Neuroendocrine tumors of unknown primary: a case report and review of tumors with gastroenteropancreatic features

Introduction

Neuroendocrine tumors (NETs) are uncommon neoplasms derived from neuroendocrine cells distributed throughout the body. Approximately two-thirds of NETs are of gastroenteropancreatic origin, arising from neuroendocrine cells of the gastrointestinal tract and pancreas (1). Of these gastroenteropancreatic cases, 30–50% present with regional or distant metastases, with the liver being the most prevalent site of metastasis (2). Recent epidemiologic data suggest that the incidence of gastroenteropancreatic NETs has increased across multiple organ sites in the United States (3). As a result, management of this subset of NETs has become an increasingly important consideration in modern clinical practice.

Approximately 8% of NETs with gastroenteropancreatic features are classified as occult, or having an unknown primary site, constituting a recognized entity in the 2022 World Health Organization/International Agency for Research on Cancer (WHO/IARC) classification framework of NETs (4,5). While certain clinical, radiographic, and immunohistochemical features may suggest a gastroenteropancreatic origin, definitive localization of the primary site remains elusive in a substantial number of patients. Understanding the implications of these NETs with occult primaries is important, as uncertainty regarding tumor origin may impact diagnostic algorithms and management.

Although NETs of unknown primary are recognized in current classifications and guidelines, there remains a lack of focused synthesis of the biological, diagnostic, and prognostic considerations for those with specifically gastroenteropancreatic features. A dedicated analysis of this subgroup may help optimize evaluation strategies and refine risk stratification. Here, we explore a case of a 19-year-old male who was diagnosed with a grade 2 well-differentiated NET of unknown primary origin, with the majority of metastatic disease burden in the liver and perigastric lymph nodes. We present this case in accordance with the CARE reporting checklist (available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0467/rc).

Case presentation

The patient was a 19-year-old white male at a tertiary academic medical center with a past medical history of celiac disease and chronic, refractory gastroesophageal reflux disease (GERD) managed on a proton pump inhibitor (PPI) for 7 years. He presented with a 6-month history of worsening GERD, intermittent abdominal pain, bloating, diarrhea, and weight loss despite following a gluten-free diet. Esophagogastroduodenoscopy and colonoscopy showed gastric erosions, duodenal ulcers, and erythema and granularity of the large bowel. Biopsies of the gastric antrum, descending duodenum, colon, and terminal ileum were all negative for metaplasia, dysplasia, and neoplasia.

Initial labs were notable for a persistent monocytosis, elevated fasting gastrin levels at 405 pg/mL, elevated chromogranin A levels at 696 ng/mL, and glucagon levels within normal limits. Computed tomography (CT) abdomen/pelvis showed multiple indeterminate hepatic hypodensities measuring up to 1.0 cm and a hypodense retrogastric lesion (Figure 1A,1B). Positron emission tomography (PET)/CT ⁶⁸Ga-DOTATATE scan showed uptake in a retrogastric lesion, concerning for a NET, notably with no visualization of radiotracer avid lesions in the liver (Figure 1C,1D). Endoscopic ultrasound (EUS) was significant for a 30 mm × 25 mm well-defined round, hypoechoic perigastric lesion in the gastrohepatic ligament, presumably a lymph node, as well as a 12 mm × 11 mm round lesion in the left liver. Pathology confirmed both lesions to be involved by grade 1 well-differentiated metastatic neuroendocrine disease. MRI Abdomen confirmed a stable 2.5 cm perigastric/peripancreatic mass and revealed multiple (at least 7) bilobar liver lesions, better visualized on this exam and concerning for metastases (Figure 2).

Figure 1 CT abdomen/pelvis with corresponding PET/CT ⁶⁸Ga-DOTATATE scan. (A) Indeterminate hepatic hypodensities (arrows). (B) No evidence of focal radiotracer uptake in hepatic lesions. (C) Retrogastric hypodense lesion (triangle). (D) Increased DOTATATE uptake in retrogastric lesion (triangle). CT, computed tomography; PET, positron emission tomography.

Figure 2 MRI abdomen W/WO contrast. (A) MRI of the abdomen confirming a 2.5 cm perigastric/peripancreatic mass (triangle) with two bilobar liver lesions (arrows), measuring up to 1 cm and concerning for metastatic disease. (B) MRI of the abdomen further demonstrating the perigastric/peripancreatic mass (triangle) with three bilobar liver lesions (arrows), concerning for metastases. MRI, magnetic resonance imaging; W/WO, with/without contrast.

