Surgical aspects of small intestinal neuroendocrine tumors

Table 1 World Health Organization classification 2019 and grading criteria for neuroendocrine neoplasms of the gastrointestinal tract

Terminology	Differentiation	Grade	Ki-67 proliferative index (%)	Mitotic index (per 10 high-power fields)
NET, G1	Well-differentiated	Low	< 3	< 2
NET, G2	Well-differentiated	Intermediate	3-20	2-20
NET, G3	Well-differentiated	High	> 20	> 20
NEC, SCNEC	Poorly differentiated	High	> 20	> 20
NEC, LCNEC	Poorly differentiated	High	> 20	> 20
Mixed neuroendocrine-non-neuroendocrine neoplasm	Well or poorly differentiated	Variable	Variable	Variable

NEC: Neuroendocrine carcinoma; SCNEC: Small cell type neuroendocrine carcinoma; LCNEC: Large cell type neuroendocrine carcinoma.

Table 2 The American Joint Committee on Cancer 8^th edition staging of small intestinal neuroendocrine tumors

Tumor	Description
TX	The primary tumor cannot be evaluated
T0	No evidence of primary tumor
T1	Tumor ≤ 1 cm and only involves the lamina propria or submucosa
T2	Tumor > 1 cm or invades muscularis propria
T3	Tumor invades through muscularis propria intoSub-serosal tissues without serosal invasion
T4	Tumor invades serosa or other organs
Lymph nodes
NX	Lymph nodes cannot be assessed
N0	No regional lymph node metastasis
N1	Lymph node metastasis < 12 nodes
N2	Lymph node metastasis ≥ 12 nodes or mesenteric masses > 2 cm
Metastases
M0	No distant metastasis
M1	Distant metastasis
M1a	Metastasis limited to the liver
M1b	Metastases in at least one extrahepatic site
M1c	Both hepatic and extrahepatic metastases
Stage
Stage I	T1, N0, M0
Stage II	T2 or T3, N0, M0
Stage III	T4, N0, M0; any T, N1 or N2, M0
Stage IV	Any T, any N, M1