Insulinoma

Etiology

The etiology of insulinoma remains largely unknown. Up to 30% of sporadic insulinomas have been identified as having somatic mutations in the YY1 and Kras genes. Insulinomas are also associated with multiple endocrine neoplasia type 1 (MEN1) syndrome, sharing common pathogenic pathways with other tumors linked to this condition. The MEN1 gene mutation on chromosome 11q13 is the most frequently identified mutation and may also be implicated in approximately one-third of sporadic insulinomas.[4]

Epidemiology

Insulinomas have an incidence of approximately 1 to 32 cases per million persons per year, with a slight female preponderance.[5] Despite their rarity, they represent the most common type of functional pancreatic neuroendocrine tumors. Insulinomas are associated with MEN1 syndrome in approximately 6% to 7% of cases.[6][7][8]

Pathophysiology

Insulinomas are characterized by their autonomous and unregulated secretion of insulin. Unlike normal beta cells, which modulate insulin secretion based on serum glucose concentrations, the neoplastic cells of insulinomas exhibit a loss of feedback inhibition. Consequently, insulin is secreted inappropriately, even during hypoglycemic states, leading to persistent hyperinsulinemia and recurrent episodes of hypoglycemia.

The pathophysiology of insulinomas centers on the disruption of glucose-insulin homeostasis. Under physiological conditions, declining blood glucose levels suppress insulin secretion to prevent hypoglycemia. However, in patients with insulinomas, the tumor cells continue to produce and release insulin regardless of serum glucose levels, exacerbating hypoglycemia. This excess insulin promotes glucose uptake by peripheral tissues, further lowering plasma glucose concentrations and intensifying the clinical manifestations.[9]

At the molecular level, insulinomas may arise from genetic mutations or dysregulations in signaling pathways that govern cell proliferation and hormone secretion. These contribute to their neoplastic nature and loss of regulatory control. While most insulinomas are benign, a minority may demonstrate malignant potential, which has implications for disease progression and management. The clinical severity of insulinomas varies considerably among patients, depending on factors, eg, tumor size, functional capacity for insulin secretion, and the patient’s metabolic state. This variability necessitates a comprehensive and individualized approach to diagnosis and treatment to optimize outcomes and minimize complications.[10][9]

Histopathology

Insulinomas can arise anywhere within the pancreas, and extra-pancreatic primary tumors are uncommon. On gross pathological examination, these tumors are typically solitary and well-demarcated lesions. The exception to this is those associated with MEN1 syndrome, which tends to be multifocal and diffuse. Microscopic evaluation reveals features characteristic of well-differentiated neuroendocrine tumors, often consisting of cells with bland nuclear features and abundant cytoplasm. The architectural patterns may vary, and amyloid deposition is occasionally observed.[11] Pathology reporting for pancreatic neuroendocrine tumors is similar to other neuroendocrine tumors. A comprehensive report includes key parameters, eg, tumor size, histologic grade, and the Ki-67 proliferation index, which is a crucial marker for assessing tumor aggressiveness. Tumor grading is based on mitotic count and Ki-67 percentage, as defined by the World Health Organization (WHO) classification system. These metrics provide critical information for predicting tumor behavior and guiding clinical management.[12]

History and Physical

Fasting hypoglycemia is the most characteristic finding of insulinoma, reported in approximately 73% of cases. About 20% of patients experience both fasting and postprandial hypoglycemic symptoms. Additionally, an increasing number of patients have been reporting exclusively postprandial hypoglycemic symptoms, which appear to be more common in men. Weight gain is a common finding in most patients with insulinoma.[13]

Hypoglycemia manifests through a variety of symptoms. Symptoms related to sympathoadrenal activation may include palpitations, tremulousness, and diaphoresis. Severe hypoglycemia can lead to neuroglycopenic symptoms, eg, blurred vision, confusion, seizures, or behavioral changes. Amnesia of the hypoglycemic episode is frequently observed.[14][13]

The diagnosis of insulinoma should be considered in patients presenting with Whipple’s triad, which comprises symptoms of hypoglycemia, documented low plasma glucose levels, and resolution of symptoms following glucose administration.[13] In rare cases, insulinoma may present with psychiatric symptoms, such as panic attacks, as reported in the literature.[13]

Evaluation

Laboratory Studies

As with all functional endocrine tumors, a biochemical diagnosis should be made before localization studies are performed.[1] Evaluation begins with the demonstration of hypoglycemia and inappropriate hyperinsulinemia. Diagnostic criteria include a plasma glucose concentration of less than 55 mg/dL, an insulin level ≥3 µU/mL, a C-peptide level ≥0.6 ng/mL, a proinsulin level ≥5 pmol/L, a beta-hydroxybutyrate level ≤2.7 mmol/L, and a negative sulfonylurea screen. These findings indicate that hypoglycemia is mediated by hyperinsulinemia. A 72-hour fasting test is considered the gold standard for diagnosing insulinoma. This test is handy when Whipple’s triad is not clinically evident or when biochemical data during spontaneous hypoglycemic episodes are unavailable.[3] 