Genetic testing was negative and multiple endocrine neoplasia type 1 (MEN1) syndrome was ruled out. Despite an elevated serum serotonin level (266 ng/mL), the patient did not exhibit cardinal features of carcinoid syndrome (e.g., flushing, diarrhea, palpitations, bronchospasm), and 24-hour urine 5-hydroxyindoleacetic acid (5-HIAA) levels were within normal limits. Echocardiogram for workup of patient’s episodic dyspnea and lightheadedness showed normal tricuspid and pulmonic valve function, making carcinoid heart disease unlikely.

The patient underwent an upper midline exploratory laparotomy and cytoreduction. Meticulous surgical exploration was conducted of the upper abdomen (including intraoperative sonography of the liver and pancreas) and manual exploration of the stomach, small intestine, peritoneum, pelvis, colon, rectum, gallbladder, and appendix. The duodenum was Kocherized to allow full mobilization, enabling complete palpation of the duodenum and pancreas, as well as thorough intraoperative ultrasound evaluation of the pancreas.

The ⁶⁸Ga-DOTATATE avid nodal mass (3.5 cm) in the gastrohepatic ligament was immediately apparent as per imaging and appeared to be separate from both the stomach and pancreas. A smaller satellite mass was also located in the gastrohepatic ligament, similarly separate from the stomach and pancreas. Resection of both perigastric nodules as well as lesser omentectomy were completed.

Intraoperative ultrasound identified at least seven nodules in the patient’s liver. These were two lesions in segment VIII near the hepatocaval confluence, a peripheral lesion in segment VI, a deeper lesion in segment VI, a deep lesion in segment IVb, and peripheral lesions in segments II and III. Lesions were localized by closely comparing sonographic findings to the patient’s preoperative MRI using anatomic landmarks. Five lesions were resected with enucleation, and the two deeper lesions were treated with microwave ablation, applied at 100 W for 2 minutes each. Of note, intraoperative liver ultrasound also identified dozens of tiny (1–2 mm) hyperechoic lesions deep in the parenchyma, deemed indeterminate lesions. Despite this, resection of the dominant lesions was estimated to achieve greater than 95% cytoreduction.

The entire abdomen, including the region of the gastrinoma triangle, was systematically explored to search for the primary tumor or other metastatic disease. No primary tumor was identified on inspection of the entire stomach, small intestine, peritoneum, pelvis, colon, and rectum. Cholecystectomy and appendectomy were performed for diagnostic and risk-reduction purposes.

Immunohistochemistry analysis on paraffin-embedded sections of the perigastric and hepatic nodules confirmed involvement by well-differentiated metastatic NET, grade 2 of 3, with approximately 5% of cells staining positive for Ki-67. The lesions stained positively for CAM 5.2, CK20, chromogranin, synaptophysin, CDX2, S100, and INSM1. Staining was negative for CK7, SATB2, and TTF-1. The larger ⁶⁸Ga-DOTATATE avid perigastric mass showed no residual lymph node tissue under microscopy, suggesting full effacement of the node by neoplastic cells, while the smaller satellite mass was confirmed to be another lymph node involved by metastatic well-differentiated NET. No carcinoma or NET was identified in the appendix or gallbladder.

The diagnosis of a grade 2 well-differentiated metastatic NET of unknown primary was made. The patient’s post-operative course was uneventful, and he was discharged home 6 days later. Due to intraoperative discovery of several indeterminate liver lesions concerning for possible miliary metastases, the patient was started on monthly lanreotide injections approximately 1 month later. After a post-operative follow-up of 6 months, the patient remained asymptomatic with normalized chromogranin A levels, downtrending serum gastrin levels, and no definite evidence of disease recurrence or progression on imaging.

All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Verbal consent was obtained from the patient for their medical information to be published in print and online with the understanding that this information may be publicly available. Written informed consent was not pursued because the case report was fully deidentified and considered minimal risk in accordance with institutional and journal guidelines..

Discussion

NETs are rare epithelial neoplasms derived from pluripotent endocrine cells throughout the body. NETs of unknown primary origin represent a particular clinical challenge due to their heterogeneity in molecular biology, clinical behavior, and prognostic features. “Carcinoid-like” symptoms or syndromes such as persistent peptic ulcer disease may lead the clinician to discover these cancers. In certain instances, however, the tumor can present in an ambiguous manner, especially if the tumor is non-functioning, presenting a difficult clinical case.

