therapeutic opportunities.

Grading of Neuroendocrine Carcinomas: Correlation of 68Ga-PET/CT Scan with Tissue Biomarkers

There is a growing need for more accurate biomarkers to facilitate the diagnosis and prognosis of patients with grade (G) 3 neuroendocrine carcinomas (NECs). In particular, the discrimination between well-differentiated neuroendocrine carcinomas (WD-NECs) and poorly differentiated neuroendocrine carcinomas (PD-NECs) is still an unmet need. We previously showed that ⁶⁸Gallium-(⁶⁸Ga-) PET/CT positivity is a prognostic factor in patients with gastroenteropancreatic (GEP) G3 NECs, correlating with a better outcome in terms of overall survival. Here, we hypothesize that ⁶⁸Ga-PET/CT could help to discriminate between WD-NECs and PD-NECs, adding complementary information to that obtained from morphologic and biologic factors. A retrospective, single-institution study was performed on 11 patients with histologically confirmed, measurable G3 large- or small-cell GEP-NECs according to the 2017 WHO classification. The staging procedures included a ⁶⁸Ga-PET/CT scan. Results of ⁶⁸Ga-PET/CT were correlated in univariate analysis with loss of tissue immunohistochemical expression of DAXX/ATRX or RB1 frequently associated with WD-NECs or PD-NECs, respectively. None of the patients with positive ⁶⁸Ga-PET/CT showed loss of RB1 expression, whereas among those (n = 6) with negative ⁶⁸Ga-PET/CT, 4 showed loss of expression. A trend towards a correlation between loss of RB1 expression and negative ⁶⁸Ga-PET/CT was observed. Our preliminary data support the hypothesis that PD-NECs carrying RB1 mutation and loss of its expression may be associated with negative ⁶⁸Ga-PET/CT. If confirmed in a larger clinical trial, ⁶⁸Ga-PET/CT would help in the stratification of G3 NECs.

Impact of Nutritional Status on Gastroenteropancreatic Neuroendocrine Tumors (GEP-NET) Aggressiveness

Neuroendocrine tumors (NETs) are rare neoplasms mostly originating from the gastroenteropancreatic tract (GEP-NETs). Data regarding nutritional status in GEP-NET patients are limited. The aim of the study was to investigate the nutritional status and adherence to the Mediterranean Diet (MD) in GEP-NET patients and to correlate them with tumor aggressiveness. A cross-sectional case-control observational study was conducted enrolling 83 patients with well-differentiated G1/G2 GEP-NETs after resection, as well as 83 healthy subjects, age, sex and body mass index-matched. Nutritional status was assessed by evaluating with Bioelectrical Impedance analysis and its phase angle (PhA), adherence to the MD according to PREDIMED score, dietary assessment, anthropometric parameters, and clinico-pathological characteristics. GEP-NET patients consumed less frequently vegetables, fruits, wine, fish/seafood, nuts, and more frequently red/processed meats, butter, cream, margarine, and soda drinks than controls. Patients with more aggressive disease presented a lower adherence to MD according to PREDIMED categories in comparison to G1, localized and free/stable disease status. A smaller PhA value and a lower PREDIMED score were significantly correlated with G2 tumor, metastases, and progressive disease. To the best of our knowledge, this is the first study reporting an association between nutritional status and tumor aggressiveness in a selected group of GEP-NETs. Moreover, higher intakes of food of MD, may represent a potential tool for prevention of tumor aggressiveness. Thus, a skilled nutritionist should be an integral part of the multidisciplinary management of GEP-NET patients.

A common classification framework for neuroendocrine neoplasms: an International Agency for Research on Cancer (IARC) and World Health Organization (WHO) expert consensus proposal

The classification of neuroendocrine neoplasms (NENs) differs between organ systems and currently causes considerable confusion. A uniform classification framework for NENs at any anatomical location may reduce inconsistencies and contradictions among the various systems currently in use. The classification suggested here is intended to allow pathologists and clinicians to manage their patients with NENs consistently, while acknowledging organ-specific differences in classification criteria, tumor biology, and prognostic factors. The classification suggested is based on a consensus conference held at the International Agency for Research on Cancer (IARC) in November 2017 and subsequent discussion with additional experts. The key feature of the new classification is a distinction between differentiated neuroendocrine tumors (NETs), also designated carcinoid tumors in some systems, and poorly differentiated NECs, as they both share common expression of neuroendocrine markers. This dichotomous morphological subdivision into NETs and NECs is supported by genetic evidence at specific anatomic sites as well as clinical, epidemiologic, histologic, and prognostic differences. In many organ systems, NETs are graded as G1, G2, or G3 based on mitotic count and/or Ki-67 labeling index, and/or the presence of necrosis; NECs are considered high grade by definition. We believe this conceptual approach can form the basis for the next generation of NEN classifications and will allow more consistent taxonomy to understand how neoplasms from different organ systems inter-relate clinically and genetically.