Imaging Studies

Preoperative localization of insulinomas using noninvasive imaging modalities achieves a success rate of approximately 75%. Contrast-enhanced CT is the most commonly employed initial imaging study, with a sensitivity of 70% to 80%. Insulinomas typically appear as small, well-circumscribed, solid masses that enhance with contrast. MRI detects approximately 85% of insulinomas, which usually enhance homogeneously following gadolinium administration. Glucagon-like peptide-1 receptor imaging is an emerging modality that has demonstrated a detection rate of 66% to 97% in small-scale studies.[15][16]

Additional Diagnostic Studies

For cases with a high clinical suspicion of insulinoma and negative noninvasive imaging results, invasive techniques such as endoscopic ultrasonography (EUS) or selective arterial calcium stimulation testing (SACST) with hepatic venous sampling may be employed. EUS demonstrates a sensitivity of 70% to 95% and is often superior to CT for tumor localization. EUS is the preferred modality when noninvasive studies are inconclusive, as it also facilitates tissue sampling, which is especially valuable for small tumors.[16][17] SACST, with a sensitivity of 93%, is another option, particularly in patients with negative imaging studies or those with multiple endocrine neoplasia type 1 (MEN1) syndrome. It involves administering intraarterial calcium administration with hepatic venous insulin sampling. Calcium does not affect normal insulin secretion but increases secretion from insulinoma cells.[15][18][19]

Histopathological analysis and immunohistochemical staining for chromogranin A, synaptophysin, and insulin are essential to confirm the diagnosis of insulinoma. The Ki-67 index should also be determined for tumor grading, especially in cases with an aggressive clinical course.

Predictors of metastatic disease and poor progression-free survival (PFS) include tumor size ≥2 cm, Ki-67 index >2%, and molecular alterations, eg, loss of chromosomal regions 3p and 6q, a gain of 12q, or evidence of chromosomal instability. Malignant insulinomas are characterized by extrapancreatic involvement, including regional lymph node metastasis or distant spread. Factors associated with malignancy include hypoglycemia that occurs with fasting lasting less than 8 hours, higher serum levels of insulin (≥28 µU/mL) and C-peptide (≥4.0 ng/mL) at the glycemic nadir, and tumor size ≥2.5 cm. The extent of hepatic metastases and lymph node involvement significantly impacts prognosis.[17][8]

Genetic testing in all young patients with insulinoma is recommended. MEN1 syndrome should be considered in individuals with a personal or family history of other endocrine disorders.

Treatment / Management

Surgical Resection

Surgical resection remains the preferred treatment modality for localized insulinoma and is also a viable option in select cases of advanced disease. Single, sporadic insulinomas are frequently curable through surgical intervention. Enucleation is suitable for patients with small (<2 cm) benign tumors that do not encroach upon or involve the main pancreatic duct. In contrast, larger or more aggressive tumors, as well as those close to the pancreatic duct, necessitate pancreatic resections, with distal pancreatectomy being the most commonly performed procedure.[3] Other types of pancreatic resections, eg, central pancreatectomy and pancreaticoduodenectomy, are rarely required. Minimally invasive surgical techniques are increasingly employed, offering comparable oncologic outcomes while reducing postoperative pain and hospital length of stay.[8] The liberal use of intraoperative ultrasound is recommended to accurately locate the tumor, assess its relationship with adjacent structures, and confirm the appropriateness of the planned surgical approach. Despite all these modalities, the tumor is sometimes not identifiable. 

Patients with MEN1 syndrome typically present with multifocal and widespread disease, which may necessitate more extensive surgical interventions. A subtotal distal pancreatectomy combined with enucleation of tumors in the pancreatic head is a commonly recommended approach for these patients.[20] 

For metastatic insulinoma, the most frequent sites of metastasis include the peripancreatic lymph nodes and liver. Surgical resection is not contraindicated if the majority of the metastatic disease can be removed. Although not curative, achieving a reduction of 70% to 90% of the tumor burden is associated with improved disease-free survival and should be pursued when feasible. Contraindications to liver resection include bilobar liver disease, impaired hepatic function, and the inability of the patient to tolerate complex surgical procedures.[21]

Ablation

For patients who are poor surgical candidates, chemical ablation of lesions using ethanol or other sclerosing agents presents an alternative treatment option. This procedure is often guided by endoscopic ultrasound to enhance precision.[22][23](B2)