Proposed mechanisms for NETs of unknown primary origin

Several hypotheses have been proposed to explain the mechanism behind occult primary NETs, particularly those with gastroenteropancreatic features. Most widely accepted is the idea that these tumors can arise from small, slow-growing primaries that are difficult to detect. For example, small or submucosal NETs of the small intestine may be overlooked in routine tissue examination. Anlauf et al. reported a case in which multiple microgastrinomas were identified in a patient with MEN1 only after complete embedding and microscopic examination of the entire pancreaticoduodenectomy specimen (6). Furthermore, CT and somatostatin receptor scans have limited sensitivity for small intestinal primaries, which often require meticulous palpation and inspection for intraoperative diagnosis.

Another area of debate concerns whether some NETs of occult primary may have arisen as primary tumors themselves. Prior literature has bolstered the existence of primary nodal or hepatobiliary NETs in patients without any other evident source of primary (7-11). Proposed mechanisms include differentiation of ectopic pancreatic or adrenal tissue, or malignant transformation of stem or multipotent neuroendocrine cells in the hepatobiliary tree (9). While hypothetical, these theories offer plausible explanations for NETs occurring in atypical locations without clear evidence of other primary.

Although speculative, spontaneous regression of the primary tumor cannot be excluded. The spontaneous regression of advanced tumors in patients has been an uncommon but well-documented phenomenon (12,13). In a study by Okamoto et al., a case of gastric gastrinoma regressed spontaneously after biopsy and resection of a metastatic lesion in the lesser omentum (14). Other reports describe instances of spontaneous regression of NETs after pregnancy (15) or cytomegalovirus infection (16), suggesting a possible immunologically mediated mechanism of regression. While a primary tumor was not identified at any point in the current case, spontaneous regression of the primary may account for a subset of NETs of unknown primary.

Ultimately, it remains unclear what the primary source of neuroendocrine malignancy was in this patient. His clinical presentation was not typical of Zollinger-Ellison syndrome, given nonspecific gastric erosions and the absence of hypertrophic rugal folds. While gastrin levels were elevated pre-surgery and trended downwards after surgery, the tumor specimens were not stained for gastrin, and the presence of secreted hormones alone is not considered diagnostic of any particular subtype. Furthermore, initial fasting gastrin levels were obtained while this patient was on prolonged PPI therapy, which is known to elevate gastrin. Accurate assessment requires PPI cessation, and normalization may take up to 1 month. Nonetheless, this case may represent a malignant PPI-associated NET given the patient’s 7-year history of use, a phenomenon well-established in prior literature (17). Overall, this case highlights the diagnostic complexity of NETs with unknown primary and underscores the need to consider occult lesions, atypical tumor origins, and potential regression in clinical decision-making.

Identification and classification

In patients undergoing evaluation for suspected NET, preoperative localization of the primary tumor relies primarily on imaging modalities. In-111 diethylenetriaminepentaacetic acid (DTPA) octreotide scintigraphy (OctreoScan) has long been the standard for somatostatin receptor (SSTR) imaging, though it is limited by false-positive findings and modest spatial resolution (18). A newer imaging technique, ⁶⁸Ga-DOTATATE PET/CT, is now considered the preferred imaging modality per NANETS and NCCN guidelines (19,20), offering whole-body detection of SSTR-expressing tumors with higher sensitivity and improved spatial resolution. Additionally, EUS has become an important complementary tool, providing high-resolution visualization of lesions along the gastrointestinal tract and facilitating tissue biopsy for histopathologic confirmation. For small rectal NETs that may not be readily visualized on colonoscopy, transrectal ultrasonography may similarly assist as an accessible, low-cost tool for preoperative evaluation (21).

Due to promiscuous marker expression, NETs cannot be reliably assigned a site of origin based on immunohistochemical staining alone. For example, NETs of gastrointestinal origin may stain positive for TTF1 (a lung marker), while NETs of pulmonary origin can be TTF1-negative, yet uptake gastroenteropancreatic NET markers. A new promising diagnostic tool is the 92-gene assay (CancerTYPE ID, Biotheranostics, Inc, San Diego, Calif), a reverse transcriptase polymerase chain reaction-based assay which has demonstrated high accuracy in predicting tumor classification when the site of origin is unclear (22).