Curative and palliative surgery in patients with neuroendocrine tumors of the gastro-entero-pancreatic (GEP) tract

The incidence of neuroendocrine tumors (NET) increases with age. Lately, the diagnosis of NET was mainly caused by early detection of small NET (<1 cm) in the rectum and stomach, which are depicted by chance during routine and prophylactic endoscopy. Also in patients with large and metastatic pancreatic and intestinal tumors thorough pathologic investigation with use of different immunohistologic markers discovers more neuroendocrine tumors with low differentiation grade (G2-G3) and more neuroendocrine carcinomas (NEC), nowadays, than in former times. While gastric and rectal NET are discovered as small (<1 cm in diameter) and mainly highly differentiated tumors, demonstrating lymph node metastases in less than 10% of the patients, the majority of pancreatic and small bowel NET have already metastasized at the time of diagnosis. This is of clinical importance, since tumor stage and differentiation grade not only influence prognosis but also surgical procedure and may define whether a combination of surgery with systemic biologic therapy, chemotherapy or local cytoreductive procedures may be used. The indication for surgery and the preferred surgical procedure will have to consider personal risk factors of each patient (i.e. general health, additional illnesses, etc.) and tumor specific factors (i.e. tumor stage, grade of differentiation, functional activity, mass and variety of loco regional as well as distant metastases etc.). Together they define, whether radical curative or only palliative surgery can be applied. Altogether surgery is the only cure for locally advanced NET and helps to increase quality of life and overall survival in many patients with metastatic neuroendocrine tumors. The question of cure versus palliative therapy sometimes only can be answered with time, however. Many different aspects and various questions concerning the indication and extent of surgery and the best therapeutic procedure are still unanswered. Therefore, a close multidisciplinary cooperation of colleagues involved in biochemical and localization diagnostics and those active in various treatment areas is warranted to search for the optimal strategy in each individual patient. How far genetic screening impacts survival remains to be seen. Since surgeons do have a central role in the treatment of NET patients, they have to understand the need for integration into such an interdisciplinary team.

Multilayered heterogeneity as an intrinsic hallmark of neuroendocrine tumors

Neuroendocrine tumors (NETs) comprise a complex and highly heterogeneous group of neoplasms that can arise all over the body, originating from neuroendocrine cells. NETs are characterized by a general lack of symptoms until they are in advanced phase, and early biomarkers are not as available and useful as required. Heterogeneity is an intrinsic, pivotal feature of NETs that derives from diverse causes and ultimately shapes tumor fate. The different layers that conform NET heterogeneity include a wide range of distinct characteristics, from the mere location of the tumor to its clinical and functional features, and from its cellular properties, to the core signaling and (epi)genetic components defining the molecular signature of the tumor. The importance of this heterogeneity resides in that it translates into a high variability among tumors and, hence, patients, which hinders a more precise diagnosis and prognosis and more efficacious treatment of these diseases. In this review, we highlight the significance of this heterogeneity as an intrinsic hallmark of NETs, its repercussion on clinical approaches and tumor management, and some of the possible factors associated to such heterogeneity, including epigenetic and genetic elements, post-transcriptional regulation, or splicing alterations. Notwithstanding, heterogeneity can also represent a valuable and actionable feature, towards improving medical approaches based on personalized medicine. We conclude that NETs can no longer be viewed as a single disease entity and that their diagnosis, prognosis and treatment must reflect and incorporate this heterogeneity.

Significance of cytodiagnosis in primary carcinoid tumor of the middle ear: A case report

Primary carcinoid tumors of the middle ear are uncommon. Cytodiagnosis of the middle ear is not usually performed as a preoperative examination because of the anatomical structure of the middle ear. To the best of our knowledge, the present study reports for the first time a case of a carcinoid tumor of the middle ear, which was preoperatively diagnosed using cytodiagnosis, and also review current literature. A 22-year-old woman complained of left-sided otalgia. A subcutaneous tumor in the left middle ear was observed. Magnetic resonance imaging revealed that the tumor was primarily present in the tympanic cavity of the middle ear and extended to the mastoid antrum and mastoid cells of the middle ear and to the external acoustic meatus. Fine needle aspiration cytology was performed from the tumor protruding into the external acoustic meatus. Cytologically, the tumor was composed of small uniform cells with small round nuclei and granular eosinophilic cytoplasm, arranged in a nested pattern. Tumor cells were immunocytochemically positive for synaptophysin and cytokeratin CAM5.2. These results indicated that this was a neuroendocrine tumor. No other tumor lesions were identified on computed tomography scans, and the tumor was considered to be the primary lesion. Tympanoplasty and mastidectomy were then performed and carcinoid tumor was diagnosed in the postoperative pathological diagnosis. To the best of our knowledge, the present study is the first to report on preoperative cytodiagnosis for carcinoid tumors of the middle ear.

Management of neuroendocrine tumors of unknown primary

Neuroendocrine neoplams (NENs) are mostly relatively indolent malignancies but a significant number have metastatic disease at diagnosis mainly to the liver. Although in the majority of such cases the primary origin of the tumor can be identified, in approximately 11-22% no primary tumor is found and such cases are designated as NENs of unknown primary origin (UPO). This has significant therapeutic implications with respect to potentially resectable hepatic disease and/or application of appropriate medical therapy, either chemotherapeutic agents or targeted treatment, as the response to various treatments varies according to the origin of the primary tumor. This lack of tumor specific orientated treatment may also account for the relatively poorer prognosis of NENs of UPO compared to metastatic NENs with a known primary site. In the majority of cases the primary tumors are located in the small bowel and the lung, but a number may still elude detection. Occasionally the presence of a functional syndrome may direct to the specific tissue of origin but in the majority of cases a number of biochemical, imaging, histopathological and molecular modalities are utilized to help identify the primary origin of the tumor and direct treatment accordingly. Several diagnostic algorithms have recently been developed to help localize an occult primary tumor; however, in a number of cases no lesion is identified even after prolonged follow-up. It is expected that the delineation of the molecular signature of the different NENs may help identify such cases and provide appropriate treatment.