Medical Management

In cases where curative surgery is not feasible, medical management is employed to control symptoms. Dietary modifications, diazoxide, and somatostatin analogs are commonly utilized. Diazoxide, which suppresses insulin secretion, is the first-line treatment for reducing hypoglycemia and has been shown to alleviate symptoms in up to 60% of patients.[24][3] While somatostatin analogs are considered first-line therapy for most neuroendocrine tumors, they are less effective for insulinomas and are typically reserved for cases refractory to diazoxide.[3](B3)

Additional therapeutic options for well-differentiated pancreatic neuroendocrine tumors include everolimus, an mTOR inhibitor, sunitinib, a tyrosine kinase inhibitor, and peptide receptor radionuclide therapy. Cytotoxic chemotherapy, employing agents such as streptozocin, doxorubicin, and temozolomide, is indicated for the treatment of malignant insulinoma.[3][25]

Liver-Directed Therapy

Given that the liver is the primary site of metastatic disease, targeted therapies for hepatic lesions play a crucial role in the management of patients ineligible for surgical resection. Hepatic artery embolization leverages the principle that liver metastases predominantly derive their blood supply from the hepatic arteries rather than the portal vein. Various techniques, including bland embolization, hepatic chemoembolization, and transarterial radioembolization, are employed to deliver therapeutic agents to the lesions.[26] For cases of limited hepatic disease, percutaneous or surgical ablation using radiofrequency or microwave ablation offers an effective treatment adjunct.[25][27][28][27]

Differential Diagnosis

Differential diagnoses that should also be considered when evaluating insulinomas include:

• Persistent hyperinsulinemic hypoglycemia of infancy
• Noninsulinoma pancreatogenous hypoglycemia syndrome
• Post-gastric bypass hypoglycemia
• Factitious use of insulin
• Sulfonylurea-induced hypoglycemia
• Insulin autoimmune hypoglycemia
• Non-islet-cell tumors that secrete insulin-like growth factors
• Nesidioblastosis[29]

Staging

Tumor Node Metastasis Staging

Researchers have developed several classification and grading systems, including WHO 2010, the European Neuroendocrine Tumor Society (ENET), and the American Joint Committee on Cancer (AJCC) for primitive neuro-ectodermal tumors (PNETs). Most of them provide essential prognostic values.[30] Newer classification systems include WHO 2017 and AJCC eighth edition. The AJCC 2017 staging includes ENETs definitions for tumor, node, metastasis (TNM) stage, and prognostic stage grouping. Further modification of this staging system is available, which might provide better prognostic information.[31] The following clinical assessment is used to define TNM stages:

Tumor (T)

• TX: Tumor cannot be assessed
• T1: Tumor limited to the pancreas, <2 cm
• T2: Tumor limited to the pancreas, 2 to 4 cm
• T3: Tumor limited to the pancreas, >4 cm; or tumor invading the duodenum or common bile duct
• T4: Tumor invasion of adjacent organs (eg, stomach, spleen, colon, adrenal gland) or the walls of large vessels (celiac axis or the superior mesenteric artery)

Nodes (N)

• NX: Regional lymph nodes cannot be assessed
• N0: No regional lymph node involvement
• N1: Regional lymph node involvement

Metastasis (M)

• M0: No distant metastasis 
• M1: Distant metastasis 
  • M1a: Metastasis confined to the liver
  • M1b: Metastasis in at least 1 extrahepatic site (eg, lung, ovary, nonregional lymph node, peritoneum, bone)
  • M1c: Both hepatic and extrahepatic metastases

Prognosis

In patients with insulinoma, the 10-year survival rate is approximately 88% following successful surgical removal, and 87.5% of patients achieve a cure, defined as being symptom-free for at least 6 months. For malignant insulinomas, the 10-year survival rate has been reported as 29% following successful surgical resection, with a 5-year survival rate of 24% in another study.[32][1]

Patients with MEN1 syndrome or malignant insulinomas exhibit higher rates of failure in initial surgical interventions or recurrence of the disease. Among patients with MEN1 syndrome, the recurrence rate is approximately 21% at both 10 and 20 years postsurgery. In contrast, patients without MEN1 syndrome experience a lower recurrence rate, estimated at 5% at 10 years and 7% at 20 years.[2]

Complications

The complications associated with insulinoma are diverse and can significantly impact patient quality of life. Persistent hypoglycemia, a hallmark of the condition, interferes with daily activities and can lead to episodes of confusion, disorientation, and loss of consciousness, thereby increasing the risk of accidents and injuries. Chronic and severe hypoglycemia is associated with irreversible neurocognitive impairment due to repeated episodes of brain glucose deprivation, which can manifest as memory deficits, reduced cognitive function, and, in extreme cases, permanent brain damage. If left untreated, the condition carries a risk of mortality due to prolonged hypoglycemic episodes. Comprehensive management and timely intervention are critical to mitigating these complications and improving patient outcomes.[1]