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Pancreatic Neuroendocrine Neoplasms (PanNENs)

Source: Robbins, Cotran & Kumar - Pathologic Basis of Disease, p. 1011-1012

Overview

The preferred term for what were historically called "islet cell tumors" is pancreatic neuroendocrine neoplasms (PanNENs). They are rare, accounting for only 2% of all pancreatic neoplasms, but they are clinically important because many secrete hormones that produce dramatic clinical syndromes.

Key general features:

Can occur anywhere in the pancreas or adjacent peripancreatic tissues
May be single or multiple, and of varying histologic grades
Graded by mitotic activity and Ki-67 proliferative index
Poorly differentiated neuroendocrine carcinomas (small cell or large cell type) have Ki-67 >20%
May be functional (hormone-secreting) or nonfunctional
Prognosis: ~90% of insulin-producing tumors are benign; 60-90% of other subtypes are malignant

Pathogenesis involves recurrent mutations in three major pathways:

MEN1 gene (also causes familial MEN syndrome type 1)
PTEN/TSC2 loss-of-function → activates mTOR signaling
ATRX/DAXX inactivation → alternative lengthening of telomeres (ALT); found in ~50% of PanNENs

1. Hyperinsulinism (Insulinoma) - p. 1011

Insulinomas are the most common PanNEN subtype and arise from beta (β) cells of the islets of Langerhans.

Pathophysiology

The tumor secretes insulin autonomously, independent of blood glucose levels, causing hypoglycemia.

Clinical Features - Whipple's Triad (classic)

Hypoglycemic episodes when blood glucose falls below 50 mg/dL
Symptoms: confusion, stupor, loss of consciousness (neurological due to glucose deprivation)
Episodes precipitated by fasting or exercise
Promptly relieved by feeding or IV glucose

Key Lab Finding

Inappropriately high circulating insulin in the setting of hypoglycemia
Elevated C-peptide confirms endogenous (not exogenous) hyperinsulinism

Morphology

Usually solitary, small (<2 cm), encapsulated, pale-to-red-brown nodules
Look like giant islets histologically - monotonous cells, minimal pleomorphism, minimal mitotic activity
Only ~10% are malignant (carcinomas)
Characteristic histologic feature: amyloid deposition (pink material on H&E)

Fig. 24.40 - Insulinoma histology showing monotonous neoplastic cells with abundant amyloid deposition (pink material), characteristic of insulinoma. The patient had episodic hypoglycemia.

Treatment: Surgical resection - usually followed by prompt reversal of hypoglycemia.

Important differential diagnoses for hypoglycemia: adrenal insufficiency, diffuse liver disease, glycogenoses, ectopic insulin production, sulfonylurea use, factitious hypoglycemia (self-injection of insulin).

2. Zollinger-Ellison Syndrome (Gastrinoma) - p. 1012

Gastrinomas arise from G cells (gastrin-secreting cells) and are the second most common functional PanNEN.

Key Anatomical Point - "Gastrinoma Triangle"

Gastrinomas are equally likely to arise in the duodenum and peripancreatic soft tissues as in the pancreas itself - all within the so-called "gastrinoma triangle."

Pathophysiology

Tumor secretes excess gastrin → stimulates parietal cells → massive gastric acid hypersecretion → severe, refractory peptic ulceration.

Clinical Features

Peptic ulcers present in 90-95% of patients
Ulcers are often multiple and occur in unusual locations (e.g., the jejunum, which is rare in ordinary peptic ulcer disease)
Ulcers are unresponsive to standard therapy - a key clinical clue
Should be suspected in patients with intractable jejunal ulcers

Morphology

>50% are locally invasive or metastatic at time of diagnosis
~25% arise in the setting of MEN-1 syndrome (multifocal in MEN-1, usually single in sporadic cases)
Histologically bland, rarely show marked anaplasia

Treatment

H+/K+-ATPase inhibitors (PPIs) to control acid hypersecretion
Surgical excision of the neoplasm when possible
Hepatic metastases carry a poor prognosis - progressive liver failure usually within 10 years

3. Other Rare Pancreatic Endocrine Neoplasms - p. 1012

Tumor	Cell	Hormone	Clinical Syndrome
Glucagonoma	α cells	Glucagon	Mild diabetes, necrolytic migratory erythema (skin rash), anemia; most common in perimenopausal/postmenopausal women
Somatostatinoma	δ cells	Somatostatin	Diabetes mellitus, cholelithiasis, steatorrhea, hypochlorhydria
VIPoma	?	Vasoactive intestinal peptide (VIP)	WDHA syndrome: Watery Diarrhea, Hypokalemia, Achlorhydria; can metastasize
Carcinoid	EC cells	Serotonin	Atypical carcinoid syndrome - exceedingly rare in pancreas
PPoma	PP cells	Pancreatic polypeptide	Present as mass lesions; even high hormone levels are asymptomatic

Multihormonal PanNENs also occur - single tumors secreting two or more hormones (e.g., insulin + glucagon + gastrin + ACTH/MSH/ADH/serotonin/norepinephrine). These must be distinguished from the MEN syndromes, where multiple hormones come from tumors in separate glands.

Summary Table

Feature	Insulinoma	Gastrinoma	Others
Cell of origin	β cell	G cell	α, δ, EC, PP cells
Main hormone	Insulin	Gastrin	Glucagon, somatostatin, VIP, etc.
Most common subtype?	Yes (#1)	#2	Rare
% Malignant	~10% (mostly benign)	>50%	High for glucagonoma/VIPoma
Key clinical clue	Hypoglycemia relieved by glucose	Refractory/jejunal ulcers	Varies by type
MEN-1 association	Yes	Yes (25%)	Yes

Robbins, Cotran & Kumar - Pathologic Basis of Disease, pp. 1011-1012

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