Imaging of gastroenteropancreatic neuroendocrine tumors

Table 1 Guide to World Health Organization classification of neuroendocrine tumors

Behaviour	Metastasis	Muscularis propria invasion	Differentiation	Size (cm)	Angioinvasion	Ki-67 (%)	Hormonal index
WHO criteria (gastrointestinal)
Benign	-	-	Well-differentiated	≤ 1	-	< 2	-
Benign/Low-grade malignant	-	-	Well-differentiated	1-2	-/+	< 2	-
Low-grade malignant	+	+	Well-differentiated	> 2	+	2-20	+
High-grade malignant	+	+	Poorly-differentiated	Any	+	> 20	-
WHO criteria (pancreas)
Benign	-	-	Well-differentiated	≤ 1	-	< 2	-/+
Benign/Low-grade malignant	-	-	Well-differentiated	> 2	-/+	< 2	-/+
Low-grade malignant	+	+	Well-differentiated	> 4	+	2-20	+
High-grade malignant	+	+	Poorly-differentiated	Any	+	> 20	-

Exceptions: Malignant duodenal gastrinomas are usually < 1 cm and confined to submucosa. Benign neuroendocrine tumors of the appendix usually invade the muscularis propria. WHO: World Health Organization.