Despite these diagnostic improvements, preoperative evaluation fails to identify the primary tumor in approximately 20% of patients (23). In these cases, laparoscopic or open surgical exploration has shown a high success rate (86.7%) in identifying occult primary tumors (23). As established by Jeffrey Norton, duodenotomy is critical when a gastrinoma is suspected, as the majority arise within the duodenal wall (24). Endoscopic transillumination of the gastric and duodenal walls may reveal additional occult lesions, though there is currently no high-level evidence demonstrating greater sensitivity compared to meticulous bimanual palpation. Finally, if comprehensive abdominal exploration fails to identify a primary tumor, bronchoscopy should be considered to assess for possible pulmonary origin.

It must be noted that most of these additional intraoperative procedures are not routinely performed, and tumors may therefore be categorized as “unknown primary” despite the absence of a truly comprehensive search. This raises the question of what should appropriately qualify as a NET of unknown primary: those in which the primary site remains occult despite comprehensive evaluation, or those in which the primary has not been identified due to a partial workup? Further guidelines regarding preoperative and intraoperative evaluation are needed to clearly define the criteria for classification of unknown primary disease in NETs.

Treatment strategies and prognostic implications

Metastatic well-differentiated NETs of unknown primary are often treated similarly to midgut tumors (20). Following surgical management, long-acting somatostatin analogs (e.g., lanreotide) are the standard of care for metastatic gastroenteropancreatic NETs (25). Further second-line systemic therapies include mTOR inhibitors (e.g., everolimus) and tyrosine kinase inhibitors (e.g., sunitinib) (26). In the case of gastroenteropancreatic NETs, patients may benefit from peptide receptor radionuclide therapy (PRRT) using ¹⁷⁷Lu-DOTATATE, a targeted therapy using radioactive Lutetium-177 to treat SSTR-positive NETs (27). Though not FDA-approved for other NETs, PRRT is considered for NETs of unknown primary. Chemotherapy regimens, such as capecitabine/temozolomide (CAPTEM), are typically reserved for poorly differentiated, high-grade NETs and are less effective in well-differentiated, low-grade NETs (28). Although immune checkpoint inhibitor monotherapy has demonstrated efficacy in several cancers, it has shown limited effectiveness in NETs, with shorter durations of disease control and benefit restricted to a subset of patients (29,30).

It is important to note that identification of a primary origin remains clinically relevant due to its potential impact on treatment planning, as certain drugs are approved for specific primary origins (e.g., sunitinib for treatment of pancreatic NETs). Primary identification is also important for eligibility for clinical trials and investigational therapies, which are often primary site-specific. Furthermore, several studies have demonstrated that resection of the primary tumor is associated with improved overall survival, regardless of liver-directed therapy (31-33). This therapeutic advantage is inherently limited in patients with unknown primaries.

The distribution of metastatic disease also has important prognostic implications. In pancreatic NETs, mortality is significantly increased in patients with liver metastases compared to those with extrahepatic disease (34). On the other hand, gastrointestinal NETs with peritoneal carcinomatosis are associated with overall worse prognosis (35). These differences underscore the importance of both tumor origin and metastatic pattern in shaping clinical outcomes.

Importantly, the designation of “unknown primary” in NETs should not necessarily be interpreted as a failure of diagnostic evaluation or clinical care. Rather, it may reflect the unique biological and anatomical characteristics of these tumors. Unknown primary status in NETs does not inherently confer a worse prognosis and should not preclude aggressive management, including cytoreductive surgery, liver-directed therapies, or systemic treatments when appropriate.

Conclusions

A comprehensive understanding of the diverse presentations of NETs of unknown primary is critical to advancing diagnostic and therapeutic strategies for this subset of patients. When feasible, comprehensive preoperative imaging and surgical exploration should be undertaken, complemented by molecular assays such as the 92-gene assay, to facilitate identification of the primary site. Metastatic burden should continue to inform prognostication and guide individualized treatment planning. Future efforts should focus on developing standardized, evidence-based preoperative and intraoperative evaluation algorithms to more consistently define and stage “unknown primary” disease.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the CARE reporting checklist. Available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0467/rc

Peer Review File: Available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0467/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0467/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Verbal consent was obtained from the patient for their medical information to be published in print and online with the understanding that this information may be publicly available. Written informed consent was not pursued because the case report was fully deidentified and considered minimal risk in accordance with institutional and journal guidelines.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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